How Do Microphones Work?
How Do Microphones Work?
The Basics
Microphones are a type of transducer - a device which converts energy from one form to another.
Microphones convert acoustical energy (sound waves) into electrical energy (the audio signal).
Different types of microphone have different ways of converting energy but they all share one thing
in common: The diaphragm. This is a thin piece of material (such as paper, plastic or aluminium)
which vibrates when it is struck by sound waves. In a typical hand-held mic like the one below, the
diaphragm is located in the head of the microphone.
Location of Microphone Diaphragm
When the diaphragm vibrates, it causes other components in the microphone to vibrate. These
vibrations are converted into an electrical current which becomes the audio signal.
Note: At the other end of the audio chain, the loudspeaker is also a transducer - it converts the
electrical energy back into acoustical energy.
Types of Microphone
There are a number of different types of microphone in common use. The differences can be
divided into two areas:
(1) The type of conversion technology they use
This refers to the technical method the mic uses to convert sound into electricity. The most
common technologies are dynamic, condenser, ribbon and crystal. Each has advantages and
disadvantages, and each is generally more suited to certain types of application. The following
pages will provide details.
(2) The type of application they are designed for
Some mics are designed for general use and can be used effectively in many different situations.
Others are very specialised and are only really useful for their intended purpose. Characteristics to
look for include directional properties, frequency response and impedance (more on these later).
Mic Level & Line Level
The electrical current generated by a microphone is very small. Referred to as mic level, this signal
is typically measured in millivolts. Before it can be used for anything serious the signal needs to be
amplified, usually to line level (typically 0.5 -2V). Being a stronger and more robust signal, line level
is the standard signal strength used by audio processing equipment and common domestic
equipment such as CD players, tape machines, VCRs, etc.
This amplification is achieved in one or more of the following ways:
•
Some microphones have tiny built-in amplifiers which boost the signal to a high mic level or
line level.
•
The mic can be fed through a small boosting amplifier, often called a line amp.
•
Sound mixers have small amplifiers in each channel. Attenuators can accommodate mics of
varying levels and adjust them all to an even line level.
•
The audio signal is fed to a power amplifier - a specialised amp which boosts the signal
enough to be fed to loudspeakers.
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Dynamic Microphones
Dynamic microphones are versatile and ideal for general-purpose use. They use a simple design
with few moving parts. They are relatively sturdy and resilient to rough handling. They are also
better suited to handling high volume levels, such as from certain musical instruments or amplifiers.
They have no internal amplifier and do not require batteries or external power.
How Dynamic Microphones Work
As you may recall from your school science, when a magnet is moved near a coil of wire an
electrical current is generated in the wire. Using this electromagnet principle, the dynamic
microphone uses a wire coil and magnet to create the audio signal.
The diaphragm is attached to the coil. When the diaphragm vibrates in response to incoming sound
waves, the coil moves backwards and forwards past the magnet. This creates a current in the coil
which is channeled from the microphone along wires. A common configuration is shown below.
Earlier we mentioned that loudspeakers perform the opposite function of microphones by
converting electrical energy into sound waves. This is demonstrated perfectly in the dynamic
microphone which is basically a loudspeaker in reverse. When you see a cross-section of a
speaker you'll see the similarity with the diagram above. If fact, some intercom systems use the
speaker as a microphone. You can also demonstrate this effect by plugging a microphone into the
headphone output of your stereo, although we don't recommend it!
Technical Notes:
Dynamics do not usually have the same flat frequency response as condensers. Instead they tend
to have tailored frequency responses for particular applications.
Neodymium magnets are more powerful than conventional magnets, meaning that neodymium
microphones can be made smaller, with more linear frequency response and higher output level.
Condenser Microphones
Condenser means capacitor, an electronic component which stores energy in the form of an
electrostatic field. The term condenser is actually obsolete but has stuck as the name for this type
of microphone, which uses a capacitor to convert acoustical energy into electrical energy.
Condenser microphones require power from a n external source. The resulting audio
signal is stronger signal than that from a dynamic. Condensers also tend to be more sensitive and
responsive than dynamics, making them well-suited to capturing subtle nuances in a sound. They
are not ideal for high-volume work, as their sensitivity makes them prone to distort.
How Condenser Microphones Work
A capacitor has two plates with a voltage between them. In the condenser mic, one of these plates
is made of very light material and acts as the diaphragm. The diaphragm vibrates when struck by
sound waves, changing the distance between the two plates and therefore changing the
capacitance. Specifically, when the plates are closer together, capacitance increases and a charge
current occurs. When the plates are further apart, capacitance decreases and a discharge current
occurs.
A voltage is required across the capacitor for this to work.
This voltage is supplied by external phantom power.
Cross-Section of a Typical Condenser Microphone
_
+
48v Phantom Power
Or 12v “T” Power
The Electret Condenser Microphone
The electret condenser mic uses a special type of capacitor which has a permanent voltage built in
during manufacture. This is somewhat like a permanent magnet, in that it doesn't require any
external power for operation. However good electret condenders mics usually include a preamplifier which does still require power @ 1.5v. This is usually supplied by an internal battery
or reduced Phantom power.
Battery
V-
Other than this difference, you can think of an electret condenser microphone as being the same
as a normal condenser.
Directional Properties
Every microphone has a property known as
directionality. This describes the microphone's
sensitivity to sound from various directions. Some microphones pick up sound equally from all
directions, others pick up sound only from one direction or a particular combination of directions.
The types of directionality are divided into three main categories:
1. Omnidirectional
Picks up sound evenly from all directions (omni means "all" or "every").
2. Unidirectional
Picks up sound predominantly from one direction. This includes cardioid and hypercardioid
microphones (see below).
3. Bidirectional
Picks up sound from two opposite directions.
To help understand a the directional properties of a particular microphone, user manuals and
promotional material often include a graphical representation of the microphone's directionality.
This graph is called a polar pattern. Some typical examples are shown below.
Omnidirectional
Captures sound equally from all directions.
Uses: Capturing ambient noise; Situations where sound is coming from
many directions; Situations where the mic position must remain fixed while
the sound source is moving.
Notes:
•
Although omnidirectional mics are very useful in the right situation, picking up sound from
every direction is not usually what you need. Omni sound is very general and unfocused - if
you are trying to capture sound from a particular subject or area it is likely to be
overwhelmed by other noise.
Cardioid
Cardioid means "heart-shaped", which is the type of pick-up pattern these
mics use. Sound is picked up mostly from the front, but to a lesser extent
the sides as well.
Uses: Emphasising sound from the direction the mic is pointed whilst
leaving some latitude for mic movement and ambient noise.
Notes:
•
The cardioid is a very versatile microphone, ideal for general use. Handheld mics are usually
cardioid.
•
There are many variations of the cardioid pattern (such as the hypercardioid below).
Hypercardioid
This is exaggerated version of the cardioid pattern. It is very directional
and eliminates most sound from the sides and rear. Due to the long thin
design of hypercardioids, they are often referred to as shotgun
microphones.
Uses: Isolating the sound from a subject or direction when there is a lot of
ambient noise; Picking up sound from a subject at a distance.
Notes:
•
By removing all the ambient noise, unidirectional sound can sometimes be a little unnatural.
It may help to add a discreet audio bed from another mic (i.e. constant background noise at
a low level).
•
You need to be careful to keep the sound consistent. If the mic doesn't stay pointed at the
subject you will lose the audio.
•
Shotguns can have an area of increased sensitivity directly to the rear.
Bidirectional
Uses a figure-of-eight pattern and picks up sound equally from two
opposite directions.
Uses: As you can imagine, there aren't a lot of situations which require this
polar pattern. One possibility would be an interview with two people facing
each other (with the mic between them).
Variable Directionality
Some microphones allow you to vary the directional characteristics by selecting omni, cardioid or
shotgun patterns.
This feature is sometimes found on video camera microphones, with the idea that you can adjust
the directionality to suit the angle of zoom, e.g. have a shotgun mic for long zooms. Some models
can even automatically follow the lens zoom angle so the directionality changes from cardioid to
shotgun as you zoom in.
Although this seems like a good idea (and can sometimes be handy), variable zoom microphones
don't perform particularly well and they often make a noise while zooming. Using different mics will
usually produce better results.
Microphone Impedance
When dealing with microphones, one consideration which is often misunderstood or overlooked is
the microphone's impedance rating. Perhaps this is because impedance isn't a "critical" factor; that
is, microphones will still continue to operate whether or not the best impedance rating is used.
However, in order to ensure the best quality and most reliable audio, attention should be paid to
getting this factor right.
If you want the short answer, here it is: Low impedance is better than high impedance.
If you're interested in understanding more, read on....
What is Impedance?
Impedance is an electronics term which measures the amount of opposition a device has to an AC
current (such as an audio signal). Technically speaking, it is the combined effect of capacitance,
inductance, and resistance on a signal.
Impedance is measured in ohms, shown with the Greek Omega symbol
microphone with the specification 600Ω has an impedance of 600 ohms.
Ω or the letter Z. A
What is Microphone Impedance?
All microphones have a specification referring to their impedance. This spec may be written on the
mic itself (perhaps alongside the directional pattern), or you may need to consult the manual or
manufacturer's website.
You will often find that mics with a hard-wired cable and 1/4" jack are high impedance, and mics
with separate balanced audio cable and XLR connector are low impedance.
There are three general classifications for microphone impedance. Different manufacturers use
slightly different guidelines but the classifications are roughly:
1. Low Impedance (less than 600Ω)
2. Medium Impedance (600Ω 10,000Ω)
3. High Impedance (greater than 10,000Ω)
Note that some microphones have the ability to select from different impedance ratings.
Which Impedance to Choose?
High impedance microphones are usually quite cheap. Their main disadvantage is that they do not
perform well over long distance cables - after about 5 or 10 metres they begin producing poor
quality audio (in particular a loss of high frequencies). In any case these mics are not a good
choice for serious work. In fact, although not completely reliable, one of the clues to a microphone's
overall quality is the impedance rating.
Low impedance microphones are usually the preferred choice.
Matching Impedance with Other Equipment
Microphones aren't the only things with impedance. Other equipment, such as the input of a sound
mixer, also has an ohms rating. Again, you may need to consult the appropriate manual or website
to find these values. Be aware that what one system calls "low impedance" may not be the same
as your low impedance microphone - you really need to see the ohms value to know exactly what
you're dealing with.
A low impedance microphone should generally be connected to an input with the same or higher
impedance. If a microphone is connected to an input with lower impedance, there will be a loss of
signal strength.
In some cases you can use a line matching transformer, which will convert a signal to a different
impedance for matching to other components.
Microphone Frequency Response
Frequency response refers to the way a microphone responds to different frequencies. It is a
characteristic of all microphones that some frequencies are exaggerated and others are attenuated
(reduced). For example, a frequency response which favours high frequencies means that the
resulting audio output will sound more trebly than the original sound.
Frequency Response Charts
A microphone's frequency response pattern is shown using a chart like the one below and referred
to as a frequency response curve. The x axis shows frequency in Hertz, the y axis shows response
in decibels. A higher value means that frequency will be exaggerated, a lower value means the
frequency is attenuated. In this example, frequencies around 5 - kHz are boosted while frequencies
above 10kHz and below 100Hz are attenuated. This is a typical response curve for a vocal
microphone.
Which Response Curve is Best?
An ideal "flat" frequency response means that the microphone is equally sensitive to all
frequencies. In this case, no frequencies would be exaggerated or reduced (the chart above would
show a flat line), resulting in a more accurate representation of the original sound. We therefore
say that a flat frequency response produces the purest audio.
In the real world a perfectly flat response is not possible and even the best "flat response"
microphones have some deviation.
More importantly, it should be noted that a flat frequency response is not always the most desirable
option. In many cases a tailored frequency response is more useful. For example, a response
pattern designed to emphasise the frequencies in a human voice would be well suited to picking up
speech in an environment with lots of low-frequency background noise.
The main thing is to avoid response patterns which emphasise the wrong frequencies. For
example, a vocal mic is a poor choice for picking up the low frequencies of a bass drum.
Frequency Response Ranges
You will often see frequency response quoted as a range between two figures. This is a simple (or
perhaps "simplistic") way to see which frequencies a microphone is capable of capturing
effectively. For example, a microphone which is said to have a frequency response of 20 Hz to 20
kHz can reproduce all frequencies within this range. Frequencies outside this range will be
reproduced to a much lesser extent or not at all. This specification makes no mention of the
response curve, or how successfully the various frequencies will be reproduced. Like many
specifications, it should be taken as a guide only.
Condenser vs Dynamic
Condenser microphones generally have flatter frequency responses than dynamic. All other things
being equal, this would usually mean that a condenser is more desirable if accurate sound is a
prime consideration.
Microphone Directionality and Polar Patterns.© homerecording.about.com
* Additional notes by Steven Rogers C.A.S.
The three basic flavors of directionality are:
• Omni-directional
• Bi-directional
• Uni-directional
Below you'll see "maps" of the pickup patterns of each of these microphone types. Picture the microphone's diaphragm
facing forward at the center of the circle. Polar pattern diagrams are drawn from this perspective and represent the directionality of the mike.
Omni-Directional Mikes.
Omni mikes are sensitive to sounds coming from all over. The polar pattern is depicted as a full circle, but in reality the
omni-directional mike's pattern is a three dimensional sphere. Imagine a beach ball on the end of a stick. It "hears" 360
degrees. * (Pattern is used with hand held microphones, and Lavaliere microphones. Never on a boom for dialog recording)
Bi-Directional, or "Figure eight", Mikes.
Bi-directional mikes are sensitive to sounds from the front and back, while being insensitive to sounds coming from the
sides. The figure eight polar pattern shows that the areas of least sensitivity (the "null" points) are at 90 and 270 degrees,
and the areas of highest sensitivity at 0 and 180 degrees. *(Used in radio and music recording. Rarely used for film dialog
recording unless used as part of a MS Stereo microphone setup combined with a Hyper -Cardioid microphone.)
Uni-Directional Mikes.
Cardioid Mikes
Uni-directional mikes are most sensitive at the front, and least sensitive at the back. The pattern is most useful in multimike setups since this type of mike can be focused on one instrument, or one part of an instrument, while excluding or
rejecting other nearby sounds. The most common uni-directional pattern is "cardioid", or heart-shaped, with the "sweet
spot" at 0 degrees, and the "null point" at 180 degrees. *(Used occasionally on a boom for dialog recording)
Hyper-Cardioid and Super-Cardioid Mikes.
Also considered uni-directional mikes, these two mike styles have two nulls. They can be very effective for Dialog recording, since they can be positioned in a way that focuses on a particular source while rejecting the sounds of the surroundings. Hyper-cardioids are standard issue on video cameras, since they focus tightly on whatever is directly in front
of them.*(Most common pattern used on a boom for dialog recording)
MICROPHONE POLAR PATTERN DETAIL
The Myth of Microphone Reach © Shure Microphones
Shure is often asked "How far away will my microphone pick up?'" or "Which microphone has the best reach?" Both
questions are based on a misunderstanding of how microphones work. This bulletin attempts to debunk the myth of microphone reach.
Myth: A microphone reaches out to capture sound
Fact: A microphone only responds to sound waves that travel to its location. A microphone measures local rapid variations in air pressure and provides an electrical output that mirrors these variations. These rapid air pressure variations are
sensed as sound if they are within the hearing frequency range of 20 - 20,000 Hertz. The microphone is stimulated only by
the sound waves that travel to its location. It cannot "reach" out and capture the sound wave from a distance.
Myth: A directional microphone enhances sound waves which approach from the front
Fact: A directional microphone merely rejects sounds from directions other than the front. An omnidirectional microphone "hears" equally well in all directions. It does not reject any sound as it is insensitive to the direction of the passing
sound wave. A unidirectional microphone "hears" well in certain directions and not so well in other directions. As an example, a cardioid microphone does not reject sound waves which approach from the front; is slightly "deaf" to sounds approaching from the left or right; and is very "deaf" to sounds approaching from the rear. So a cardioid microphone appears
to enhance sound waves from the front, by being far less sensitive to sound waves from the left, right, and rear.
Myth: A microphone has a reach specification that can be measured in feet or meters
Fact: A microphone's effectiveness at different distances is primarily dependent on the background noise level. Let's use
an example. For a nature film, you want to record the call of a wolf in the wilderness of Canada.
The recording conditions are superb. There is no wind; the closest town is 100 miles away; your equipment is a state-ofthe-art DAT machine (Digital Audio Tape) with a quiet mic preamplifier. Your microphone is an omnidirectional dynamic. You spot a wolf about 1/2 mile away and start the recorder. The wolf howls for two minutes and you obtain a fantastic
recording. [If you believe in microphone reach, your microphone has a reach of at least 1/2 mile.] The next week you are
back in New York City. A wolf escapes from the Bronx Zoo, takes the subway to Manhattan, and is now howling on 6th
Avenue at 59th Street, which happens to be 1/2 mile from your studio. You immediately activate your DAT recorder and
hang the same omnidirectional microphone outside your window to record the wolf, but much to your surprise all you
hear is traffic and wind noise when you play back the tape. The wolf cannot be heard even though you saw it howling
through your binoculars. Did the reach of the microphone somehow change? No, only the ambient noise conditions
changed.
Myth: A shotgun microphone is like a zoom lens on a camera
Fact: Using a shotgun mic is like taking a photo with the lens aimed down a cardboard tube. The image being photographed is not brought any closer, but the unwanted images to the sides are reduced or eliminated. A shotgun mic seems
to bring the desired sound source closer because the unwanted sounds from the side and rear are attenuated. It is tempting
to compare light with sound. Light waves and sound waves do have similarities, but many more differences. The most important difference is in wavelength. The wavelength of light is measured in millionths of an inch. The wavelength of
sound is measured in inches and feet. For a zoom lens to be effective, its diameter must be hundreds of thousands times
larger than the wavelength. A typical camera zoom lens is approximately 3 inches in diameter. But a "zoom lens" for
sound waves (using the same ratio) would have to be tens, even hundreds, of miles in diameter. There is an acoustical device, called a parabolic reflector, that can be combined with a microphone for distant pickup. However, it too will be limited by unwanted ambient noise, and to be effective at lower frequencies, the parabolic reflector must be six to eight feet
in diameter. It is the extremely long wavelengths that make sound so difficult to control, manipulate, and focus.
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