AUDIO GLOSSARY
AUDIO GLOSSARY
5-way binding post
A terminal often used on Subwoofers and amplifiers that can accommodate five different
connection options: banana plugs, ring and spade terminals, bare wire and through-hole
pins.
A/B test
A form of listening test in which two sounds are compared with each other in a rapid A/B
fashion. Ideally, the listeners would not know the identity of the two products being
compared. See Double-Blind Listening Test
A/D Converter
A device that accepts an analog signal at its input, and outputs a digital version of the
signal.
A/V
Audio/Video
A/V receiver
Combines the following functions in one device: audio- and video-signal switcher; surround
sound decoder; multichannel amplifier; preamplifier, including tone controls and other
effects; and radio tuner
Absorption
In acoustical absorption, sound energy is converted into heat. See: Resistive Absorbers,
Diaphragmatic Absorbers, Resonant Absorbers, Absorption Coefficient, Sabine
Absorption Coefficient
The proportion of incident sound that is absorbed by a surface. Usually this is expressed as
a number between 0 and 1, or a percentage. In most materials, the amount af absorption
changes with the angle of incidence. The specification is normally a 'random incidence'
measurement.
AC
see Alternating Current
AC Line Conditioner
or Protector
A device inserted between the wall outlet and your equipment to isolate it from voltage
spikes and unwanted high frequency signals that may be picked up by the power lines. High
quality audio equipment may already have some of this kind of isolation built in. See also:
AC Voltage Stabilizer
AC Voltage Stabilizer
A device inserted between the wall outlet and your equipment to isolate it from voltage
spikes and unwanted high frequency signals that may be picked up by the power lines. High
quality audio equipment may already have some of this kind of isolation built in. See also:
AC Voltage Stabilizer
AC-3
The first descriptor for what is now called Dolby Digital.
Acoustic Suspension
A closed box loudspeaker enclosure in which the compliance (spring) of the air inside the
box is a substantial portion of the total compliance of the system, including the mechanical
suspension of the woofer. See: Suspension, Compliance
Acoustical
Interference
When two or more sounds arriving from different directions combine at a point in space,
e.g. at an ear or microphone, those components which are in step with each other (in
phase) will add (constructive interference) and those that are out-of-phase will subtract, or
cancel each other (destructive interference). See: In Phase, Out-of-Phase, Comb Filter.
Acoustics
The physical science dealing with how sound is produced, propagated, manipulated and
perceived. See also: Room Acoustics
Active Crossover
An analog or digital device performing high-pass, low-pass and bandpass functions ahead of
power amplifiers driving the transducers in a loudspeaker
Active Loudspeaker
A loudspeaker which has a built-in power amplifier for at least one driver, usually the
woofer or subwoofer. It may also have amplifiers for mid and high frequency drivers. See:
Powered Tower
Active Matrix Decoder See: Matrix Encode/Decode
AES/EBU
The two-channel digital audio communication process that was standardized by the Audio
Engineering Society (AES) and the European Broadcasting Union (EBU).
Aftermarket
The sale of components for installation after the customer has purchased the original item.
For example, aftermarket car audio equipment can embellish or replace the originally
supplied system
Algorithm
A step-by-step procedure for solving a problem or accomplishing a task, usually by using a
microprocessor or DSP (see below). In digital audio, algorithms are often used for sound
processing, data compression and surround sound en-/decoding.
Alternate Channel
Selectivity
A measure of the ability of a radio tuner to reject information from a radio station close to
the frequency of the one being listened to
Alternating Current
An electric current that moves in both directions, cycling at a regular rate, as in 60 Hz
home power, or at a variable rate as in an electrical music signal being supplied to a
speaker. Also used to describe the accompanying alternating voltages, cycling between
positive and negative
AM
see Amplitude Modulation
AM Rejection
A specification describing to how well a radio tuner can ignore changes in the amplitude of
an FM signal, such as those caused by propagation effects and interference
Ambience
In audio this refers to the reflected and reverberant sound characteristics of an acoustic
space. All rooms can be acoustically 'live' or 'dead'. Large rooms can be flattering to musical
performances (concert hall) or hostile (gymnasium).
Ammeter
A device used to measure current flow in amperes
Amperage
The magnitude of an electrical current as expressed in amperes.
Ampere
Unit of measurement for the electric current flowing through a circuit. Abbreviated: amp.
Ampere-hour
A measure of the quantity of electricity delivered by a battery determined by multiplying
the integrated current in amperes by the duration of the current flow in hours
Ampere-hour
capacity
Rating for a battery describing current in amperes that can be drawn over a period of time
in hours before the discharge limit is exceeded
Amplification
An increase in signal level.
Amplifier
A device that increases the magnitude of the voltage, current or power in an electronic
system. In audio systems, preamplifiers and surround processors amplify voltages. Power
amplifiers amplify both voltage and current, therefore providing more power output in order
to drive loudspeakers
Amplitude
The magnitude of an electrical signal (voltage, current or power), sound (sound pressure or
intensity), or movement of a mechanical device such as a loudspeaker diaphragm.
Amplitude Modulation
A method of radio broadcasting in which the radio carrier frequency is amplitude modulated
by the audio signal. Typically limited in bandwidth, and susceptible to interference and
static. However, it propagates well over long distances and around hills and buildings.
Abbreviated AM.
Analog
An electrical signal in which the voltage (or current) waveform has the same form as the
original acoustical sound waves. See also Digital
Analog-to-digital
converter
See A/D converter.
Anamorphic
A film or video format in which a widescreen image has been "squeezed" horizontally
(either with lenses or by digital manipulation) to fit a standard 4:3 aspect ratio. Correct
picture geometry is restored on playback by "unsqueezing" the image into its original
aspect ratio. Anamorphic DVDs are sometimes marked "Enhanced for widescreen TVs."
See: Widescreen, Aspect Ratio
Anechoic
Without echoes, reflections or reverberation.
Anechoic Chamber
A room without echoes or reflections that is used for precise acoustical measurements, not
contaminated by normal room acoustical factors, including noise. It is the ultimate 'dead'
room. See Reverberation
Anode
The electrically positive terminal of a battery, or the plate of a vacuum tube
Aperture Ratio
In digital display devices, this is a measure indicating the percentage of the available area
that is used for active pixels. Some displays have obvious inactive 'frames' around
individual pixels, leading to the description 'screen-door' effect when viewing such a display
from insufficient distance
Aquaplas
A water based compound high in particulates which is used to adjust the mass of, and to
add damping to, a variety of loudspeaker diaphragm materials
Articulation /
Articulation Index
Having to do with the intelligibility of speech. This is measured using listeners who try to
identify randomly presented 'nonsense' words and phrases. The Articulation Index is the
percentage of correct identifications. Used mainly in large venues.
Artifact
In video, the degradation of picture details when a video decoding system cannot keep up
with frame-by-frame changes. Video compression systems (codecs) rely on the common
reality that only portions of a picture change from frame to frame. If a lot of the picture is
changing (as in panning across a crowd of people) decoders may not be able to keep up,
and the result is that portions of the picture may momentarily revert to large blocks of
color. Other artifacts can be seen in jagged edges, 'staircasing' of guitar strings, etc.
Fortunately, most are brief.
Artificial
Reverberation
Synthesized reflected sounds intended to add to a recording the acoustical impression of
being in a specific size and kind of room, such as a concert hall, stadium, club, etc.
Inexpensive versions tend to be very artificial sounding. The best versions are hardly
distinguishable from the real thing, and in fact are used in numerous concert performing
spaces to improve on less-than-perfect natural acoustics. Also, the electronic reverberation
added to close-miked recordings
Aspect Ratio
The width-to-height ratio of a visual image. Standard NTSC television sets have an aspect
ratio of 4:3 (4 units wide by 3 units high). Widescreen television sets have an aspect ratio
of 16:9. Many films are produced with even wider ratios. Pictures with aspect ratios
different from the display will show dark bars at the top and bottom, or at the sides. See:
Widescreen, NTSC
ATRAC
Acoustic Transformation Adaptive Coding. A perceptual encoding system used in the Sony
MiniDisc format. See: Perceptual Coding
Attack
In music, the onset of a sound or note.
Attenuation
A reduction in signal or sound level
Audio Frequency
Range
The range of human hearing commonly accepted as 20 to 20,000 Hertz (cycles per second)
Audio Interconnect
Cable
A shielded wire used to link the audio signal output of one device to the input of another
audio device
Audio Oscillator
A test instrument that produces single frequency tones for measuring the performance of
audio devices.
Audio Signal
An audio frequency signal in electronic form or after conversion to sound
Audiometer
A closed box loudspeaker enclosure in which the compliance (spring) of the air inside the
box is a substantial portion of the total compliance of the system, including the mechanical
suspension of the woofer. See: Suspension, Compliance
Audiophile
Anyone interested in the reproduction of sound.
Auto Convergence
An automated feature for aligning the red, green and blue guns of a CRT projector or rearprojection television. See: Convergence
Auto Turn-on
A signal sensing circuit designed to turn an audio component on when it receives a music
signal.
A-weighting
See Frequency Weighting
Axial Modes
The acoustical resonances in rooms that occur between opposing parallel surfaces: walls, or
floor and ceiling. See Room Resonances
Azimuth
In tape recorders, the angle made by the magnetic gap in the record/play heads and the
direction of tape motion. The playback azimuth must equal the record azimuth or there will
be a loss of high frequencies on playback
Background Noise
In listening, it is any sound (normally unwanted) that is not part of the sound track being
auditioned. In measurement, it is extraneous sound that, if too loud, can cause errors in the
measured data
Baffle
The board to which loudspeaker drivers are mounted in a loudspeaker enclosure
Balance Control
In stereo systems, a control to adjust relative sound levels in the left and right
loudspeakers. In multichannel systems there is a front-back balance adjustment. In car
audio, the front-back adjustment is called a 'fader'
Balanced Connection
A method of interconnecting audio components using a three-wire cable in which there are
two signal wires and one ground wire, all of which may be inside a cylindrical shield that is
also grounded. The two signal conductors both have the identical impedances to the
common ground terminal, hence the name Balanced. Because of this such interconnections
are highly immune to ground-originated noises and hum. It is widely used in professional
audio systems which routinely have very long cable runs and very complex interconnections
of signal and power grounds. The XLR plug is commonly used. See: Unbalanced Connection
Banana Plug and Jack A largish, pin-type, connector commonly used to connect loudspeakers to power amplifiers
Bandpass
An adjective describing a system or device that operates only over a specified range of
frequencies. In audio, there are bandpass filters, as in loudspeaker crossovers, and
bandpass loudspeakers as in bandpass subwoofers. In all cases, frequencies outside of the
pass-band range are severely attenuated
Bandpass Enclosure
A form of low-frequency loudspeaker enclosure in which chambers, drivers, and internal
and external ports are configured in an integrated acoustical design that behaves as an
acoustical bandpass filter, exhibiting natural low-and high-pass limits to the acoustical
output. See: Low-Pass Filter, High-Pass Filter.
Bandwidth
The difference between the upper and lower usable frequency limits of a circuit, a device, or
a communications (radio, TV or digital data transmission) channel
Barrel Distortion
In video, a form of geometric distortion in which straight lines bulge or bow outward toward
the edges of the screen, e.g. making a square look barrel shaped. See: Pincushion
Distortion
Basket
The outer frame of a loudspeaker driver that supports the magnet, the diaphragm and
other moving parts.
Bass
Low-frequency audio signals. Frequencies below approximately 300Hz (below 100Hz in
cars)
Bass Boost/Cut
An electrical filter designed to increase or decrease the amount of bass reproduced by an
audio system
Bass Control
A tone control allowing the user to boost or cut the low frequency portion of the audio
signal.
Bass Management
A function in a multichannel surround processor that combines the low bass frequencies
from all of the channels (including the LFE channel) in a recording, and directs it to the
appropriate loudspeakers. To do this, the customer must tell it the number, kind (small or
large), and placement of loudspeakers, and whether there is a subwoofer in the system.
See: LFE
Bass Reflex
A low-frequency loudspeaker enclosure design in which the volume of the enclosure and the
dimensions and shape of a vent, or port, form a Helmholtz resonance which is designed to
integrate with the performance parameters of a woofer. Such systems are characterized by
increased acoustical output at certain frequencies near the system resonance, less output
at lower frequencies, and the potential for reduced distortion at high sound levels. See:
Helmholtz Resonance
Beaming
See Directivity
Beats
The combination or difference tone heard when two closely spaced frequencies combine in
the ear. It is an intermodulation effect, or distortion
Bi-Amping
In loudspeakers that have the appropriate connections, bi-amping is the practice of using
two power amplifiers to drive a single loudspeaker, one for the woofer and one for the
mid/high frequency section. Normally, the internal passive crossovers are used so that the
manufacturer's system design is left intact (assuming that the amplifiers gains are identical,
and they are in phase). In large professional systems an external electronic crossover is
often used, in which case it is essential to equalize the system afterwards, since the user is
really designing the loudspeaker system from the ground up. See: Bi-Wiring
Binaural
Literally, two ears, or listening through two ears. Also used to describe recordings made
with a dummy head microphone, with microphones at each ear location, intended for
playback through headphones or two loudspeaker with acoustical crosstalk cancellation so
that the sound of each loudspeaker is heard primarily by only one ear. See Monaural
Binding post terminal
A terminal that provides several connection options. See also 5-way binding post
Bipole
A loudspeaker system with forward and backwards firing transducers connected in the same
polarity (in phase). The directional pattern is almost omnidirectional at frequencies up to a
few thousand Hz, beaming slightly forward and back at the highest frequencies
Bit
A binary digit, the basic unit of digital data of all kinds. It has two states, off and on, high
or low, zero or one. In the encoding of audio signals, each bit translates into approximately
6 dB of useable dynamic range. Thus, the 16-bit encoding of CD's permits a maximum
dynamic range of 16 x 6 = 96 dB, accompanied by very low distortion and noise. Current
technology permits higher bit rates, and so we are seeing even wider dynamic ranges, e.g.
24 bits, which is useful for professional audio, and more than enough for a delivery medium
Bi-Wiring
In loudspeakers that have the appropriate connections, bi-wiring is the practice of using
two sets of wires from a single amplifier to a loudspeaker system, one for the woofer and
one for the mid/high frequency section. See: Bi-Amping
Black Level
The level of brightness at the darkest part of a visual image
Blind Testing
See Double-Blind Listening Test
Blooming
Video distortion in CRT display devices, caused by excessive white level (contrast) settings
resulting in images that have lost focus or sharpness
BNC
A high quality bayonet (push and turn to lock) connector used with some high frequency
video and digital signal cables
Bookshelf Speaker
A small loudspeaker system, capable of fitting on a bookshelf or in a cavity in A/V furniture.
However, from a sound quality perspective, shelves and cavitities are the worst possible
locations for such loudspeakers. They sound best when mounted on stands away from walls
and other objects
Boomy
Bass reproduction in which certain notes are exaggerated or prolonged by one or more
resonances. See Resonance
Boundary Effects
In acoustics, the interactions between loudspeakers and listeners, and the room walls, floor
and ceiling. Often refers to the interactions with boundaries that are adjacent to the source
or receiver
Breakup
When a loudspeaker diaphragm fails to move in a pistonic fashion, and flexes, or breaks up,
distorting the sound
Bridging
Combining the outputs of two amplification channels to provide one more powerful channel.
Note that bridging may raise the minimum load impedance that the amplifier can safely
drive
Bright, brightness
In video displays, the Brightness control is used to set the black level, which determines
how dark the black portions of a picture are. In describing sound quality, it refers to an
excess of high frequency sound. It can be caused by electronics or loudspeakers with
excess output at high frequencies, or by rooms that are too reflective, or live, at high
frequencies.
Broadband
A term implying very wide bandwidth. In the context of data communications it implies a
connection with higher data rates (wider bandwidth) than telephone modems
Bus
A signal distribution system, normally employing wire, optical fiber, which enables many
components to be operated from a single control unit without each being individually linked
to it. Some bus systems have two-way communication.
B-weighting
See Frequency Weighting
Capacitance
The property of a circuit element that permits it to store charge. The number of electrons
(charge) it can hold under a given electrical pressure (voltage) is called its capacitance
which is measured in farads, or fractions thereof, e.g. microfarads
Capacitor
Two metallic surfaces separated by an insulator creating a device that can store an electric
charge. It cannot pass DC and will pass AC signals with an impedance that decreases as
frequency increases. This property makes capacitors useful in filter and crossover designs.
Capture Ratio
A specification describing the ability of an FM radio tuner to lock on to one station when
there is another on the same frequency that is only slightly less strong
Cathode
The electrically negative terminal of a battery, or the electron source of a vacuum tube.
Cathode Ray Tube
See CRT.
CATV
Cable television
CAV
Constant Angular Velocity. A method of recording a disc (e.g. laserdisc) in which the
rotational rate is kept constant from beginning to end. Each rotation can be contrived to
hold the same amount of information, such as a single frame of a picture, making perfect
'pause' functions possible in an analog medium. It is wasteful of space, however, as the
density of recorded data reduces with increasing distance from the center of the disc. See:
CLV
Caveat Emptor
From the Latin: let the buyer beware. It is a principle in commerce that, without a
warranty, the buyer takes the risk
CD
CD = Compact Disc. An optical disc format for storing digital signals, developed jointly by
Sony and Philips.
CD-DA
CD-DA = Compact Disc Digital Audio, the original PCM digital music storage format, defined
by the Red Book standard
CD-R
A disc in the CD format that can be recorded once. Defined by the Orange Book standard
CD-ROM
A Read-Only Memory (ROM) in the CD format, used for storing computer data. Defined by
the Yellow Book standard
CD-RW
A disc in the CD format that can be recorded many times. It is rewritable
CEA-2006
An amplifier rating standard provided by the Cnsumer Electronics Association and adopted
my many reputable makers of car-audio amplifiers. CEA-2006 attempts to provide a basis
for rating amplifers using RMS output power and a specific load impedance to ensure a
useful rating.
Center channel
A channel driving a loudspeaker located midway between the front left and right
loudspeakers. It does most of the work in movie and television audio, anchoring dialogue to
the screen. In multichannel music, the featured artist can have a private channel. Must
sound as good as, and as similar as possible to, the left and right loudspeakers. In
automobiles it is especially important to solidly anchor the center portion of the front
soundstage for both the driver and the passenger. See Channel.
C-Format
See VHS-C
Channel
A signal path. Stereo consists of two channels, starting from the signal source, and ending
at the loudspeakers. Multichannel audio can have 5, 6 or 7 channels, plus a so-called .1
channel for low bass sound effects
Channel Separation
See: Separation
Chassis
The metal frame of a vehicle, or the metal base on which an electrical circuit is constructed
Chroma
The color information in a video signal consisting of hue and saturation. See: Chrominance,
Saturation, Hue
Chrominance
The color (hue and saturation) of light, independent of luminance (brightness), or that
portion of a video signal that carries this information. Designated by the symbol 'C'. See:
Luminance
Circuit Breaker
A device designed to protect other electronic devices by opening the circuit when the
system is drawing too much power. Since it uses relay contacts, it can be reset manually or
automatically. See: Fuse
Circumaural
Headphones
Headphones having cushions that surround the external ear, resting on the sides of the
head, not on the ear itself. See: Supraaural Headphones, In-Ear Headphones
Class A
An amplifier design in which both positive and negative polarities of an audio waveform
pass through devices that are active (conducting current) at all times. All low-level analog
amplifiers are of this type. Class A power amplifiers are relatively uncommon because they
generate a lot of heat, even when no signal is being reproduced, thus requiring enormous
heat sinks and many output devices if large amounts of power are needed. See: Class AB,
Class B
Class AB
Basically a Class B power amplifier in which the positive and negative output devices never
completely shut off, leaving a residual Class A activity at low signal levels and to smooth
the transition from one polarity to the other. Less efficient, and therefore hotter, than Class
B, but much cooler running than Class A. The most common form of power amplification.
See: Class A, Class B
Class B
A power amplifier design in which positive and negative polarities of an audio waveform
pass through separate output devices that conduct only when needed. It is difficult to
eliminate all distortions created when the devices transition from one polarity to the other
(crossover distortion), which has relegated this design to low quality audio and other
applications of power amplification where efficiency and cost are of greatest importance
Class D
An amplifier design in which the positive and negative output voltages are defined by a
pulse-width-modulated output section where the transistors are biased to be either on or
off and provide the full voltage available from the power supply directly to an output filter.
The output filter turns the pulses into an analog waveform suitable for driving loudspeakers
Clipping
Distortion that occurs when an amplifier is driven beyond its voltage, current or power limit.
Clipping describes the "cutting off" of signal peaks when the amplifier's limit is exceeded by
playing the system too loud. Clipped signals have excess high frequencies, putting tweeters
at risk, and some amplifiers become unstable when overdriven, and this also can result in
damage to the loudspeakers. So, small amplifiers driven too hard can be more dangerous
to loudspeakers than large amplifiers driven within their design range.
Closed Captions
Text captions that can be made visible on demand. Open captions are a permanent part of
the video signal
CLV
Constant Linear Velocity. A method of recording a disc (e.g. laserdisc) in which the
rotational rate is varied from start to finish so as to maintain a constant velocity of the
'groove' being read by the playback laser. This results in a more uniform data density over
the surface of the disc, from inside to outside, allowing for longer recordings. See: CAV
CMMD™
Working in collaboration with metallurgy specialists, the resourceful Infinity transducer
engineers identified a special combination of materials that exhibit a remarkably useful set
of mechanical properties. Infinity’s new Ceramic Metal Matrix Diaphragms are much stiffer
than standard metal diaphragms, moving the natural modes significantly upwards in
frequency. At the same time, CMMD™ cones have more damping than metal cones, making
this an excellent cone material for all transducers: woofers, midranges and tweeters
Coaxial Cable
A transmission line in which a central conductor is located within a cylindrical outer
conductor, separated by insulation. Can be designed to have specific characteristic
impedances at radio and TV frequencies to minimize losses. Sometimes used in the much
less demanding role of interconnect cables at audio frequencies, where they function as
simple shielded wires
Coaxial Loudspeakers
Loudspeakers in which the tweeter is located on the central axis of the woofer, and the two
are combined in a single structural unit
Codec
A combination encoder and decoder for any kind of digital signals, audio or video. See:
Perceptual Coding, Compression
Coherence
In listening, it describes a kind of perceptual realism in sounds. In measurements, it is a
measure of the correlation between the phases of two or more signals.
Coil
Turns of wire used to create inductance for use in an electrical circuit, or to create a
magnetic flux when current is passed through it, or to respond to a changing magnetic field.
An electrical impedance of a coil increases with frequency. See also Inductance, Inductor,
Voice Coil
Color Temperature
A measure of the color of light. In video, the underlying basis for a picture. A low
temperature would be associated with a reddish picture, while a high temperature would
yield one more bluish. NTSC standards require 6500 degrees Kelvin
Coloration
In listening evaluations, a perceived characteristic of a sound that was not in the original
recording. Coloration modifies the timbre of voices and musical instruments, and is
therefore not a good thing.
Comb Filter
AUDIO: A characteristic of a frequency response curve that appears when a sound is
combined with a delayed version of itself. The name comes from the orderly, repeated,
bump-dip-bump-dip visual appearance, looking like a comb. Comb filtering that occurs in a
simple signal path can be annoying. However, rooms are very complicated comb filters, due
to the many reflected sounds, and common experience tells us that this is usually perceived
as a benefit e.g. concert halls. VIDEO: a filter that separates a composite video signal into
luminance (black and white) and chrominance (color) components. See: Luminance,
Chrominance, Frequency Response, Acoustical Interference
Compliance
The force required to move an elastically suspended object a certain distance - e.g. the
'stiffness' of a spring. The diaphragm suspensions in loudspeakers have mechanical
compliance. The air inside an enclosure has acoustical compliance
Component Video
A video signal consisting of three components: red/green/blue (RGB) or a Color Difference
method going by one of several names: Y,U,V or Y, Pb, Pr or Y, B-Y, R-Y. The latter is the
method of video storage on DVD's and the component connection is the preferred way to
communicate video infomation to displays. The green, blue and red cables may be
terminated in either RCA or BNC plugs. See: Chrominance, Luminance, RCA, S-Video, BNC
Composite Video
A video signal in which the chrominance and luminance signals are combined, along with
synchronizing signals. These tend to use cables, looking like yellow audio cables, with RCA
connectors. This is the most basic form of video found on virtually all TV's, VCR's, etc. See:
Chrominance, Luminance, RCA, S-Video, Component Video
Compression
AUDIO: A reduction in the dynamic range of a system. This can be done deliberately, as is
common in radio broadcasts, or it can be the result of overdriving something, such as a
loudspeaker where, above a certain level, the device does not respond proportionally to
increases in signal level (see Power Compression). ACOUSTICS: A momentary increase in
pressure as a sound wave passes. See: Rarefaction, Sound Wave. DIGITAL DATA: Reducing
the amount of digital data required to store audio or video. Data compression can be
lossless or lossy, depending on whether the reconstruction is exact or not. Perceptual
coding is used in lossy systems. See: Perceptual Coding
Compression Driver
A loudspeaker driver especially designed to drive a horn. The name derives from the fact
that the sound radiating from a diaphragm is forced through a phase plug to a smaller
opening (i.e. compressed) at the throat, or beginning, of the horn expansion. See: Phase
Plug
Cone
A cone-shaped diaphragm of a loudspeaker that vibrates and radiates sound. Loosely used
to describe all diaphragms, some of which have other profiles, such as domes
Constant Directivity
Horn
A horn design that has a relatively constant angular dispersion of sound over its useful
frequency range
Convergence
In front or rear projection devices using combined red, green and blue images to form the
final picture, it is necessary that all three be perfectly aligned, or converged, with each
other in order to maximize the sharpness and color accuracy of the picture. It may be
necessary to converge the pictures at many locations all over the picture area. A special
test signal is used
Critical Band
In hearing perception, it is a band, or range, of frequencies over which the ears tend to
combine sounds for purposes of detection or loudness perception. It is not the frequency
resolution of hearing, as it relates to the perception of timbre, which is much higher
Critical Distance
The distance from a sound source in a room at which the direct sound and the reverberant
sound are equal in level
Crossover
Electrical filters that direct the appropriate frequencies to the woofer, midrange, tweeter,
etc. in a loudspeaker system. The crossover frequency is the frequency at which the
loudspeaker driver being turned off (e.g. a woofer) is at the same sound level as the one
being turned on (e.g. a tweeter).
Crossover Frequency
See crossover
Crossover Slope
The rate, expressed in dB per octave, at which audio signals are attenuated as frequencies
move into the crossover range. A high attenuation rate, e.g. 24 dB/octave means that there
is little interaction between adjacent loudspeaker drivers. Low attenuation rates, e.g. 6
dB/octave allow adjacent drivers to operate simultaneously over a wide frequency range.
See: Crossover, Octave
Crosstalk
Unwanted sound from one channel that leaks into another
CRT
An abbreviation for cathode ray tube. A device in which a finely focused beam of electrons
scans the phosphor coated front surface of a picture tube causing changes in brightness
according to the current flow in the beam. Multiple beams can energize different colored
phosphors for color video displays. Used in direct view televisions and projection units.
Curvilinear Cone
A diaphragm with a reducing curvature from center to edge, used to achieve specific
performance objectives
C-weighting
See Frequency Weighting
cycles per second
The frequency at which a periodic event, such as alternating current, occurs. Now called
Hertz, abbreviated Hz
D/A Converter
A device that accepts a digital signal as an input and converts it to analog form at its output
D2B Optical
This Digital Data Bus (D2B) was developed by Becker in 1995 in collaboration with Philips
and C&C Electronics. D2B Optical was the first fiber-optic data transmission system to be
used for audio and control data in vehicles with a data rate of approximately 5Mbit/s.
DAB
DAB (Digital Audio Broadcasting) is a technology standard for broadcasting audio using
digital radio transmission. The term digital radio was first used for DAB in 2001
DAC
See D/A Converter.
Damping
Reducing the energy in a vibrating or resonating system by adding electrically, mechanically
or acoustically lossy materials or devices
Damping Factor
In power amplifiers, it is a measure of the output impedance of the device. Expressed as a
number arrived at by dividing the impedance into 8 ohms. For example, an amplifier with
an output impedance of 0.04 ohms would have a damping factor of 8/.04=200. This, and
higher numbers are common for solid state amps. Tube amplifiers have much higher output
impedances and lower damping factors. In practice, the output impedance of the amplifier
has almost no effect on loudspeaker damping, but it can have a significant effect on the
frequency response of loudspeakers, most of which have frequency-dependent impedances.
Within reason, higher numbers are better
DAT
Digital Audio Tape. A popular version is R-DAT which employs rotary heads, as is done in
VHS video recorders, but on tiny cassettes. Signals are digitally encoded
dB
see Decibel.
DBO
Dynamic Bass Optimization)A high pass filter with variable cutoff frequency and variable Q,
designed to enhance the performance of subwoofers mounted in sealed and vented
enclosures. DBO provides bass boost and protection from overexcursion at the lowest
frequencies simultaneously
DC
Direct Current. A condition in which the polarity of the voltage is constant, and current
flows only in one direction. Batteries and rectified AC power supplies are examples of DC
power sources
Decade
A factor of ten. In frequency, humans can hear a range of about 3 decades: 20-200, 2002000,2000-20,000 Hz
Decibel
A logarithmic measure of relative voltage, current or power. A decibel is one-tenth of a bel,
abbreviated dB. In terms of power, 3 dB = 2x, 10 dB = 10x. In terms of current or voltage:
6 dB = 2x. In terms of perceived loudness: 1 dB is just audible, a 10 dB sound level change
represents double or half loudness
Demo
Demonstration
Diaphragm
The vibrating portion of a loudspeaker driver that radiates sound. In woofers, diaphragms
are commonly conical in shape. Tweeters are often dome shaped. Midrange speakers can
be either
Diaphragmatic
Absorbers
Sound absorbers that remove energy from a sound field by making it do work, moving a
surface. Most effective at low frequencies. Vibrations in walls, floors, ceilings, windows, etc
are all indications of diaphragmatic absorption at work. Sometimes it is necessary to add
custom built units to absorb more energy at specific low frequencies in order to control
room resonances. See: Resistive absorbers, Reactive Absorbers
Die Cast Aluminum
Baskets
Driver baskets that are cast from aluminum. Cast baskets are much stronger than stamped
steel and allow for tighter production tolerances and precise driver operation under even
the most demanding situations
Diffraction
When the direction of a sound wave is changed by obstacles or geometric changes in the
sound path. Not to be confused with reflection or refraction. Hearing sound around a corner
is an example. In loudspeaker systems, sound is diffracted by the edges of a baffle, in an
amount depending on wavelength and the size and shape of the edge. It is a factor in the
design.
Diffuse Sound Field
A sound field in a room that, at any point in the room, has equal energy arriving from all
directions. This is an idealized concept that, in practical listening spaces, is never perfectly
realized
Diffuser
An acoustical device, or surface that, by its shape causes incoming sounds to be reflected
outward in many different directions
Diffusion
A lack of directional order in a sound field. More reflections from more directions add
diffusion
Digital
The representation of a quantity in numeric form, normally in binary. In audio, this means
that waveforms of sounds are sampled at very high frequencies, and each sample is stored
in numeric form, so that the waveform can subsequently be reconstructed. See A/D
converter, D/A converter, Analog
Digital Light
Processing
Developed by Texas Instruments, the heart of this image projection device is a
semiconductor chip, operating as a light switch. It contains a rectangular array of hingemounted microscopic mirrors; each of these micromirrors measures less than one-fifth the
width of a human hair, and corresponds to one pixel in a projected image. Single-chip
devices use a color wheel to reproduce color, while three-chip devices dedicate a chip to
each of the primary colors, red, green and blue
Digital Multimeter
A meter that measures voltage, current, and resistance, and displays the result in
numerical form
Digital Satellite
System
A broadcast system in which digital video and audio signals are delivered to customers'
homes by means of signals radiated from a satellite in stationary orbit and received by a
small dish antenna
Digital Signal
Processor
see DSP
Digital Television
A generic term that describes any of several systems capable of delivering digital video
signals. High Definition (HDTV), Intermediate Definition (IDTV), Enhanced-Definition
(EDTV) and standard definition (SDTV) are some of the systems. See: HDTV, IDTV, EDTV,
SDTV.
Digital-to-analog
converter
See D/A Converter
D-ILA
D-ILA = Direct Drive Image Light Amplifier. A development of the original light valve in
which the reflectivity of the polarized liquid crystal surface is controlled directly by pixels
arrayed on a semiconductor backing. Each pixel can be addressed individually. Hence an
alternative description has evolved Liquid Crystal on Silicon, or LCOS. The technology
allows for a very high aperture ratio, meaning that the pixels are harder to see. See: ILA,
Light Valve, Aperture Ratio, Projector
DIN
Stands for: Deutsches Institut für Normung e.V., the German Institute for Standardization.
In audio we find such things as DIN connectors, and car audio head units with DIN specified
dimensions.
Diode
An electronic device that blocks current flow in one direction, but allows it in the other.
Dipole
Classic definition: a loudspeaker that radiates sound from both the front and back faces of
the diaphragm. The forward and backward radiated sounds are therefore of opposite
polarity (out-of-phase), and they cancel each other when they meet at the +/- 90 degree
axes, producing acoustic nulls. The radiation pattern is nominally that of a figure 8. This
also applies to microphones, in terms of their directional sensitivity. Contemporary usage: a
loudspeaker with bidirectionally aimed drivers that are connected out-of-phase. Since the
drivers are not coincident, the null is not perfect, and the off axis frequency response can
be very irregular
Direct Sound
Sound that arrives at a listener's ear (or a microphone) directly from the sound source,
without reflection
Direct Subwoofer
Input
A connection bypassing the internal crossover of a subwoofer, to allow the connection of
A/V receivers and surround processors that already contain their own subwoofer crossovers
and adjustments. See: Crossover
Directional
Interconnects/Wire
Since audio is an alternating (AC) signal, overall the electrons in wires will spend exactly
half of the time moving in one direction, and half of the time moving in the other. They end
up where they started. Since there is no net flow of current, there can be no directional
preference in the conducting wires. If there were directional behavior in interconnects or
speaker wire, we would have the beginnings of diode behavior, meaning that one half of the
audio signal would be distorted - clearly not a good idea. This is part of the 'smoke' of
audio. There is, however, one legitimate situation where the orientation of an interconnect
matters, and that is if one ground connection has been broken to eliminate a ground loop.
In this case the end with the attached ground should be plugged into the signal source.
See: Alternating Current, Ground Loop, Unbalanced Connection, Diode
Directivity
A measure of the angular dispersion of sound radiating from a loudspeaker. It is known
that, for good sound in rooms, loudspeaker system directivity should be relatively constant
over most of the frequency range. See: Directivity Index
Directivity Index
A numerical representation of the sound dispersion characteristics of a loudspeaker,
expressed in dB. It is the difference between the measured on-axis frequency response and
the sound power. 0 dB describes an omnidirectional loudspeaker, radiating sound equally in
all directions. Increasing numbers describe an increasing bias for sound radiated in the
forward direction. See On Axis, Sound Power, Directivity
Discrete
In the audio context, discrete refers to sound recordings in which all channels are stored
separately. Each channel is completely independent of each other channel
Discrete 5.1 media
Discrete 5.1 media include DVD-Audio, DVD-Video and DTS-CDs. The popular 5.1-channel
digital formats, like Dolby® Digital and DTS®, have three front channels (left, center and
right) and two surround channels, all of which are full bandwidth, and a “.1” channel that is
low-frequency only, and used for occasional special effects
Discrete Circuitry
The use of separate components such as transistors, resistors, capacitors, and diodes in an
electronic circuit instead of IC's (Integrated Circuits) in which these components are
fabricated in microscopic size on a silicon chip. Performance and economic factors usually
determine which alternative is chosen
Discrete Logic 7®
technology
See Logic 7 technology
Distortion
When an audio signal has been changed by the nonlinear behavior of the microphones,
electronics, and loudspeakers. The nonlinearities, whether acoustical, mechanical or
electrical, change the audio signals which are passed through them. See: Linear Distortion,
Non-Linear Distortion
Diversity Reception
An antenna system, occasionally used in cars, employing two or more antennas in a system
that constantly seeks out the one(s) with the strongest signal. Advantageous in difficult FM
reception areas, like downtown in cities where multipath problems abound. See: Multipath
DLP
See: Digital Light Processing
DMM
see Digital Multimeter.
Dolby Pro Logic®
An active matrix decoder for Dolby® Surround (Lt, Rt) signals outputting four audio
channels (left, center, right and surround). See: Matrix Encode/Decode, Dolby® Surround,
Dolby Pro Logic® II
Dolby Pro Logic® II
A development of Dolby Pro Logic® in which there is some separation introduced into the
two surround audio channels, as well as several other playback options, some of which
apply to the playback of stereo music
Dolby®
See Dolby® Digital; Dolby® Pro Logic®; Dolby ProLogic® II; Dolby® Stereo; Dolby®
Surround; Dolby® B, C and S
Dolby® B, C and S
Noise reduction systems developed by Dolby Laboratories, and widely used in tape
record/play devices
Dolby® Digital
An audio signal encoding/decoding system, developed by Dolby Laboratories, that uses
perceptual coding to reduce the data rate required to transmit, and the amount of digital
space required to store, digital audio signals. Different consumer and professional versions
use different amounts of compression, and therefore have different levels of audio quality.
It can store mono, stereo and multichannel formats. See Perceptual Coding
An audio signal encoding/decoding system, developed by Dolby Laboratories, that uses
perceptual coding to reduce the data rate required to transmit, and the amount of digital
Dolby® Digital (audio
space required to store, digital audio signals. It can store mono, stereo and multichannel
coding 3/AC-3)
formats. Different consumer and professional versions use different amounts of
compression and, therefore, have different levels of audio quality. See Perceptual coding
Dolby® Stereo
The name commonly given to Dolby® Surround as it is used in movie theaters
Dolby® Surround
A multichannel audio signal that has been matrix encoded for storage in two channels,
called Left total and Right total (Lt,Rt). This is not a discrete system, as it incorporates a lot
of crosstalk (sounds from one channel leaked into the others). The name also was used in
early decoders that did not incorporate active steering to enhance the channel separation.
See: Crosstalk, Channel Separation.
Dome Loudspeaker
A loudspeaker driver having a diaphragm shaped like a dome. Commonly used for tweeters
and midrange units
Double-Blind
Listening Test
A listening test in which the listeners are not aware of the product identities (the first
'blind') and the person conducting the test also is unaware of, or has no control over, which
products are being auditioned at any given time (the second 'blind'), and therefore cannot
influence the results
Downmix
Generally, the process of combining several channels of information into a smaller number
of channels. Specifically, a feature of some Dolby Digital playback devices, allowing users to
convert the multichannel digital audio into a two-channel Dolby ProLogic compatible signal
(Lt, Rt), or a stereo signal, or a mono signal. In all of these downmixes the LFE channel is
not included, therefore the downmixed signals are not exactly the same as the original.
See: Dolby Digital, LFE, Lt, Rt
downmixing
The process of converting a program created in a multichannel format so that it can be
played through a system with fewer channels, e.g. 5.1 to stereo
DRC
See: Dynamic Range Contol
Driver
Another name for a raw speaker or transducer, such as a woofer, midrange or tweeter.
See: Compression Driver
DRM
DRM (Digital Radio Mondiale) is a digital radio system for short-, medium- and long-wave.
Compared to conventional AM radio, DRM can fit more channels, at higher quality, into a
given bandwidth
DSP
Digital Signal Processing. Any form of manipulation performed on an audio or video signal
while it is in digital form. The term DSP acquired an unfortunate reputation when, in the
early days, it came to be associated with artificial reverberation (hall, stadium, etc.) effects
that could be added in during playback. Many of these effects were not good. Now, the
quality of DSP processing is undisputed, and limited only by the competence of the
programmers.
DSS
See: Digital Satellite System
DTCP
DTCP (Digital Transmission Content Protection) is a digital encryption method that enables
the transport of content-protected audio and video signals via a digital transmission channel
from a source (e.g., DVD player) to an end point (e.g., amplifier or display).
DTS®
An audio signal encoding/decoding system, developed by DTS, Inc., that uses perceptual
coding to reduce the data rate required to transmit, and the amount of digital space
required to store, digital audio signals. It can store mono, stereo and multichannel formats.
Different consumer and professional versions use different amounts of compression and,
therefore, have different levels of audio quality. See Perceptual coding
DTS-CDs
DTS-CDs are music CDs produced with five discrete channels for playback in multichannel
format through suitable decoders
DTV
See: Digital Television
Dual Cone
A speaker that includes a small cone attached to the apex of the woofer cone. At high
frequencies, where the woofer cone is unable to respond to the movement of the voice coil
fast enough, the small cone vibrates and helps to augment the high frequency response of
the speaker. Dual-cone speakers are often used in enrty-level car-audio systems
Dual Voice Coil
A speaker that includes two voice coils wound on the same voice coil former. The two voice
coils, connected in either series or parallel provide a choice between two total impedance
values
Dubbing
In film sound, the act of mixing or re-recording components that make up a sound track is
called dubbing
Dull
In sound quality: the opposite of 'bright', implying a deficiency of high frequency sounds.
DVC (Dynamic
Volume Control)
In car audio systems, this adjusts the volume and frequency response of the playback to
compensate for the auditory masking effects of road, aerodynamic and mechanical noises in
a moving vehicle
D-VCR
A video cassette recorder (VCR) which can record digital audio and video signals on
standard VHS tape
DVD
Initially interpreted as meaning Digital Video Disc, but now that there are several uses for
the medium, it is more popularly known as the Digital Versatile Disc
DVD, DVD-Video
DVD initially stood for Digital Video Disc but, now that there are several uses for the
medium, it is more popularly known as the Digital Versatile Disc
DVD-A (DVD-Audio)
An audio version of DVD, in which multiple digital audio channels can be stored in
uncompressed PCM form, or using a lossless compression algorithm (the digital audio
signals are reconstructed without alteration). The massive storage capacity of DVD allows
for many options
DVD-Audio
An audio version of DVD, in which multiple digital audio channels can be stored in
uncompressed PCM form, or using a lossless compression algorithm (the digital audio
signals are reconstructed without alteration). The massive storage capacity of DVD allows
for many options
DVD-Video
DVD with a special directory structure for files that contain uncompressed or compressed
audio signals, and still or moving images plus optional subtitles. Usually protected by
various digital copy protection measures
D-VHS
See: D-VCR
DVI
Digital Visual Interface. A standard created to convert analog signals into digital signals to
interface with both analog and digital monitors. It handles bandwidths in excess of 160 MHz
and thus supports UXGA and HDTV. Common in computers and some television and video
products. Currently (2003) being combined with HDCP, a copy-protection scheme, for
connections between some video sources and displays. See: HDCP, UXGA, HDTV.
DVR
Digital Video Recorder. A device that records and plays back streaming audio and video
signals using a digital hard drive for storage
DX
A short form for 'distance', referring to radio reception. Some radios have local/DX settings
to optimize performance when close to or far from the transmitters
Dynamic Range
The difference between the loudest and softest sounds that can be reproduced by a device
or format. Usually expressed in dB
Dynamic Range
Control
DRC is a feature of some Dolby Digital decoders which enables users to reduce the dynamic
range of the sound so that late-night listening need not disturb others in the house, or
neighbors. See: Dynamic Range
Early Reflections
Those sounds that arrive at a listener's ears after one reflection from a room boundary.
Echo
A reflected sound that arrives after a delay sufficient for a listener to distinguish it as a
discrete event in time. In rooms this is modified by the reverberation time of the space
Edge-driven Dome
Tweeter
A tweeter that uses a voice coil wound around a voice coil former that has the same
diameter as the tweeter's radiating area. Edge-driven tweeters often provide higher power
handling than balanced, or W-Dome tweeters
Edgewound Ribbon
Voice Coil
A loudspeaker voice coil that is made from wire flattened into a metal ribbon and then
wound on edge. This maximizes the amount of wire in the magnetic gap, improving the
performance of a loudspeaker motor
EDTV
See: Enhanced-Definition Television
Efficiency
The measure of a device's ability to convert input power to work. Expressed as a
percentage. See: Sensitivity
Electrolytic Capacitor
A common form of capacitor consisting of a conductive film in an electrolyte. Tends to be
used for larger values of capacitance
Electromagnet
A magnet consisting of wire wound around a soft iron core. It becomes an active magnet
when current is passed through the wire, and it ceases to be a magnet when the current
stops. See: Magnet
Electronic Crossover
An electrical filter designed to limit the band of frequencies that is sent to the input of an
amplifier connected to a loudspeaker designed to reproduce only a certain band of
frequencies, i.e. a subwoofer, midrange driver or tweeter. See also Crossover
Electrostatic
Commonly referring to loudspeakers in which a thin conductive membrane serves as the
diaphragm driven by electrostatic forces generated between it and fixed perforated
electrodes in front of and behind it. Requires a polarizing voltage, and high voltages to drive
it. Electrically capacitive. Usually radiating a dipole sound field. Small maximum
displacements limit low frequency output and necessitate large diaphragms, and
consequent directivity issues. See: Dipole, Directivity
Elliptical Oblate
Spheroidal™ (EOS™)
waveguides
Developed for the JBL Pro LSR studio monitors, EOS™ waveguides evenly disperse high
frequencies for more precise stereo imaging over a wider listening area
EMC
EMC (Electromagnetic Compatibility) refers to electromagnetic disturbance that occurs
when different electronic devices are connected. See EMI
EMI
Electromagnetic interference created when a device radiates electromagnetic waves that
are received by other devices. Some kinds of electric motors, car ignitions and computers
are notorious for radiating EMI. AM radio is a common indicator of EMI.
Enclosure
The box, or other shape of volume, that accommodates the transducers in a loudspeaker
system. Normally, its shape is dictated by the acoustical needs of the mid and high
frequency drivers, and the volume is determined by the design chosen to complement the
performance of the woofer. There are several options for low-frequency enclosures, closed,
bass reflex, passive radiator, etc. See: Transducer, Driver, Woofer, Bass Reflex, Acoustic
Suspension
Enhanced-Definition
Television
A broadcast system in which existing equipment is used to transmit an enhanced signal. It
looks no different on standard television receivers but is enhanced when viewed on those
with the extra feature
EOS™ Waveguide
Developed for the JBL Pro LSR studio monitors, EOS™ waveguides evenly disperse high
frequencies for more precise stereo imaging over a wider listening area
EQ
See: Equalizer
Equal Loudness
Contours
Graphs displaying contours of equal perceived loudness for single frequencies or narrow
band sounds. Each contour is referenced to 1 kHz, and shows the variations in physical
sound level required to maintain equal perceived loudness at all audible frequencies. There
have been several determinations, for different sounds, under different listening conditions.
Fletcher and Munson, Robinson and Dadson, and Stevens are among the best known
investigators. See Phon.
Equalization
The process of using an equalizer to correct for problems in an audio system. The difficulty
is to know what the problem is caused by, and whether it is the kind of problem correctable
with an equalizer. Measurements are usually advised.
equalization
A feature of the THX Home program, in which high frequencies are attenuated to
compensate for excessive high frequencies in many movies. Problem is that it is a fixed
equalization, and not all movies need the same amount of compensation, or any at all.
Concert programs usually need none. It is a feature that should be easily accessible for
turning on and off, but often is not. A treble control is an imperfect alternative as they
usually start attenuating at too low a frequency.
Equalizer
A device consisting of adjustable filters that can change the frequency response of an audio
system. Equalizers can compensate for frequency response aberrations in loudspeakers,
loudspeaker/room combinations, and also for adjusting the tonal balance of recordings. See
also: Graphic Equalizer, Parametric Equalizer, Tone Controls
Excursion
The in and out movement of a loudspeaker diaphragm
Extended Pole Piece
A form of loudspeaker motor design in which the front of the pole piece is extended to
improve the uniformity of the magnetic field within which the voice coil moves. This reduces
distortion at large excursions
EzSet™
EzSet™ uses a built-in sound pressure measurement and calibration system that lets you
automatically balance speaker channel levels for optimum surround sound enjoyment –
regardless of the speaker type or room conditions
F Connector
A cylindrical, often threaded, connector in which the central wire in a coaxial cable is used
as the center pin. Widely used for cable and antenna connections in television.
Fader
In car audio, the front-to-back sound level adjustment
Far Field
Moving away from a sound source, the far field is the region in which the sound level drops
6 dB for each doubling of distance. The larger the sound source, the farther away the far
field begins. When measuring loudspeakers it is necessary to be in the far field, otherwise
the frequency response will be different for every measuring distance. Normally, 2 meters
or more is required. See Sensitivity
Farad
The basic unit of capacitance
Fast Fourier
Transform
A computationally very efficient way to calculate a Fourier Transform. See: Fourier
Transform
Feedback
In Amplifiers: the practice of connecting (feeding back) a portion of the output signal to the
input so that it can be compared to the input signal and errors corrected. The signal must
be inverted (negative feedback) to prevent oscillation, or uncontrolled, very loud, howling
(positive feedback). Positive feedback is sometimes experienced in public address systems
as a ringing or howling when too much of the amplified sound is picked up by the
microphones
FEM (Finite Element
Method)
FEM is a form of computer-aided modeling used in engineering. It is used to solve problems
in many disciplines. The area or volume to be analyzed is divided into a large number of
small elements, and the behavior of each element is modeled by solving the large system of
simultaneous equations. FEM is used to predict the behavior of materials and assemblies
during the design phase, speeding the process and allowing for experimenting with different
alternatives without having to construct prototypes
FFT
See: Fast Fourier Transform
Fidelity
See: High Fidelity
Field
In a video display of interlaced scan lines there are two fields, presented alternately, one
consisting of even-numbered lines, and the other of the odd-numbered lines. See:
Interlace.
Fill
Polyfill, glass or mineral fiber or something similar used inside a loudspeaker enclosure.
Filling a closed box makes it seem acoustically larger to the woofer, and in all boxes, fill
damps standing waves within the box.
Flat Response
Refers to a flat, or linear, frequency response, meaning that an audio component can
reproduce all audio frequencies at the same, correct, level. See Frequency Response
Fletcher and Munson
Scientific pioneers in the investigation of perceived loudness as a function of frequency and
sound level. See: Loudness Contours','Glossary',775,400,'right')" class="link">Equal
Loudness Contours
Flush mount
A speaker mounting system that places the front of the speaker on nearly the same plane
as the speaker's baffle
Flutter
Acoustical: a rapid succession of reflected sounds occurring between two parallel surfaces,
normally stimulated by a transient sound such as a hand clap. Electronic: in recording
media, especially analog tape, a periodic variation in pitch caused by uneven motion of the
tape.
FM
see Frequency Modulation
Foley Effects
Sound effects of all kinds, wind, footsteps, door slams, telephones, gunshots, etc. that are
added to a movie soundtrack
Foot-Lambert
A measure of picture brightness. 1 Foot-Lambert is 1 lumen per square foot of screen
surface. The SMPTE (Society of Motion Picture and Television Engineers) specification for a
film-type movie theater is 16 foot-Lamberts of brightness, a level not always achieved.
Direct-view televisions can be as high as 30 foot-Lamberts
Fourier Transform
Developed by a French mathematician in the early 1800's, the Fourier transform basically
separates a waveform into sinusoids (pure tones) of different frequencies, storing the data
as amplitude and phase as a function of frequency. These sinusoids can be added together
to reconstruct the original waveform. The transformation, therefore, is between the timedomain waveform and the frequency-domain spectrum of a signal or sound. Both are
complete descriptions of the signal, and one can be computationally converted into the
other. See: Transform, Spectrum, Transfer Function. Impulse Response, FFT, Spectrum
Analyzer.
Frame
In video, the portion of a signal that contains all of the scan lines that comprise one image.
Interlaced systems have two fields per frame. See: Interlaced, Field.
Free Air Resonance
The natural resonance frequency of a loudspeaker driver when it is suspended in "free
space".
Free Field
See Anechoic.
Frequency
The number of vibrations or cycles completed by a signal in one second. Frequency is
expressed in cycles, or more commonly, Hertz (Hz).
A method of radio broadcasting in which the radio carrier frequency is frequency modulated
by the audio signal. Capable of high sound quality, and relatively immune to interference
Frequency Modulation
and static. However, it propagates poorly over long distances and suffers from multipath
interference and shadow zone problems in cities and hilly areas
Frequency Response
A measure of the amplitude vs. frequency performance of an audio component, measured
from its input to its output. A perfect electronic device should have a flat, or linear,
frequency response over its useful frequency range, indicating that it reproduces all
frequencies at the correct level. Loudspeakers are more complicated since the output is
sound which is radiated in all directions. It is necessary to measure the frequency response
at many locations all around the loudspeaker in order to be able to predict how it may
sound in a room. For loudspeakers, there is no single frequency response measurement
that is completely descriptive of its performance. See: Transfer Function, Spectrum
Frequency Weighting
Used in measurement of overall sound levels to take account of the frequency-dependent
loudness characteristics of ears at different sound levels. A-weighted measurements
attenuate sounds below about 1000 Hz (low sound levels), B-weighting rolls off frequencies
below about 200 Hz (middle sound levels), and C-weighting rolls off below about 50 Hz
(high sound levels). All three weightings roll off high frequencies above about 7 kHz. Sound
levels measured using these weightings are designated dB (A), dB(B), dB(C), as opposed to
unweighted (linear) measurements of sound pressure level (SPL).
Front Projector
A projector that delivers an image to the front surface of a reflective screen allowing the
audience to view the reflected image. See: Rear Projector, RPTV.
Fundamental
The lowest in a series of harmonically related sounds. In musical sounds, it is the basis for
all higher harmonics. In acoustically or mechanically resonant systems, it is the lowest of all
resonant modes. See: Missing Fundamental
Fuse
A device designed to protect other electronic devices by melting and opening the circuit
when the system is drawing too much power. If a fuse blows it must be replaced with an
identical part, otherwise future protection may be compromised. See: Circuit Breaker
Fusion Zone
A horribly abused term, evolving from misinterpretations of the Haas effect, implying that
all sounds arriving within an interval of time, ranging from about 20 to 50 milliseconds, are
perceptually fused or integrated. This is wildly simplistic at best, and totally wrong at worst,
because within these time limits there are several levels of clearly distinguishable
perceptions including spaciousness, timbre change, image shift, multiple sound images and,
at large delays, echoes. See: Haas Effect
Gain
AUDIO: The difference in amplitude between the input to a device, and the output. The
amount of amplification applied to a signal. VIDEO: the difference between the brightness
of an image projected on a screen, and the brightness of the reflected light. Screens can
have gains greater or less than one. The amount of gain is related to the uniformity of
brightness as viewers move away from the direct axis. High-gain screens tend to be more
directional. The color of the screen material is another factor.
Graphic Equalizer
An equalizer consisting of several identical percentage-bandwidth (e.g. fractional octave)
filters covering the Frequency Range audio frequency range or portion thereof. For
example: 30 one-third-octave filters would cover the frequency range from 20 Hz to 20
kHz. See: Parametric Equalizer.
Gray Scale
A measure of how well a video display tracks the shades of gray between perfect black and
white. It is an assessment of the quality of the high resolution black and white picture that
underlies the lower resolution color information.
Ground
An electrical connection to the earth, or a conductor or chassis serving as a common
reference.
Ground Loop
When multiple ground connections are made at separated points in an electrical system
there is the possibility of voltages existing between the connecting points. The common
result is hum and/or noises introduced into the audio signal. See: Balanced Connections,
Unbalanced Connections.
Haas
Effect
Haas, a German researcher, determined that a delayed sound could be as much as 10 dB higher in level
than the direct sound before it was judged to be as loud as the direct sound. This means that, in a
public-address system, the sound from a delayed loudspeaker could be that much louder than the voice
of the person speaking, before the audience was attracted to the loudspeaker rather than the person.
See: Precedence Effect, Localization, Direct Sound, Fusion Zone.
Harmoni
c
A tone that is a whole-number multiple of the original, or fundamental, tone. Numerically, the first
harmonic is the fundamental. Harmonics are abundant in musical sounds, helping to give instruments
and voices their distinctive qualities. When harmonics occur as a result of nonlinear distortion, they
change the timbre of musical sounds and voices. See: Missing Fundamental, Overtones.
Harmoni
c
Distortio
n
The form of distortion that occurs when a nonlinear device is driven with a pure tone. See Harmonic,
Non-Linear Distortion, Distortion.
HD
Radio™
One of DAB’s competitors, HD Radio is a digital radio system for transmitting audio on the FM and AM
bands. It was developed in the USA by iBiquity, a company founded specifically to develop this
technology. In contrast to DAB, HD Radio is not only a digital system, but one that can be operated as a
hybrid system with analog AM and FM radio.
HDCD
High Definition Compatible Digital. An encode/decode process aimed at enhancing the performance of
conventional 16-bit audio signals on compact discs.
HDCP
High-Bandwidth Digital Content Protection is a specification developed by Intel Corporation to protect
digital entertainment content across the DVI interface. See: DVI.
HDTV
See: High Definition Television
Head
Unit
A factory or aftermarket car radio, possibly including a CD or cassette tape player.
Headroo
m
The difference between the maximum, or peak, signal levels in a program, and the maximum level that
the audio component is capable of. E.g. a movie sound track requires 100 watt peak output to satisfy a
listener, but the listener is using a 200 watt power amplifier. There is 100 watts of headroom. Headroom
is inaudible, but it is there in case it is needed. See: Loudness.
Hearing
Level
A measure of how the hearing threshold level for a specific ear relates to the statistical normal hearing
threshold for people in general. Audiometers are calibrated to show 0 dB for normal, and some number
of dB for hearing thresholds that have been elevated because of hearing loss. See Audiometer, Hearing
Loss, Hearing Threshold
Hearing
Loss
A reduction in hearing sensitivity, or the ability to hear small sounds, that is caused by exposure to
excessively loud sounds, age, illness, drugs, etc. Hearing loss can be temporary or permanent.
Repeated temporary losses often lead to some amount of permanent loss.
Hearing
Threshol
d
The smallest sound that can be heard. Humans are most sensitive in the middle frequency range,
showing progressively reduced sensitivity at low frequencies and greatly reduced sensitivity above about
20 kHz. This characteristic is highly individual, and it changes with age and exposure to loud sounds.
Hearing
Threshol
d Level
See Hearing Level.
Heat
dissipati
on
The property by which a device transfers potentially damaging heat to the surrounding air.
Helmholt
z
Absorber
A sound absorbing panel designed with holes or slots acting as the masses in a Helmholtz resonance
with the volume of air behind the panel.
Helmholt
z
Resonan
ce
The 'classic' acoustical resonator (named after the acoustic research pioneer) consisting of a mass of air
bouncing on a volume of air acting as a compliance, or spring. The common example is the soda bottle
resonance, excited by blowing across the mouth. The air in the neck of the bottle is the mass, the bottle
is the compliance. Drinking more soda lowers the resonant frequency. Many practical applications. See:
Bass Reflex, Resonant absorbers, Helmholtz Absorbers.
Hertz
The basic unit of frequency also called cycles per second. The number of full cycles completed by an
alternating signal in one second.
High
Definitio
n
Televisio
n
A video system with approximately twice the horizontal and vertical resolution of conventional NTSC
television, and presented in a 16:9 aspect ratio. See: NTSC, Aspect Ratio, Resolution
High
Fidelity
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HighPass
Filter
An acoustical device or electrical network that blocks frequencies below a designated point, and allows
higher frequencies to pass.
Horn
An expanding acoustical waveguide in front of a loudspeaker driver, or compression driver, that shapes
the wavefront of the sound as it radiates away from the source. This allows designers to control vertical
and horizontal dispersion characteristics, essential to achieving good audience coverage in sound
reinforcement systems, and increasingly used to finesse the frequency-dependent directivity of
consumer loudspeakers to improve sound quality. Being acoustical transformers, horns can also improve
the acoustical efficiency of the drivers. See: Compression Driver, Driver, Directivity, Throat.
Hue
The quality of a color, e.g. a red hue, green hue, blue hue. Tint.
HVAC
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Hz
Abbreviation for Hertz. See Hertz.
IDTV
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ILA
Image Light Amplifier. See: Light Valve, D-ILA.
Image
See Imaging
Imaging
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I-Mount™
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Impedance
In electronics: the opposition to alternating current flow in a circuit or device. Properly expressed as
a complex quantity, it is also simplified as a magnitude only. The latter is commonly used in
describing the impedance of loudspeaker drivers and systems. There are also acoustical and
mechanical impedances.
ImprovedDefinition
Television
A television receiver that can improve the appearance of a standard video signal by employing
signal processing techniques such as line doubling.
Impulse
Response
A measure of the time domain response of a system, input-to-output, to a very brief transient
signal at its input. The Fourier transform of this time-domain waveform is the frequency-domain
transfer function. See: Transfer Function, Fourier Transform, FFT.
Inductance
The property of a circuit whereby a change in current causes a change in voltage. The unit of
inductance is the Henry: the amount of inductance required to generate one volt of induced voltage
when the current is changing at the rate of one ampere per second. See Inductor, Coil.
Inductive
Coupling
When signals or noises are coupled magnetically into a wire or cable. This is the principle that
allows transformers to function, a good thing, and that can let unwanted signals creep into parts of
an audio system, a bad thing.
Inductor
See Coil, Inductance.
In-Ear
Headphones
Tiny headphones that are inserted directly into the ear canal. See: Circumaural Headphones,
Supraaural Headphones.
In theory, an infinitely large flat baffle, used to isolate the sounds radiated by the front and back
surfaces of a loudspeaker diaphragm. In practice, a baffle that is large compared to the longest
Infinite Baffle
wavelength of sound radiated by the loudspeaker. Also, incorrectly, used to describe a closed-box
loudspeaker enclosure.
Infotainment
A media device that provides a combination of music or video playback and navigation, telephone,
traffic, cellular communication and internet access
Infrared
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Infrasound
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In-phase
Two signals having the same waveforms that are in perfect synchronization with each other. There
is no phase difference between them at any frequency. The signals will add (constructively
interfere). See: Acoustical Interference.
Input
Sensitivity
The voltage required at the input of an electronic device in order to produce the rated electrical
output. For example: in power amplifiers, the input voltage needed to produce the rated steadystate output power.
Intensity
See Sound Intensity.
Interference
See: Acoustical Interference
Interlace
A video display made up of two alternating fields, one that scans the even-numbered lines followed
by one that scans the odd-numbered lines. The field repetition rate is normally tied to the local AC
power line frequency e.g. 60 fields/second in North America, thus presenting a complete picture, or
frame, 30 times per second. See: NTSC, PAL, SECAM, Scan Line.
Distortion created when a nonlinear device is driven by multiple tones. Intermodulation distortion
products are complex multiples and submultiples of the test signals, making them more easily
Intermodulati
audible than the harmonic distortion products which are simultaneously created in such a test.
on Distortion
Music, being a complex signal, generates abundant intermodulation distortion when processed by a
nonlinear device. See: Harmonic Distortion, Non-Linear Distortion.
IntermountII
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InverseSquare Law
In a free sound field (no reflections) the sound level of a non-directional source will diminish by 6
dB for every doubling of distance from the source. In rooms this can only apply to the direct sound
from some kinds of sources. See also: Far Field
In-Wall
Loudspeaker
A loudspeaker designed to function best when it is mounted flush with the surface of a wall or
ceiling. Special mounting hardware is used, and some models incorporate mechanical vibration
isolation to minimize the coupling of vibration to the wall itself.
IR Remote
A remote control that communicates by means of Infrared (IR) light. Such controls need line of
sight to the devices being controlled, or good optical reflecting surfaces to help the light signal to
get there. See: RF remote.
Kaladex® Tweeter
This breakthrough tweeter material is ideal for the new GTO Series. It is smooth and
natural, and has incredible UV- and moisture-rejection properties, making it ideal for
automotive use. In addition, these are not commonly used w-domes, but actually edgedriven domes with a larger voice–coil diameter, yielding increased power handling and lowend response.
Karaoke
Audio devices that allow the connection of a microphone for sing-along activities. Special
videos are available with missing vocal tracks, and showing the lyrics as a subtitle.
Keystone
Distortion/Correction
In front projectors, if the projector is tilted up or down relative to its design axis, the image
will take on a trapeziodal shape, being narrower at the top or bottom than it should be. The
common distortion comes from an upward tilt, making the image larger at the top, like the
wedge-shaped keystone at the top of a stone arch - the last one to be placed and the one
that holds the arch up. Some display devices allow this to be corrected, but there may be
some picture degradation.
Kilo-
A prefix meaning thousand. For example: one thousand Hertz is 1 kilohertz, abbreviated 1
kHz.
Laserdisc
An optical disc, 12 or 8 inches in diameter, that can store video in analog form along with
analog and digital audio signals in stereo and multichannel formats. The analog signals are
frequency modulated. See: FM, CAV, CLV
LCOS
Liquid Crystal on Silicon. A digital video display device using a liquid crystal surface the
reflectivity of which is controlled by a pixellated solid state control device underneath it.
See: D-ILA, Projector.
LED
Liquid Crystal Display is one in which the reflectivity and/or transparency can be changed
by the application of a voltage. It is divided into many tiny independently conrolled pixels.
They are very common: digital watches, calculators, dashboards, computer screens, rear
and front projection video displays, etc. Since they are used as light transmission devices in
most applications, there are losses, and getting the control voltages to each of the pixels
presents challenges in maximizing the aperture ratio. See: Aperture Ratio, Projector.
LEDE
Low-Noise Blocking Converter. The active receiving element in a satellite dish antenna.
Some systems may have two or three LNB's on one dish, each aiming at a different
satellite.
Letterbox
The method of displaying a widescreen image on a standard 4:3 aspect ratio display. The
width of the image is the width of the display, but the height is less, meaning that there are
black bars above and below - like a letterbox.
LFE
See: Low-Frequency Effects
Light Valve
A video display device in which a powerful light source is modulated by a 'valve' controlled
by a much smaller, less powerful one, thus making it possible to project a very large image.
The valve is a polarized liquid crystal device in which one surface reflects a powerful light in
proportion to the amount of much weaker light falling on the reverse side. The contolling
light can be an image created by a CRT. Three valves, red, blue and green, are necessary
for color projection. Being analog, there is no pixel structure. Also known a ILA (Image
Light Amplifier) by its creator Hughes/JVC. See: D-ILA, LCOS, Projector.
Line Doubler
A video processor that can double the number of lines in a scanned display, making each of
the lines smaller, and therefore less visible. One common form of line doubler converts an
interlaced picture into a progressively scanned picture. See: Interlace, Progressive Scan,
IDTV
Line Level
A low-level audio signal. The kind of signal that is typically communicated between
components using shielded interconnects with RCA type connectors. Sometimes called
preamp level, to distinguish it from speaker level signals. See: Speaker Level
Linear Distortion
Changes to the amplitude vs. frequency characteristic and the phase vs. frequency
characteristic of a signal. Such distortions are assumed to be constant at all signal/sound
levels. See: Non-Linear Distortion, Distortion.
Localization
Assigning a perceived direction to a sound. A full description would include horizontal angle
(azimuth), vertical angle (elevation) and distance. The size of the perceived sound image
may also be a consideration.
Logarithm
In common logarithms, representing a number by the power to which 10 must be raised to
equal it. For example, 10 to the exponent (or power) 2 = 10 squared = 100. The log of 100
is therefore 2.
Logic 7
An active matrix multichannel audio processing system, originally developed by Lexicon®
and now used in several Harman products. It converts, or upmixes, two-channel stereo
signals into five- or seven-channel signals, allowing listeners to better experience the
surrounding ambient sounds that are already in the stereo mixes. Discrete Logic 7 expands
this capability to include upmixes of 5.1-channel recordings to seven channels. All of these
systems may have modes optimized for movie and music soundtracks, and for conventional
stereo music recordings. It is especially designed to maintain a solid front soundstage,
while expanding the perceived ambience beyond the room walls. Automotive versions
address the unique circumstances of listening within cars, with special processing,
loudspeaker design and placement all contributing to a greatly enhanced listening
experience. See Matrix Encode/Decode, upmix.
Logic 7®
An active matrix multichannel audio processing system, originally developed by Lexicon®
and now used in several Harman products. It converts, or upmixes, two-channel stereo
signals into five- or seven-channel signals, allowing listeners to better experience the
surrounding ambient sounds that are already in the stereo mixes. Discrete Logic 7 expands
this capability to include upmixes of 5.1-channel recordings to seven channels. All of these
systems may have modes optimized for movie and music soundtracks, and for conventional
stereo music recordings. It is especially designed to maintain a solid front soundstage,
while expanding the perceived ambience beyond the room walls. Automotive versions
address the unique circumstances of listening within cars, with special processing,
loudspeaker design and placement all contributing to a greatly enhanced listening
experience. See Matrix Encode/Decode, upmix.
Logic 7® technology
An active matrix multichannel audio processing system, originally developed by Lexicon®
and now used in several Harman products. It converts, or upmixes, two-channel stereo
signals into five- or seven-channel signals, allowing listeners to better experience the
surrounding ambient sounds that are already in the stereo mixes. Discrete Logic 7 expands
this capability to include upmixes of 5.1-channel recordings to seven channels. All of these
systems may have modes optimized for movie and music soundtracks, and for conventional
stereo music recordings. It is especially designed to maintain a solid front soundstage,
while expanding the perceived ambience beyond the room walls. Automotive versions
address the unique circumstances of listening within cars, with special processing,
loudspeaker design and placement all contributing to a greatly enhanced listening
experience. See Matrix Encode/Decode, upmix.
Loudness
The perceptual correlate of sound level. Subjective perception of loudness is highly nonlinear. Doubling or halving loudness requires about 10 dB change in sound level at middle
and high frequencies. At low frequencies, it can be as little as 4 dB. The smallest audible
change in overall loudness level is about 1 dB. 3 dB is just nicely audible. Loudness also
depends on the frequency, bandwidth and duration of the sound. See: Loudness
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Loudness Contours
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Loudness Control
A separate control or, more commonly, a switch-selected addition to a volume control, that
causes bass frequencies to be amplified as the overall volume is turned down. The idea is to
compensate for the fact that the ear becomes progressively less sensitive to bass at low
sound levels. See: Loudness Contours','Glossary',775,400,'right')" class="link">Equal
Loudness Contours.
Loudspeaker
An transducer that converts an electrical signal into sound.
Loudspeaker System
A set of transducers, with a crossover network, in an enclosure. A two-way system has a
woofer and a tweeter, a three-way system adds a mid-range, and so on. Up to a point,
more transducers allow a designer more capability in designing a good sounding system,
but there are no guarantees. More transducers should, however, allow for higher sound
levels. See: Woofer, Tweeter, Crossover, Enclosure.
Low Frequency
Generally refers to sounds below about 300 Hz.
Low Frequency
Effects
The 0.1 channel in 5.1-channel Dolby Digital signals. All 5 main channels are full range, so
this additional channel, covering the frequency range 3 Hz to 120 Hz is there to
accommodate very loud low frequency special effects sounds, such as explosions. It is
included in the bass managed subwoofer outputs of surround processors and receivers, but
is discarded in the two-channel downmixes of Dolby Digital that occur in many DVD players.
See: Downmix, Dolby Digital, Bass Management
Low Pass Filter
An electronic or acoustical device designed to pass all frequencies lower than the design
frequency, and to attenuate, or block, all higher frequencies.
LP recordings
LP = Long Playing - relative to the 78 rpm discs of the previous generation. The familiar 12inch diameter vinyl discs, containing analogue recordings played back at 33 1/3 rpm.
Almost all are two-channel stereo, although some were encoded in four channel formats
during the Quadraphonic era in the late 1970's.
LP recordings
LP = Long Playing - relative to the 78 rpm discs of the previous generation. The familiar 12inch diameter vinyl discs, containing analogue recordings played back at 33 1/3 rpm.
Almost all are two-channel stereo, although some were encoded in four channel formats
during the Quadraphonic era in the late 1970's.
Lt, Rt
Left total, right total. Names given to the left and right channels of a two-channel audio
signal which contains Dolby Surround encoded information. See: Dolby Surround.
Luminance
Brightness, or the black and white content of a picture. The symbol 'Y' designates the
luminance signal. Combined with chrominance (C) to complete a color video picture. See:
Chrominance, Chroma.
M.T.S.
Multichannel Television Sound. The common method of broadcasting stereo audio with a
television signal.
Magnet
Steel or other magnetic material that has been magnetized. A permanent magnet will hold
its magnetization indefinitely. Used in loudspeakers to provide the static magnetic field
within which the voice coil operates. See Voice Coil, Electromagnet.
Magnetic Shielding
A design of loudspeaker motor in which the stray magnetic field is suppressed to avoid
distortions to CRT, or other magnetically sensitive, video displays.
Masking
Audio: A perceptual phenomenon in which the presence of one sound reduces our ability to
hear another. Loud music may mask a doorbell. The music that creates distortion in a
nonlinear device may prevent us from hearing all of the distortion created by the problem.
VIDEO: The black area surrounding the picture area in a front projection screen. See
Simultaneous Masking, Temporal Masking
Matrix
Encode/Decode
A method of electrically combining, or encoding, several channels of information into a
smaller number of channels for communication or storage, e.g. Dolby Surround. The
reverse matrix can be used to decode the original channels on the other end. However,
channel separation between some of the channels can be as low as 3 dB, so the
reconstruction is imperfect. Active or steered matrix decoders can enhance the separation
when not all channels are operating simultaneously, making the effect seem to be more
discrete (e.g. Logic 7, Dolby ProLogic). See: Channel Separation, Dolby ProLogic, Logic 7,
Discrete.
Mega-
A prefix meaning million. For example: one million Hertz is 1 megahertz, abbreviated 1
MHz.
Membrane Absorbers
See: Diaphragmatic Absorbers.
Micro-
A prefix meaning one-millionth. For example: one-millionth of a Farad is 1 microfarad,
abbreviated 1 (NEED SYMBOL FOR MICRO)farad.
Midrange
Frequencies between 300Hz and 3000Hz. See Midrange driver.
Midrange Driver
A speaker that is designed to reproduce middle frequencies, from about 300 Hz to about
3000 Hz. This is where most speech and much musical information lies.
Milli -
A prefix meaning one-thousandth. For example, a milliampere is one-thousandth of an
ampere. Abbreviated ma.
Missing Fundamental
A perceptual phenomenon in which listeners may perceive a low pitched sound when there
is no physical sound energy at that frequency. Musical notes are combinations of a
fundamental (the lowest harmonic) and a collection of harmonics. If only the fundamental is
missing, as it might be in a sound system with poor, or no bass output, humans can still
perceive the correct pitch for the note. Of course, the timbre is changed, and the tactile
bass sensations are missing, but the essential tune will be there. See: Harmonic,
Fundamental.
Mixing
In audio recordings of all kinds, it is common for several 'tracks' of single instruments,
voices, or sound effects to be combined, or mixed, into the final product. At the same time,
equalization, reverberation and other electronic enhancements can be made to the
recording.
MLP
MLP (Meridian Lossless Packing), also known as Packed PCM (PPCM) or Dolby MLP
Lossless™, is a proprietary lossless compression algorithm for digital sound recordings. The
format was specifically designed for high-resolution audio data and is mandatory for DVDAudio.
MMD
Derived from our patented CMMD technology, Infinity's new Metal Matrix Diaphragms
(MMD) continue the Infinity tradition of using advanced materials to improve sonic
accuracy. By adonizing both sides of an aluminum core, we're able to significantly improve
cone performance and outperform cones made of traditional materials.
Mode
See: Room Resonances.
Monaural
Literally, one ear, or listening through only one ear. Commonly used to describe a single
channel or monophonic system or device, e.g. a single channel, monaural, amplifier. See
Monophonic.
Monophonic
A single channel system or component.
Monopole
Classic definition: a sound source that is small compared to any wavelength that it radiates,
and is therefore uniformly omnidirectional. Contemporary usage: a conventional forwardfiring loudspeaker. Most consumer loudspeakers are small compared to the wavelengths at
very low frequencies and so, at least at those frequencies, they qualify as true monopoles.
MOST®
MOST® (Media Oriented Systems Transport). Digital network for the optimized
transmission of data (particularly all multimedia data used in vehicles) via fiber-optic or
electric cables. As with D2B Optical, the defining characteristic of MOST is synchronous data
transmission, which coordinates sound and image signals. Many European and Asian vehicle
manufacturers use MOST. The system was developed in 1997 by the MOST Cooperation, a
consortium of Harman/Becker, Audi, BMW, DaimlerChrysler and SMSC; the consortium also
works with other industry partners.
MOST® (Media
Oriented Systems
Transport)
Digital network for the optimized transmission of data (particularly all multimedia data used
in vehicles) via fiber-optic or electric cables. As with D2B Optical, the defining characteristic
of MOST is synchronous data transmission, which coordinates sound and image signals.
Many European and Asian vehicle manufacturers use MOST. The system was developed in
1997 by the MOST Cooperation, a consortium of Harman/Becker, Audi, BMW,
DaimlerChrysler and SMSC; the consortium also works with other industry partners.
Motor
The assembly on the back of a loudspeaker, consisting of a magnet, iron magnetic circuit
elements, and a voice coil which together cause the diaphragm to move in response to an
electrical audio signal from a power amplifier.
Moving Coil
Loudspeaker
A conventional loudspeaker driver in which a diaphragm is driven by a voice coil. See:
Motor, Voice Coil
MPEG
MPEG (Motion Picture Experts Group) develops standards for the compression of digital
audio and video signals. There are different MPEG standards, depending on the application.
MPEG-2 is the video standard used for DVDs; the correct name for the popular term MP3 is
MPEG-2 audio, layer 3.
MPEG (Motion Picture
Experts Group)
MPEG develops standards for the compression of digital audio and video signals. There are
different MPEG standards, depending on the application. MPEG-2 is the video standard used
for DVDs; the correct name for the popular term MP3 is MPEG-2 audio, layer 3.
MPEG Audio
MPEG-1 is a perceptual coding algorithm for two audio channels. MPEG-2 does multichannel
audio. See: Perceptual Coding
MPEG Video
MPEG-1 is a data compression algorithm for low-quality video, such as that on Video CD.
MPEG-2 is used for DVD and HDTV. See:
Multichannel
A sound recording/reproduction system with more than two channels and loudspeakers.
Current systems have 5, 6 or 7 channels plus a low-frequency effects (LFE) channel.
Multichannel sound can also be simulated from two-channel sources. See Logic 7, Dolby
ProLogic Plus.
Multimeter
An analogue or digital meter capable of measuring voltage, current and resistance.
Multipath
When a radio or television signal arrives at a receiver from two or more different paths (at
least one of which is a reflection) the signals interfere with each other causing distortion in
FM audio and ghosting in television pictures. See: Diversity Reception.
Multiroom
A feature of custom whole-house systems, and of some AV Receivers, allowing sound to be
delivered to loudspeakers in other rooms, without interfering with what is happening in the
main entertainment room.
Multi-way
loudspeaker
A loudspeaker system in which different drivers handle different parts of the frequency
range. E.g. two-way = woofer + tweeter, three-way = woofer + midrange + tweeter.
Music Power
A non-standard power rating used mostly for amplifiers, trying to bridge the gap between
the relatively low steady-state (RMS) power rating, and the much higher peak power rating.
Both are repeatable, but unrealistic in terms of representing what happens with music.
However, since music is so variable, there can be an infinite number of music power
ratings.
Mute, Muting
A button or control that, in one operation, reduces the sound level by a fixed amount.
Depending on the equipment, the sound can be completely turned off, or simply attenuated
by a chosen amount. Useful for relief from commercials and for answering the telephone.
Navigation
A device that uses GPS (Global Positioning System) to provide route mapping and directions
NC Rating
See: Noise Criteria
Negative Feedback
See Feedback.
Neodymium
A rare earth magnet that is several times more powerful than conventional materials such
as ferrite. This allows for very compact loudspeaker motors, which can be designed to be
self shielding to minimize interference with CRT video displays. See Motor, Magnetic
Shielding.
Noise Cancellation
The process by which a signal processor samples ambient noise through a microphone and
inserts the inverse of the noise into the playback signal. Often used in industrial settings to
reduce noise from machinery or other steady-state noise. It's now being applied to in-car
audio systems to reduce ambient noise caused by the movement of the vehicle.
Noise Criteria
In acoustics, normally used to describe a rating system for evaluating noise from heating,
ventilating and air conditioning (HVAC) systems. It has come to be used as a basis for
evaluating and establishing requirements for background noise in listening spaces. Based
on a set of sound level vs. frequency curves known as the NC criteria. The lowest curve that
the spectrum of the measured sound does not exceed is defined as the noise criterion (e.g.
NC-30) for that room. There are other variations on this theme, adopted for different
purposes, or in different parts of the world (e.g. NR - noise rating - in Europe).
Noise Reduction
Coefficient
A simplified rating of the sound absorbing properties of materials, in which the sound
absorption coefficients at 250, 500, 1000 and 2000 Hz are averaged. Abbreviated: NRC.
Obviously this is of no value in evaluating what happens at low, or very high frequencies. It
is a speech-oriented measure. See: Sound Absorption Coefficient.
Nominal Impedance
The stated impedance rating of a speaker, used by manufacturers to represent the load the
speaker will present to an amplifier. In fact, the impedance of most loudspeakers varies
considerably with frequency, so the nominal impedance is only a very rough guide. In
practice, the minimum impedance also needs to be known.
Non-Linear Distortion
An undesirable characteristic of a device in which audio signals are modified, and modified
differently at different sound levels. Usually, the problem increases in magnitude with
increasing level, although there are some problems that are more noticeable at low levels.
The audible significance of the nonlinearity is evaluated by applying test signals and
measuring the harmonic and intermodulation distortion products that are added. See:
Linear Distortion, Harmonic Distortion, Intermodulation Distortion.
NR Rating
SEE: Noise Criteria.
NRC
See: Noise Reduction Coefficient.
NTSC
Stands for National Television Standards Committee, which set the television broadcast
standard for North America, also used in much of Central and South America, the Carribean
Islands, Japan and Taiwan. It is a 525 line interlaced raster-scanned system displayed at 60
fields/second. Of these, about 480 lines contain picture information. See: PAL, SECAM,
Television Systems
O.E.
Original Equipment see also OEM
Oblique Modes
See Room Resonances.
Octave
A doubling or halving of frequency. Example: 80Hz is one octave above 40Hz. In
measurements and audio analysis, it is common to examine what is happening within
octave or fractional-octave bands. 1/3 octave band analysis is very common, with new
instruments capable of even higher resolutions. See: Real-Time Analyzer, Spectrum
Analyzer.
OEM
Original Equipment Manufacturer. When one company supplies systems or components to
another manufacturer for installation at the time of manufacture. For example, Harman
supplies many automobile manufacturers with audio, video, navigation and other systems
and components on an OEM basis. See: Aftermarket.
Ohm
Basic unit for measuring resistance and impedance.
Ohm`s Law
The basic relationship between current, voltage, and resistance. Ohm's law states that
voltage = current x resistance, current = voltage/resistance, and resistance =
voltage/current.
Omnidirectional
Referring to microphones or loudspeakers, having equal sound sensitivity or output,
respectively, in all directions.
On Axis
In a loudspeaker it is the imaginary axis that projects perpendicular to the plane of the
loudspeaker drivers. Normally it originates at the tweeter axis, or at a point close to the
tweeter and midrange drivers.
Oscillator
See: Audio Oscillator
OTA
Meaning Over The Air. See: Terrestrial.
Out-of-phase
Two signals having the same waveforms that are in perfect synchronization with each other
but with a perfect 180-degree phase shift at all frequencies. Equivalent to a polarity
inversion of one signal. The signals will cancel each other (destructively interfere). See:
Acoustical Interference, Polarity Inversion.
Overtones
Sounds with frequencies that are higher than the fundamental frequency and that normally
occur with the fundamental. The first overtone is the second harmonic, since the first
harmonic is the fundamental. See: Harmonic.
PAL
The television broadcast coding standard used in England, Europe, and many countries in
the rest of the world. It has 625 lines displayed at 50 fields/second. Of these, about 576
lines contain picture information. See: NTSC, SECAM, Television Systems.
Pan and Scan
A method of converting widescreen programs and movies for viewing on a standard 4:3
aspect ratio television. It involves rerecording the program while panning and scanning
(sweeping left and right) with a camera, selecting the portion of the picture to be shown in
the reduced size. Obviously, the result is not the same movie that the director created, but
it fills the entire TV screen. Letterboxing is the alternative. See: Letterbox, widescreen,
aspect ratio.
Parallel Connection
Connecting two or more devices across the same terminals so that each device carries the
full applied voltage. In loudspeakers, parallel connection of two identical units halves the
impedance seen by the power amplifier, which can create problems with amplifiers not able
to drive low impedances. For example, two 6-ohm loudspeakers operating together will
present a 3-ohm load, which is below the safe operating load range for many power
amplifiers. This is a consideration in the A, B, A+B loudspeaker switching feature of A/V
receivers. See Series Connection.
Parametric Equalizer
An equalizer consisting of several filters each of which can be adjusted in each of three
parameters: frequency, bandwidth or Q, and amplitude, allowing it to closely match the
properties of resonances which are to be attenuated. See:Resonance, Graphic Equalizer.
Particle Velocity
Air is constantly in motion due to wind or thermal convection currents. The particle velocity
of interest in audio is that which is added by the alternating pressure fluctuations of sound
as it passes through. Due to sound alone, particles stay in place and simply vibrate.
Pass Band
See Bandpass.
Passive Radiator
Often called a "drone cone." A diaphragm with a suspension but no motor assembly.
Passive radiators can be substituted for a vent or port in a speaker.
PCM
A straightforward, uncompressed coding method in which analog signals are sampled at
regular intervals, and each sample is represented by a digital number representing the
amplitude at that instant in time. Sampling is done at a frequency at least two times the
highest frequency of interest, and the digital number must have a sufficient number of bits
to capture and reconstruct the essential qualities of the audio signal (dynamic range, low
distortion and noise, etc.). For example, typical CDs are recorded using a sampling
frequency of 44.1kHz, using a 16-bit digital number. See Bit, Compression.
Peak
In time: the instantaneous maximum value of a time-varying quantity. E.g. peak power in
power amplifiers. In frequency response: an elevated output that is concentrated over a
narrow range of frequencies, describing a peak in the frequency response curve.
Perceptual Coding
A method of processing a digital audio signal in which knowledge of auditory masking is
used to predict what portions of the signal would not be heard by normal listeners. These
portions are then either discarded, or more simply encoded, so that the data rate used to
communicate the signal is reduced, as is the storage capacity. Several such "lossy" coding
systems exist, and they all sound excellent at high data rates, degrading by different
amounts in different ways as the data rate is reduced.
Perforated Panel
Absorber
A form of acoustically resonant absorber in which a Helmholtz resonance is created with the
air mass in the perforations reacting with the compliance of the air behind the perforated
panel. See Resonant Absorber, Helmholtz Resonance..
Period
The time interval between identical points in a periodic signal or sound. For a sinusoid
(single frequency) the period (in seconds) is calculated as 1 / frequency (in Hz). See
Periodic.
Periodic
In this context, a sound wave that repeats itself exactly at regular intervals. The simplest of
these is a pure tone, a single frequency.
Phantom Center
In stereo or surround-sound systems, a phantom center image is perceived when the same
sound is radiated from left and right loudspeakers, and the listener is exactly equidistant
from both loudspeakers. In multichannel systems, it may not always be possible to use a
real center channel loudspeaker, in which case, a phantom center can be selected (or will
occur automatically when no center loudspeaker is indicated to the surround processor
during setup). A real center loudspeaker is advantageous because the sound image is heard
to be in the same place by listeners in different locations. See: Center Channel,
Stereophonic.
Phase
For a single frequency, a periodic signal, the repetition period is divided into 360 degrees.
Phase is measured as angular degrees within the period. For a single frequency, there is an
equivalent time for a given phase shift, but the time for a given phase shift will be different
for every frequency because the period is different for every frequency. 180-degree phase
shift is equivalent to a polarity reversal. See Phase Shift.
Phase Adjustment
(Subwoofers)
Because they are in different locations in a room, the sound from a subwoofer and that
from the satellite loudspeakers do not always combine properly, resulting in too much or
too little sound in the crossover region. This is a control allowing the user to make small
adjustments to the phase of the subwoofer in the hope that the situation can be improved.
Sometimes it can. See: Phase, Crossover.
Phase Plug
In a compression driver for a horn, this is a device conveying sound energy from the large
diaphragm to the smaller throat of a horn in such a way that, over most of the operating
frequency range, sound originating at all parts of the diaphragm arrive at the throat in
phase. See: Horn, Compression Driver, In Phase, Throat.
Phase Response
A measurement of phase as a function of frequency from the input to the output of an audio
component. See Frequency Response, Transfer Function.
Phase Reversal
See Polarity Inversion
Phase Shift
A change in phase angle between the input and output of a device or system. This would
normally be specified at several frequencies, or as a continuous curve as a function of
frequency.
Phon
A measure of perceived loudness originated by Fletcher and Munson. The loudness level, in
phons, is the sound pressure level of a 1 kHz pure tone that is judged to be equally loud.
Each of the equal-loudness contours, therefore, is identified in phons. See Equal-Loudness
Contours
Phono
Input/Preamplifier
An input designed to accept the small voltage signals from a phonograph cartridge, to
amplify them to normal line levels and to perform the inverse RIAA equalization necessary
to produce properly balanced low and high frequency sounds. See: RIAA
Pincushion Distortion
Geometric distortion in video pictures in which straight lines take on a concave shape,
curving inward from the screen edges between the picture corners. See: Barrel Distortion
Pink Noise
Random noise having a continuous spectrum and energy distributed equally on a
percentage-bandwidth (e.g. octave, 1/3-octave) basis. Commonly used for audio
measurements since the energy distribution is close to that of music. See: White Noise
Pitch
The perceived quality of sound that is most closely related to frequency, but is also
influenced by sound level. The position of a note on a musical scale.
Pixel
The smallest element in a picture. An individual dot in a picture composed of dots, as in all
digital video systems.
PLUGE
Stands for: Picture Line-Up Generation Equipment. A test pattern used for setting the black
level (brightness at the darkest part of an image) of a video display.
Polarity
Being electrically positive or negative, as in the terminals of a battery or DC power supply.
Being, at an instant in time, positive or negative, relative to zero voltage, in an AC signal.
Polarity Inversion
In an AC signal path, to reverse the connections (as in a loudspeaker), or to invert the
waveform (as in electronic circuitry). Sometimes confusingly called Phase Reversal meaning that the instantaneous phase at all frequencies is simultaneously reversed by 180
degrees. See Phase, Out-of-Phase.
Pole Piece
The cylindrical core of a loudspeaker motor, one end of which forms the inside of the
magnetic gap within which the voice coil moves.
Polypropylene
Thermoplastic used in speaker diaphragms. Polypropylene is weather resistant and well
damped.
Port
A vent or tube that forms part of a resonant system in a bass reflex loudspeaker enclosure.
See Bass Reflex.
Potentiometer
A resistor with an adjustable tap, used for many purposes in electronics, including the
common one of volume control.
Power
The amount of energy delivered or used by a device or system, expressed in Watts. In
audio, power ratings of amplifiers and loudspeakers are subject to a lot of variation and
uncertainty because of the very large difference in the long-term, steady-state, power
rating, and the transient, or momentary, power rating, which can be several times larger.
Further confusion is added when ratings at a single frequency (say, 1 kHz) are compared
with the more realistic 20 - 20 kHz rating. In multichannel amplifiers, there is the further
variable of ratings done with a single channel operating vs. ratings with all channels
operating. The result, for consumers, is that advertised power ratings are often almost
meaningless.
Power Compression
In loudspeakers at high sound levels, an effect in which the acoustical output of the device
increases less than the electrical input to the device. Caused mainly by the heating of the
voice coil, and the consequent increase in electrical resistance. In multi-way systems,
power compression affects the different drivers differently, causing the loudspeaker system
to have a different spectral balance at different sound levels.
Power Handling
The amount of power that may be applied to a device without causing destruction. May be
expressed as "RMS" or as "continuous average" to indicate how much power can be applied
for long periods. It mnay also be expressed as "peak" to indicate how much power can be
applied for extremely short periods--the beat of a drum, for instance. See also Power
Power Rating
See: Power.
Powered Tower
A floor-standing loudspeaker with an amplified woofer or subwoofer.
Pre-amp
See Preamplifier
Preamplifier
An audio component that selects signal sources, and provides volume and tone
compensation functions. There may be special gain stages for phono cartridge inputs. It has
line-level outputs to drive a power amplifier. Normally a stereo device, its multichannel
equivalent is a surround processor.
Precedence Effect
In sound localization, the fact that the first arriving sound (the direct sound) normally
dominates our sense of direction. This allows us to correctly identify the direction of a
source of sound in a reverberant space. See: Direct Sound, Haas Effect.
Progressive Scan
A video display that scans all lines sequentially in each pass. A Line Doubler can create a
progressively scanned image from an interlaced scan signal. See: Line Doubler.
Progressive spider
A speaker's spider that provides increasing force (to restore the cone to the resting
position) as it stretches
Projector
There are two ways to get an image on a screen. Shine light through an image forming
element, or let light reflect from an image forming element. Both are very common. CRT
projectors use an electron beam from behind and LCD devices rely on transmitted light to
create the image, which is then communicated to a screen by lenses. Light valve (ILA), and
D_ILA/LCOS devices rely on light reflecting off an image element, which is then focused on
a screen using lenses. See: Light Valve, ILA, D-ILA, LCOS, CRT.
Psychoacoustics
The branch of acoustics that relates the physical dimensions of sound with the perceived
dimensions - the relationships between what we measure and what we hear.
Pulse-Code
Modulation
A straightforward, uncompressed, coding method in which analog signals are sampled at
regular intervals and each sample is represented by a digital number representing the
amplitude at that instant in time. Sampling is done at a frequency at least 2 times the
highest frequency of interest, and the digital number must have a sufficient number of bits
to capture and reconstruct the essential qualities of the audio signal (dynamic range, low
distortion and noise, etc.). E.G. normal CD's are recorded using a sampling frequency of
44.1 kHz using a 16 bit digital number. See: Bit, Compression.
Pure Tone
A single frequency having a line spectrum. The periodic waveform is that of simple
harmonic motion, a sinusoid. See: Spectrum.
PVR
Personal Video Recorder. A digital hard-drive video recorder (DVR) which, when combined
with a program guide, can record one's personal favorite television programs. See: DVR.
Q
The 'quality factor' of a resonance, related to the amount of damping or losses in the
resonating system. High-Q resonances have little damping. They are very frequency
specific, or narrow bandwidth, and they ring energetically. Increasing amounts of damping
lower the Q, resulting in lower amplitude, wider bandwidth effects in frequency responses,
and less ringing in the time responses.
Quadraphonic
Four channel recodings, promoted during the late 1970's, some on tape, but mostly on
matrix-encoded two-channel LP's. There was even an attempt to popularize a discrete four
channel LP format using signals recorded up to about 50 kHz. It all collapsed because the
industry could not agree on a single standard. Some of the clever matrix technology was
incorporated in Dolby Stereo and Dolby ProLogic for movie sound tracks.
R.A.B.O.S.™
Room Adaptive Bass Optimization System (R.A.B.O.S.) utilizes a single band of parametric
equalization to tame the most prominent room, mode or bass peak. These bass peaks
exaggerate the sounds at their frequency resulting in bass performance that is often sloppy
or boomy. By reducing the level of this peak, you are able to hear deeper, more detailed
low frequencies. The sound-pressure-level meter included with the R.A.B.O.S. accessory kit
allows the listener or installer to easily make the measurements required in order to
properly calibrate the RABOS settings for your particular listening room.
Rarefaction
A momentary pressure reduction as a sound wave passes. See: Compression, Sound Wave.
Raster
The collection of horizontal lines that make up a video display. E.g. in NTSC there are 525
lines in a frame (two interlaced fields), of which 480 are visible. See: Frame, Interlaced,
NTSC.
RCA Plug/Socket
Radio Corporation of America, which gave its name to the very common plug and socket
used for line level audio and many video interconnections.
RDS
Radio Data System. RDS is digital data coded into FM transmissions which displays
information such as station name, time, program information and traffic announcements
alphanumerically on the receiver visual display. Obviously, an RDS compatible radio
receiver is required.
Reactive Absorbers
Sound Absorbers that function because of the interactions of mass, compliance and
damping. E.g. Helmholtz absorbers, Diaphragmatic Absorbers. See: Helmholtz Absorbers.
Diaphragmatic Absorbers, Resistive Absorbers.
Real-Time Analyzer
A measuring instrument that presents spectral information, in 'real time', in the form of
levels in octave or fractional/octave bands covering the audio bandwidth. Traditional
instruments used 1/3-octave frequency resolution, but nowadays higher resolutions are
needed, especially at low frequencies, in order to identify and correct room acoustic
problems. See: Octave, Spectrum Analyzer..
Rear Projection
Television
A rear projector which, by using special lenses and a mirror, delivers an image to the rear
of a translucent screen in a (relatively) compact enclosure. See: Rear Projector.
Rear Projector
A projector that delivers a left-right reversed image to the rear surface of a translucent
screen allowing the audience to view the transmitted image. See: Front Projector, RPTV.
Reflection
When sound changes direction due to reflection (angle of incidence = angle of reflection)
from a surface that is large compared to a wavelength.
Reflection Free Zone
Location within a listening room where, for a specified interval of time following the arrival
of the direct sound, reflected sounds have been attenuated by a specified amount. The
purpose is to place the listener in a predominantly direct sound field for that interval of
time. Used in some recording control rooms and listening rooms. Effectiveness, and need,
depend on the directivity and quality of the loudspeakers, and on the 'tastes' of the listener
with respect to soundstage and imaging. See: Direct Sound, Directivity, Imaging,
Soundstage.
Refraction
When sound changes direction as it passes through media exhibiting different speeds of
sound. E.g. warm air near the ground and cooling with altitude, will cause sound waves to
bend upwards. The inverse will cause sound waves to bend downwards, allowing us to hear
sounds over great distances.
Resistance
Electrical: The opposition to direct current flow offered by a device or material. Measured in
ohms. There are also acoustical and mechanical equivalents to resistance.
Resistive Absorbers
Sound absorbers, like fiberglass, drapery, upholstery, etc. that remove energy from a
sound field by making it difficult for the air molecules to move in the 'fibrous tangle'. The
sound energy is converted to heat. Most effective when located in regions of high particle
velocity, meaning that, when placed on a surface, thick layers will be required to be
effective at middle to low frequencies.
Resolution
VIDEO: in digital displays, it is the number of pixels along the width and height of the
picture. In any display, it is an assessment of the clarity of details in the picture. This can
be different for stationary and moving objects, and the perceived resolution can be different
from that which is technically defined. AUDIO: a loosely defined term used to describe
perceptions of small details in music. See: Pixel
Resonance
A 'natural frequency' for an acoustical, electrical or mechanical system, at which the system
exhibits elevated vibration amplitude and sustained activity (ringing) after the input
excitation is removed. The frequency bandwidth over which the resonant activity exists, and
the duration of ringing, are related to the amount of damping in the system. This is
described as the 'Q' of the resonance. See Q, Ringing, Bandwidth.
Resonant Absorbers
Sound absorbers that are mechanically resonant (e.g. diaphragmatic absorbers) or
acoustically resonant (Helmholtz absorbers), being most effective at and around their tuned
frequencies.
Reverberation
In a room, it is the sound that has been reflected many times from many objects and
surfaces. In large rooms and concert halls the reverberation can last long enough to be
heard as a gradual decaying of sound after the source has stopped radiating. So called 'live'
rooms have a lot of reverberation. 'Dead' rooms have little.
Reverberation Time
The time it takes for reverberation to decay by 60 dB (roughly, to inaudibility) after the
sound source has ceased. See: Sabine.
RF Audio/Video
An antenna or cable-like connection provided to allow audio and video signals to be
communicated to TV's which have only this kind of input. Many VCR's and cable boxes
provide these RF (audio and video are modulated on a VHF carrier) outputs. Technically, it
is the worst form of connection. Use only when absolutely necessary.
RF remote
A remote control that communicates by means of radio frequency (RF) waves. These can
travel through walls, and other objects, like a cordless telephone, thus making them very
useful in a home. See: IR remote.
RGB
RGB = Red, Green, and Blue the three 'primary' colors which, in different combinations, can
form all other colors. Also the three connectors and cables that can be used to
communicate picture information in this format.
RIAA
Recording Industry Association of America. Also, the name of the standardized equalization
curvess used in making and playing back LP recordings.
Ringing
The natural tendency of an activated resonant system remain active after the excitation is
removed, decaying gradually over time. The decay time decreases with increasing damping.
See Resonance, Q, Tinnitus.
RMS
See Root Mean Square.
Roll-off
The attenuation of frequencies above or below a certain point.
Room Acoustics
The collection of physical properties in a room that collectively give it a recognizable
character when listening to live or reproduced sounds.
Room Proportions
The ratio of length to width to height of a room. Significant because this ratio determines
the distribution, in the frequency domain, of all of the acoustical resonances in a simple
rectangular room. Large openings and non-parallel surfaces upset the relationship.
Room Resonances
Acoustical resonances in rooms that occur between opposite parallel surfaces (axial modes),
among four surfaces, avoiding a pair of opposing parallel surfaces, e.g. the four walls, or
two walls and the floor and ceiling (tangential modes), and among all surfaces (oblique
modes). See Standing Waves.
Root Mean Square
Electrical: the square root of the time average of the squared (voltage or current) over one
cycle of a periodic signal. Also called the 'effective value', it is used to calculate power in AC
signals. Thus, the RMS power rating of a power amplifier is calculated using the RMS values
of voltage and current, and represents its ability to produce continuous power, as opposed
to momentary or peak power.
RPTV
See: Rear-Projection Television
RT60
See: Reverberation Time.
RTA
See: Real-Time Analyzer.
Sabin
A unit of measure for sound absorption. 1 Sabin = the absorption of one square foot of a
surface having 100% absorption e.g. an open window). See: Absorption Coefficient, Sabine.
Sabine
Wallace Clement Sabine, a founder of modern architectural acoustics (1868-1919).
Developed a relationship between reverberation time and the amount of acoustical
absorption in a room. Designed Boston Symphony Hall, considered to be one of the best
concert halls in the world. See: Reverberation Time, Sabin, Absorption.
Satellite Loudspeaker
A loudspeaker designed to function with a subwoofer, and therefore it has limited lowfrequency output. At higher crossover frequencies, the satellites can be very small, the size
of a coffee cup, but maximum loudness is limited and the risk of localizing the subwoofer
increases. At lower crossover frequencies, say 80 Hz (a common high-quality standard),
satellites need to be "bookshelf" size or larger, but the whole system has more dynamic
range and potentially higher sound quality. See: Subwoofer, Crossover, Localization,
Dynamic Range.
Saturation
The extent to which a pure color has been diluted with white. Highly saturated colors have
little white in them. Lower levels of saturation look washed out with the white content.
Scan Line
One horizontal line in the raster of a video display. See: Raster.
Scan Rate
The frequency with which a video display reproduces scan lines. Standard NTSC television
has a scan rate of 15,734 lines per second (525 lines per frame multiplied by 29.97 frames
per second).
SDARS
SDARS (Satellite Digital Audio Radio Services) is an umbrella term for satellite-supported
radio systems in North America. There are currently two different systems: Sirius Satellite
Radio and XM Radio.
Sealed Enclosure
See Acoustic Suspension
SECAM
The French original, SEquential Couleur Avec Memoire means SEquential Color with
Memory. A television broadcast standard used in France, several European/Mediterranean
and African countries, and others. 625 lines displayed at 50 fields/second. Of these, about
576 lines contain picture information. See: PAL, NTSC, Television Systems.
Sensitivity
A standardized measure of the sound output of a loudspeaker for a known input signal.
Originally, the input power was 1 watt. Nowadays, the input is standardized to 2.83 volts (1
watt into 8 ohms). Measurements are made on axis in an anechoic space, at a distance that
places the microphone in the far-field of the loudspeaker system, and then the sound
pressure level is calculated for a microphone distance of 1 meter. A measurement distance
of 1 meter is too close for all but single drivers and very small loudspeaker systems. See
also: Far Field, Input Sensitivity.
Separation
A measure of the independence of the channels in a multichannel system. It tells us how
much unwanted signal leaks into any of the channels. In practice, from the point of view of
maintaining good sound image localization, 20 dB or more channel separation is required.
Series Connection
A circuit where components are wired sequentially, in head-to-tail fashion, across a pair of
terminals. The terminal voltage is then divided between them in proportion to their
impedance characteristics. Each of two identical loudspeakers would receive half of the
voltage, and half of the power, delivered to the series combination. This connection is often
used in A/V receivers in A, B, A+B loudspeaker switching, to prevent confronting the power
amplifiers with loads they cannot drive. The tradeoff is reduced maximum power to each
loudspeaker. See Parallel Connection.
Series-Parallel
Connection
A circuit where there are components wired in series and also in parallel to get the desired
result out of the combination. This is often used in multi-loudspeaker systems to optimize
the impedance seen by an amplifier.
Servo Control
Subwoofer
A powered subwoofer that uses feedback from the motion of the voice coil to control the
power provided to the speaker by the amplifier.
Set-Top Box
A device, including a tuner, for receiving terrestrial, cable or satellite signals and feeding
them to the television display. Set-top boxes may include other features, like games,
storage, web access, etc. See: Tuner, Terrestrial.
Shielded Loudspeaker
A loudspeaker in which the stray magnetic field has been minimized so that it can be
located near a CRT video display without distorting the picture.
Short Circuit
A direct connection, usually accidental, between the positive and negative portions or
terminals of a circuit, bypassing some or all of the components of the circuit and preventing
operation. Connecting both leads of a loudspeaker wire together presents the power
amplifier with a short circuit, causing it deliver far more current than it was designed to.
With luck it will sense the danger and protectively shut down. If not it will lapse into an
expensive silence.
Simultaneous
Masking
Masking that occurs when a loud sound prevents us from hearing other, smaller, sounds
that occur simultaneously. See Masking, Threshold Shift.
Sine Wave
See: Pure Tone.
Sinusoid
See: Pure Tone.
Slat Absorber
An acoustically resonant absorber in which the air mass in the spaces between the slats
reacts with the compliance of the air behind the slats to form a Helmholtz resonator. See:
Resonant Absorber, Helmholtz Resonance.
SLM
See Sound Level Meter.
Sone
The unit of perceived loudness. One sone is the loudness of a 1000 Hz tone at a sound
pressure level of 40 dB. Two sones would be twice as loud and, at middle and high
frequencies, that would require an increase in sound level of about 10 dB. At low
frequencies it would require less. See: Loudness, Equal-Loudness Contours, Sound Pressure
Level.
Sound
There are two definitions. Physically, sound consists of pressure fluctuations, at any
frequency, that propagate in an elastic medium, such as air. Perceptually, sound is the
human response to those physical pressure fluctuations, which are normally considered to
occur in the frequency range 20 Hz to 20 kHz.
Sound Absorption
Coefficient
See: Absorption Coefficient.
Sound Intensity
A measure of the sound energy propagated in a specific direction. Measured as
watts/square meter over an area normal to the direction of propagation. The term is often
incorrectly used when sound level, or sound pressure level is really intended.
Sound Level
Sound pressure in decibels, usually referenced to the standard level near the hearing
threshold at middle frequencies, and almost always weighted according to one of the
standard frequency weighting contours, A, B or C. Such sound levels should be described as
dB (A), for example. See Frequency Weighting.
Sound Level Meter
A meter that measures sound pressure levels, with or without frequency weighting, and
with the option of fast or slow averaging times. See Frequency Weighting.
Sound Power
A measurement of the total sound radiated from a loudspeaker without regard to direction.
Ideally measured at many points on an imaginary sphere surrounding the loudspeaker. One
of several factors used in predicting how a loudspeaker may sound in a room.
Sound Pressure Level
A measurement in which the sound pressure is expressed as so many decibels (dB) above a
standard reference pressure level, which is close to the threshold of hearing at middle
frequencies. SPL, as it is called, is a standardized measurement, and is always done with
the measuring instrument set to the "linear" mode, with all frequencies represented
equally. Measurements made with A, B, C, or other weightings, are called Sound Level
measurements. See Frequency Weighting, Hearing Threshold, Sound Level.
Sound Ray
A graphic way of illustrating the direction in which sound is propagating. It is commonly
used to illustrate how sound reflects in rooms (angle of incidence equals angle of reflection)
where it works well at frequencies where the wavelengths are small compared to the
dimensions of the reflecting surfaces.
Sound Transmission
Class
A rating number used to classify the sound attenuation capabilities of various walls, floors,
ceiling, doors, windows, etc. It is measured over the frequency range 125 - 4000 Hz, as it
is intended to evaluate speech privacy, and so it tells us absolutely nothing about isolating
the bass frequencies - one of the most serious problems with home entertainment systems.
Abbreviated: STC, and it is specified in dB, larger numbers being better.
Sound Wave
A pressure fluctuation that propagates in air, or any other elastic medium. See:
Compression, Rarefaction.
Soundstage
A general term embracing the perceived dimensions of direction and space pertaining to
musicians performing in an acoustic space. Usually used in discussions of stereo and
multichannel reproduction of recordings. Dimensions like width, depth, 'air' or space, and
the directional clarity and size of individual performers are matters of interest. See:
Ambience, Localization.
Spaciousness
An important property of concert halls, in which reflections from side walls (lateral
reflections) generate a desirable sensation of space. One of the reasons for multichannel
reproduction in homes and in cars, where the objective may be to recreate these
impressions.
Spatial
An attribute of an audio system's playback that creates a sense of space within or different
from the listening space
Speaker
see Loudspeaker
Speaker Level
The signal that comes out of a power amplifer, intended to drive a loudspeaker. Sometimes
speaker level signals are used to provide input to an amplified subwoofer from a receiver
having no line level subwoofer output. See: Line Level
Spectrum
The distribution of energy in a signal or sound, displayed as amplitude as a function of
frequency. Strictly, it is the result of a spectrum analysis on a signal, but it is loosely used
to describe certain applications of frequency response data. Periodic signals tend to have
spectra in which the energy is concentrated in one or more lines (a line spectrum).
Discontinuous or random signals may have continuous spectra. See: Frequency Response,
Spectrum, Spectrum Analyzer.
Spectrum Analyzer
A measuring instrument capable of displaying the distribution of energy in a signal or
sound. With contemporary techniques, frequency resolution can be almost whatever one
wants, from 1 Hz (or many other fixed-bandwith options) to a variety of percentagebandwidth options, such as fractional-octave displays. Most employ FFT methods, and some
can display results in 'real time', others not. See: FFT, Real-Time Analyzer.
Speed of Sound
In air, at 72 degrees F (22 degrees C), it is 1131 feet/second, or 345 meters/second. All
audio frequencies travel at the same speed. In denser, stiffer materials, sound travels at
higher speeds.
Spider
The part of the speaker attached to the junction of the diaphragm and voice coil that keeps
the voice coil aligned in the gap and helps to suspend the cone.
SPL
See Sound Pressure Level.
SSP™ Networks
Straight-Line Signal Path™ Networks
Standing Waves
Associated with room resonances, at low frequencies, these can result in large variations in
sound level at different frequencies at different locations in the room. The bass sounds
different as one moves around a room. See Resonance.
Starfish Tweeter
Mounting System
A tweeter mounting system included in car audio component speaker systems that includes
a mounting cup with several break-off mounting tabs. The system facilitates mounting in
factory tweeter locations.
STC
See Sound Transmission Class
Stereophonic
Literally, solid sound. In theory, stereophonic systems could have any number of channels,
but now the term is used to describe systems having two-channels. Stereo is designed for a
single listener located equidistant from the left and right loudspeakers. Moving to the side,
away from this location, causes the stereo image (soundstage) to collapse towards the
nearer loudspeaker. See: Phantom Center.
Straight-Line Signal
Path™
SSP (STRAIGHT-LINE SIGNAL PATH™) NETWORKS • This crossover design provides the
shortest route from the input terminals to the transducers. • Minimizes signal loss and
noise. Benefit: Ensures that a strong, precise signal arrives at each driver.
Subwoofer
A loudspeaker system especially designed to reproduce only bass frequencies. Generally
used only below 80-100 Hz.
Superposition
An important property of sound fields in which, what happens at a point in space is the
vector sum of all separate events. It means that one can analyze individual components,
e.g. direct and reflected sounds, and then mathematically add them together to determine
the final result, exactly as happens in the physical world.
Supraaural
Headphones
Headphones that sit directly on the ears. See: circumaural headphones, in-ear headphones.
Surround
In loudspeaker drivers: the flexible support surrounding a loudspeaker diaphragm in the
rigid frame, usually a half roll or corrugated shape. In audio systems: describing any of
several ways to provide multichannel sound that surrounds the listeners with sound.
Surround Processor
An audio component that selects signal sources, provides volume and tone compensation
functions, and decodes and/or provides setup and calibration functions for several
multichannel surround-sound modes.
Surround Sound
A generic term that describes any of several systems capable of delivering multichannel
audio that includes channels placed to the sides and/or rear of the listener.
Surround Sound 5.1
A multichannel sound system, with five main channels and a low-frequency effects (LFE)
subwoofer channel. Surround sound was developed from cinema technology. All six
channels are discrete, meaning they are stored and played back separately.
Surround Speaker
A speaker typically matched in size to the midrange speakers used in a system. Its primary
function is to “carry” sound throughout the listening area so that the listener feels
enveloped in sound. The size and material of a surround speaker should closely match the
midrange speakers in the system; this ensures that the “moving” sound doesn’t waver, or
change timbre, thereby guaranteeing a rich, full-range surround-sound experience.
Suspension
In a loudspeaker: the compliant surround and spider that support the diaphragm while
allowing it to move freely in and out. See Spider, Surround
SVGA
Super Video Graphics Array. A computer display with 800 x 600 pixel image resolution.
See: Pixel, VGA, XGA, SXGA.
S-VHS
A VHS format video cassette recorder that can record and play back s-video signals. See:
S-Video
S-Video
A video signal in which the chrominance (C) and luminance (Y) signals are kept separate.
They are stored separately on S-VHS and DVD media, and using this output means that the
two components do not need to be combined by the player and then separated by a comb
filter in the video display. If the source, e.g. laserdisc, stores a composite signal (Y & C
combined) then the decision to use the s-video connection is based on whether the comb
filter in the player is better than the comb filter in the TV. See: Chrominance, Luminance,
Composite, Comb Filter.
SXGA
Super Extended Graphics Array. A computer display with 1280 x 1024 pixel image
resolution. See: Pixel, VGA, XGA, SXGA.
Symmetrical Field
Geometry
A speaker motor assembly design intended to maintain uniform magnetic force throughout
the travel of the voice coil. The objective is to reduce distortion at large diaphragm
excursions.
Tangential Modes
See Room Resonances.
TDS
See: Time Delay Spectrometry.
Television Systems
Three incompatible systems exist in the world. NTSC has 525 lines, presented at 60
fields/second (about 480 picture lines), while both PAL and SECAM have 625 lines
presented at 50 fields/second (about 576 picture lines). All of these are used for broadcast,
cable and satellite transmission, as well as for DVD-video. See: NTSC, PAL, SECAM, Field,
Raster, Scan Lines.
Temporal Masking
Masking that occurs when a loud transient sound prevents us from hearing sounds that
occur just earlier (backward masking) and just later (forward masking) in time. See
Masking, Threshold Shift.
Terrestrial
Jargon for radio and television signals that propagate normally from a transmitter on the
ground (terrestrial) to a conventional antenna. Also called over the air (OTA).
Three-Way
Loudspeaker
See: Loudspeaker System.
Threshold of Pain
When sound levels are sufficiently high as to cause discomfort, or pain, in the ears of
listeners. Very approximately, 110+ dB, as it varies with individual listeners and with the
kind of sound.
Threshold Shift
A change, normally an elevation, in the hearing threshold. It can be permanent, as in
hearing loss, or temporary, as occurs when the audibility of some sounds is reduced by the
presence of others, in masking. See Masking, Hearing Loss.
Throat
In a horn loudspeaker, this is the smallest part, where the sound leaves the driver begins
its travel down the expanding flare of the horn itself. See: Horn, Compression Driver, Phase
Plug.
THX
THX was created by George Lucas to apply performance standards to cinema sound
systems ensuring the playback quality of film sound tracks. Named after his first feature
film, THX 1138. Expanded into home audio, and other areas. THX is now a private licensing
company issuing certifications, paid for by manufacturers, that products meet certain
performance standards, and incorporate certain functions, some of which are proprietary.
There are levels of certification, from very basic to high. It is a reassurance to customers,
but it needs to be noted that non-THX-certified products can also offer performance that is
as good or better.
Timbre
The essential distinctive perceived quality of a sound, separate from loudness and pitch.
That which allows us to recognize whether the same note has been played by a piano or a
guitar, a clarinet or a trumpet, or sung by Pavarotti or Sinatra.
Time-Delay
A measurement system that allows one to examine events in time as well as frequency, and
Spectrometry
to isolate a direct sound from later reflected sounds in a normal room, thus approximating
anechoic measurements. There are trade-off's though. As the measurement time window is
shortened, e.g. to eliminate later reflections, the frequency resolution of the measurement
is reduced. An anechoic measurement space is still desirable. See: Anechoic, Direct Sound.
Tinnitus
A ringing in the ears that accompanies temporary hearing loss after exposure to loud
sounds, or that becomes permanent for some individuals unfortunate enough to incur
permanent hearing loss.
Tone Burst
A time sample of a single frequency (pure tone) which contains some number of complete
cycles. The burst may be turned on gradually or abruptly, all of which affects the spectrum
of the signal and how it sounds. Once a pure tone is interrupted, it no longer has a single
frequency spectrum.
Tone Control
A simple filter that can boost or cut portions of the Frequency
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tonal balance of reproduced sound. See Bass Control, Treble Control, Equalizer.
Transducer
A device that converts energy from one form to another. For example, a microphone
converts acoustical energy into electrical energy, while a loudspeaker does the reverse.
Transfer Function
An input-to-output measurement of performance that includes both amplitude and phase as
functions of frequency. The Fourier transform of the transfer function is the impulse (time)
response of the system. See: Frequency Response, Phase Response, Fourier Transform,
FFT.
Transform
A computational procedure or set of rules by which a signal can be converted from one form
into another. See Fourier Transform
Transformer
An electronic component consisting of two or more coils of wire in close proximity, allowing
the transfer of energy from one to the other by means of inductive coupling. Many
applications, including electrical isolation, voltage transformation, impedance matching, etc.
Transient Response
A measure of the time domain response of a system, input-to-output, to a very brief
transient signal at its input. It is widely believed that the time-domain behavior of a device
is independent of the frequency response. However, the Fourier transform of this timedomain waveform is the frequency-domain transfer function. See: Impulse Response,
Transfer Function, Fourier Transform, FFT.
Transparency
The objective of every maker of windows, and conscientious loudspeaker manufacturers,
who attempt to provide a transparent - i.e. unmodified - rendering of an audio work of art.
Treble
High-frequency audio signals
Treble Control
A tone control allowing the user to boost or cut the high frequency portion of the audio
signal.
Tuned Port Bass
Enclosure
See Bass Reflex
Tuner
A device that tunes, or selects, radio or television stations from broadcast signals received
on a terrestrial antenna, by cable or satellite.
Tweeter
A loudspeaker driver optimized to reproduce high frequencies, typically above about 2kHz
to 4kHz.
Two-Way
Loudspeaker
See: Loudspeaker System.
Ultrasound
Sound waves with frequencies higher than those that are normally audible by humans.
Typically, sounds above about 20 kHz.
Unbalanced
Connection
An interconnection between audio components in which there are two conductors, signal
and ground. The 'unbalanced' aspect of this connection is that, since ground is the
reference, each of the two signal leads has a very different (unbalanced) impedance
relationship to to the reference. This is of no consequence unless there are noises or hum
potentials in the grounding of the interconnected components. Commonly employs a coaxial
cable, in which the outer cylindrical shield serves as the ground connection. If hum
problems are found, it is possible to selectively break the ground connection, and thus
break the ground loop. See: Balanced Connection, Coaxial, Ground, Ground Loop.
Unipivot Tweeter
A tweeter, mounted at an angle atop a midrange speaker's polepiece that can be rotated so
that it points at the listening position.
Upmixing
The process of converting a program created in one format so that it can be played in
another that uses more channels. This is often done in recording studios under the guidance
of a sound engineer and/or musicians. If it is done by a fixed algorithm or process, it is
sometimes called "blind" upmixing.
UXGA
Ultra Extended Graphics Array. A computer display with up to 1600 x 1200 pixel image
resolution. See: Pixel, VGA, XGA, SVGA, SXGA
VCR
Video Cassette Recorder
Vent
See Port.
VGA
Video Graphics Array. The basic computer display standard, having 640 x 480 pixel image
resolution. See: Pixel, XGA, SVGA,SXGA.
VHS
VHS = Video Home System. An analog video cassette recording (VCR) system developed by
Matsushita (Panasonic, JVC) in 1976.
VHS-C
Developed for video cameras, it is a physically small tape cassette, in the VHS format,
which can record up to about 30 minutes. With an adapter it can be played on any VHS
machine. See: VHS
VHS-C
Developed for video cameras, it is a physically small tape cassette, in the VHS format,
which can record up to about 30 minutes. With an adapter it can be played on any VHS
machine. See: VHS
Video Shielded
See Shielded Loudspeaker
VMAx®
VMAx® creates a three-dimensional sound field using only conventional left and right front
speakers. The result is sound that fills a room and puts you at front row, center.
Voice Coil
A coil of wire attached to the rear of the speaker diaphragm. When a current is passed
through the voice coil there is an inductive interaction with the stationary magnetic field
generated by the magnet, causing the diaphragm to move. This is the basis of the
loudspeaker motor. See Magnet, Motor.
Volt
See Voltage
Voltage
Electrical potential difference measured in volts.
Volume
Audio: loosely, the loudness of sound, and the control that allows us to vary it.
Loudspeakers: The cubic measure of space in a speaker enclosure.
VOM
Volt, Ohm, Milliammeter. An analog meter that measures voltage, resistance and current.
Watt
The basic unit for electrical or acoustical power. See: Power
Wattage
Amount of electrical power expressed in watts. See: Power.
Wave
See Sound Wave
Waveform
The amplitude of a sound or electrical wave displayed as a function of time. The 'shape' of a
wave
Wavelength
The distance traveled by a single-frequency sound as it propagates. In air, it can be
calculated by dividing the speed of sound (1130 ft/s, 345 m/s) by the frequency.
W-dome tweeter
A tweeter that includes a voice coil that is substantially smaller in diameter than the
tweeter's diaphragm. W-dome tweeters often look like little cones with the voice coil former
attached to the inner diameter.
White Noise
Random Noise having a continuous spectrum and constant energy per cycle. Compared to
pink noise, white noise sound bright. See: Pink Noise
Widescreen
A video display that is wider than the standard 4:3 aspect ratio although, in the context of
DVD and HDTV, widescreen refers specifically to a 16:9 aspect ratio. See: Aspect Ratio.
Woofer
A loudspeaker driver designed to reproduce bass frequencies.
XGA
Extended Graphics Array. A computer display with either 640 x 480 or 1024 x 768 pixel
image resolution. See: Pixel, VGA, SVGA, SXGA.
XLR
A professional grade three-pin connector conveying balanced line level audio signals
between components. See: Balanced Connection, Unbalanced Connection.
Y-adapter
A signal connecting cable designed to provide signal to two devices rather than one. It looks
like the letter "Y", hence its name
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