Sound and Music Science
Sound and Music Science
Electronic Reinforcement of Sound
Sound Reinforcement
z Reinforcement
can be done of either the
direct, early or reverberant sound
z Reinforcement of the reverberant sound
can increase the reverberation time and
create a feeling of liveness
z Reinforcement of the direct sound may
also increase the reverberant level
Sound Sources in a room
zA
source tends to be more directional at
high frequencies where the wavelength
of the sound is comparable to the
dimensions of the source
z The power of a source is expressed in
watts, and given by the following
equation where W0 = 10-12 W
z
LW = 10log W / W0
Sound Sources in a room continued
z The
directivity factor Q is defined as the
ratio of the sound intensity at a distance
r in front of a source to the sound
intensity averaged over all directions
z Sound that radiates in all directions has
a directivity of Q = 1
Sound Sources in a room continued
z Sound
pressure level depends on the
power and the directivity of the source,
its distance and the strength of the
reflected sound
Sound Fields
zA
sound field refers to the distribution of
sound in space
z The near field is where the distance is
small compared to the dimensions of the
source and the sound radiates strongly
z Further away is the free field where
sound level decreases by 6 dB each time
the distance r is doubled
Sound Fields continued
z Reverberant
field is where the
contribution from the reflected sound
takes on increasing importance and the
sound does not decrease with increasing
distance
Sound Fields continued
Intelligibility of Speech
z The
following measures the percentage
articulation loss for consonants, where k
is the constant for each listener that
indicates listening ability (1.5% for the
best listener to 12.5% for the poorest)
Intelligibility of Speech continued
z The
distance D beyond which the
articulation loss remains a constant
value is given by
z The
percentage articulation loss is then
given by:
Intelligibility of Speech continued
z If
the main problem is background noise,
the intelligibility can be improved with
electronic reinforcement
z If the problem is with a poor ratio of
direct sound to reverberant sound,
speakers with high directivity factors
must be used
Power Considerations
z The
previous equation can determine the
power capacity of a sound system
z The desirable levels of direct and
reverberant sound must also be
determined
Chart for determining Sound
Pressure Level in a Room
Power Considerations continued
z For
speech, a sound pressure level of
65- 70 dB will be adequate, provided that
is it at least 25 dB above the noise level
in the room
z Music spans a wider dynamic range, and
so peak levels of 90 -100 dB or more
may be required
z Values above that are attainable but not
without distortion
Loudspeaker Placement
z The
goal is to give the best coverage
over the listening area
z The designs are either a large single
source or a distribution of small sources
throughout the room
z A single source generally consists of a
cluster of loudspeakers with directivity
factors Q selected to give the best
coverage of the audience
Loudspeaker Placement continued
Loudspeaker Placement continued
z In
this arrangement, vertical
displacement of the source is not
particularly distracting because of our
inability to localize sound in the vertical
plane
z Speakers will be aimed towards the
listeners at the back of the hall
Loudspeaker Placement continued
z Another
arrangement, which works for
long rooms with low ceilings, is an
overhead distributed-speaker system
z Each speaker covers 60 to 90 degrees
z If the room is long, electronic time delays
would be necessary to prevent an echo
Loudspeaker Placement continued
Loudspeaker Placement continued
z Loudspeakers
should not be placed
along side walls as they will cross fire
and cause the listener to hear sound
from several speakers at the same time
z One should also avoid putting one
speaker on each side of the stage area
or front wall, as listeners will hear the
sound from one the speakers before they
hear the direct sound
Loudspeaker Placement continued
Loudspeaker Placement continued
z If
this arrangement is necessary, sound
to both loudspeakers should be delayed
so that the direct sound is heard first
Equalization
z Equalization
serves two important
functions:
Results in a more natural sound by
compensating for predominant room
resonances
z Permits the sound system to be operated at
a higher level by suppressing acoustic
feedback oscillation
z
Equalization continued
z It
is desirable to use 8 to 10 filters, each
an octave wide, to control the sound
z 1/3 octave filters are available but they
are more expensive
z Looking at the frequency response of the
sound system before equalization would
shows peaks corresponding to the room
resonances
Equalization continued
z As
the amplifier gain is increased,
acoustic feedback causes the system to
oscillate at the highest of these peaks
z Equalization may look like a flat line
z Tunable band-reject filters with narrow
bands (1/6 or 1/10 of an octave) are
used to reduce prominent resonances
Equalization continued
Equalization continued
zA
frequency shifter, which shifts the
frequencies in the microphone signal by
an inaudible amount, typically from 3-5
Hz, can be used to suppress acoustic
feedback
z This shift is enough to avoid feedback
due to room resonances that are narrow
but prominent
Time Delay
zA
sound system with loudspeakers at
several positions in a large auditorium
generally requires some time delay
z This is necessary to preserve the
apparent direction of the sound source
by ensuring the direct sound is properly
reinforced
Enhancement of Reverberation
z Many
techniques are employed to be able
to adjust the reverberation time
z One of these places a loudspeaker and a
microphone in a reverberation chamber
(a small room 300 to 3000 m3) with highly
reflective walls, ceiling and floor
z By amplifying the reverberant sound in
this chamber and feeding it back to the
main auditorium, the reverberation time is
increased
Enhancement of Reverberation
continued
z Another
method is to use a spring or a
thin plate with a transducer at each end
z
The sound will travel back and forth many
times on the spring before dying out, which
approximates the decay of reverberant
sound
z DSP
methods are also possible
Enhancement of Reverberation
continued
z Assisted
resonance systems are used to
boost lower frequencies
z It uses independent channels, each with
its own tuned resonator spaced over a
desired frequency range
z A microphone can be placed inside a
tuned Helmholtz resonator at lower
frequencies
z Tuned pipes can be used for frequencies
above 300 Hz
Enhancement of Reverberation
continued
Microphone Placement
z Varying
the microphone-to-source
distance affects the timbre of the sound,
especially if it has a cardioid pickup
pattern
z If a microphone is placed a small
distance from a large surface,
cancellation of certain frequencies can
occur due to interference between the
direct and reflected sound waves
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