A
microphone, colloquially called a
mic or
mike (both pronounced
/ˈmaɪk/), is an acoustic-to-electric
transducer or
sensor that
converts
sound into an
electrical signal. In 1876,
Emile Berliner invented the first microphone
used as a telephone voice transmitter. Microphones are used in many
applications such as
telephones,
tape recorders,
hearing
aids,
motion picture production,
live and recorded
audio
engineering, in
radio and
television broadcasting and in computers for
recording voice,
VoIP, and for
non-acoustic purposes such as ultrasonic checking.
The most common design today uses a thin membrane which vibrates in
response to sound pressure. This movement is subsequently
translated into an electrical signal. Most microphones in use today
for audio use electromagnetic induction (dynamic microphone),
capacitance change (condenser microphone, pictured right),
piezoelectric generation, or light modulation to produce the signal
from mechanical vibration.
Varieties
The sensitive transducer element of a microphone is called its
element or
capsule. A complete microphone also
includes a housing, some means of bringing the signal from the
element to other equipment, and often an electronic circuit to
adapt the output of the capsule to the equipment being driven.
Microphones are referred to by their
transducer principle, such as condenser, dynamic,
etc., and by their directional characteristics. Sometimes other
characteristics such as diaphragm size, intended use or orientation
of the principal sound input to the principal axis (end- or
side-address) of the microphone are used to describe the
microphone.
Condenser, capacitor or electrostatic microphone

Inside the Oktava 319 condenser
microphone
In a condenser microphone, also known as a capacitor or
electrostatic microphone, the
diaphragm acts as one plate of a
capacitor, and the vibrations produce
changes in the distance between the plates. There are two methods
of extracting an audio output from the transducer thus formed:
DC-biased and radio frequency (RF) or high frequency (HF) condenser
microphones. With a DC-biased microphone, the plates are
bias with a fixed charge (
Q). The
voltage maintained across the capacitor
plates changes with the vibrations in the air, according to the
capacitance equation (C = Q / V), where Q = charge in
coulombs, C = capacitance in
farads and V = potential difference in
volts. The capacitance of the plates is inversely
proportional to the distance between them for a parallel-plate
capacitor. (See
capacitance for
details.) The assembly of fixed and movable plates is called an
"element" or "capsule."
A nearly constant charge is maintained on the capacitor. As the
capacitance changes, the charge across the capacitor does change
very slightly, but at audible frequencies it is sensibly constant.
The capacitance of the capsule (around 5–100
pF) and the value of the bias resistor (100 megohms to
tens of gigohms) form a filter which is highpass for the audio
signal, and lowpass for the bias voltage. Note that the time
constant of an
RC circuit equals the
product of the resistance and capacitance.
Within the time-frame of the capacitance change (as much as
50 ms at 20 Hz audio signal), the charge is practically
constant and the voltage across the capacitor changes
instantaneously to reflect the change in capacitance. The voltage
across the capacitor varies above and below the bias voltage. The
voltage difference between the bias and the capacitor is seen
across the series resistor. The voltage across the resistor is
amplified for performance or recording.
RF condenser microphones use a comparatively low RF voltage,
generated by a low-noise oscillator. The oscillator may either be
amplitude modulated by the capacitance changes produced by the
sound waves moving the capsule diaphragm, or the capsule may be
part of a
resonant circuit that
modulates the frequency of the oscillator signal. Demodulation
yields a low-noise audio frequency signal with a very low source
impedance. The absence of a high bias voltage permits the use of a
diaphragm with looser tension, which may be used to achieve wider
frequency response due to higher compliance. The RF biasing process
results in a lower electrical impedance capsule, a useful byproduct
of which is that RF condenser microphones can be operated in damp
weather conditions which could create problems in DC-biased
microphones whose insulating surfaces have become contaminated. The
Sennheiser "MKH" series of microphones
use the RF biasing technique.
Condenser microphones span the range from telephone transmitters
through inexpensive karaoke microphones to high-fidelity recording
microphones. They generally produce a high-quality audio signal and
are now the popular choice in laboratory and studio recording
applications. The inherent suitability of this technology is due to
the very small mass that must be moved by the incident sound wave,
unlike other microphone types which require the sound wave to do
more work. They require a power source, provided either via
microphone outputs as
phantom power or
from a small battery. Power is necessary for establishing the
capacitor plate voltage, and is also needed to power the microphone
electronics (impedance conversion in the case of electret and
DC-polarized microphones, demodulation or detection in the case of
RF/HF microphones). Condenser microphones are also available with
two diaphragms, the signals from which can be electrically
connected such as to provide a range of polar patterns (see below),
such as cardioid, omnidirectional and figure-eight. It is also
possible to vary the pattern smoothly with some microphones, for
example the
Røde NT2000 or CAD M179.
Electret condenser microphone

First patent on foil electret
microphone by G.
Sessler et al. (pages 1 to 3)
An
electret microphone is a relatively new type of capacitor
microphone invented at Bell laboratories
in 1962 by Gerhard
Sessler and Jim
West.The externally-applied charge described above under
condenser microphones is replaced by a permanent charge in an
electret material. An
electret is a
ferroelectric material that has been
permanently
electrically charged or
polarized. The name comes from
electrostatic and
magn
et; a static charge is embedded in an electret by
alignment of the static charges in the material, much the way a
magnet is made by aligning the magnetic domains in a piece of
iron.
Due to their good performance and ease of manufacture, hence low
cost, the vast majority of microphones made today are electret
microphones; a semiconductor manufacturer estimates annual
production at over one billion units. Nearly all cell-phone,
computer, PDA and headset microphones are electret types. They are
used in many applications, from high-quality recording and
lavalier use to built-in microphones in
small
sound recording devices and
telephones. Though electret microphones were once considered low
quality, the best ones can now rival traditional condenser
microphones in every respect and can even offer the long-term
stability and ultra-flat response needed for a measurement
microphone. Unlike other capacitor microphones, they require no
polarizing voltage, but often contain an integrated
preamplifier which does require power (often
incorrectly called polarizing power or bias). This preamplifier is
frequently
phantom powered in sound
reinforcement and studio applications. Microphones designed for
Personal Computer (PC) use,
sometimes called multimedia microphones, use a stereo 3.5 mm
plug (though a mono source) with the ring receiving power via a
resistor from (normally) a 5 V supply in the computer;
unfortunately, a number of incompatible dynamic microphones are
fitted with 3.5 mm plugs too. While few electret microphones
rival the best DC-polarized units in terms of noise level, this is
not due to any inherent limitation of the electret. Rather, mass
production techniques needed to produce microphones cheaply don't
lend themselves to the precision needed to produce the highest
quality microphones, due to the tight tolerances required in
internal dimensions. These tolerances are the same for all
condenser microphones, whether the DC, RF or electret technology is
used.
Dynamic microphone
Dynamic microphones work via
electromagnetic induction. They
are robust, relatively inexpensive and resistant to moisture. This,
coupled with their potentially high gain before feedback makes them
ideal for on-stage use.
Moving-coil microphones use
the same dynamic principle as in a
loudspeaker, only reversed. A small movable
induction coil, positioned in the
magnetic field of a
permanent magnet, is attached to the
diaphragm. When sound enters
through the windscreen of the microphone, the sound wave moves the
diaphragm. When the diaphragm vibrates, the coil moves in the
magnetic field, producing a varying
current in the coil through
electromagnetic induction. A
single dynamic membrane will not respond linearly to all audio
frequencies. Some microphones for this reason utilize multiple
membranes for the different parts of the audio spectrum and then
combine the resulting signals. Combining the multiple signals
correctly is difficult and designs that do this are rare and tend
to be expensive. There are on the other hand several designs that
are more specifically aimed towards isolated parts of the audio
spectrum. The
AKG D 112, for example,
is designed for bass response rather than treble. In audio
engineering several kinds of microphones are often used at the same
time to get the best result.
Ribbon microphones use a thin,
usually corrugated metal ribbon suspended in a magnetic field. The
ribbon is electrically connected to the microphone's output, and
its vibration within the magnetic field generates the electrical
signal. Ribbon microphones are similar to moving coil microphones
in the sense that both produce sound by means of magnetic
induction. Basic ribbon microphones detect sound in a
bidirectional (also called
figure-eight) pattern because the ribbon, which is open to sound
both front and back, responds to the
pressure gradient rather than the
sound pressure. Though the symmetrical
front and rear pickup can be a nuisance in normal stereo recording,
the high side rejection can be used to advantage by positioning a
ribbon microphone horizontally, for example above cymbals, so that
the rear lobe picks up only sound from the cymbals. Crossed figure
8, or
Blumlein pair, stereo recording
is gaining in popularity, and the figure 8 response of a ribbon
microphone is ideal for that application.
Other directional patterns are produced by enclosing one side of
the ribbon in an acoustic trap or baffle, allowing sound to reach
only one side. The classic
RCA
Type 77-DX microphone has several externally-adjustable
positions of the internal baffle, allowing the selection of several
response patterns ranging from "Figure-8" to "Unidirectional". Such
older ribbon microphones, some of which still give very high
quality sound reproduction, were once valued for this reason, but a
good low-frequency response could only be obtained if the ribbon
was suspended very loosely, and this made them fragile. Modern
ribbon materials, including new nanomaterials have now been
introduced that eliminate those concerns, and even improve the
effective dynamic range of ribbon microphones at low frequencies.
Protective wind screens can reduce the danger of damaging a vintage
ribbon, and also reduce plosive artifacts in the recording.
Properly designed wind screens produce negligible treble
attenuation. In common with other classes of dynamic microphone,
ribbon microphones don't require
phantom
power; in fact, this voltage can damage some older ribbon
microphones. Some new modern ribbon microphone designs incorporate
a preamplifier and, therefore, do require phantom power, and
circuits of modern passive ribbon microphones,
i.e., those
without the aforementioned preamplifier, are specifically designed
to resist damage to the ribbon and transformer by phantom power.
Also there are new ribbon materials available that are immune to
wind blasts and phantom power.
Carbon microphone
A carbon microphone is a capsule containing carbon granules pressed
between two metal plates. A voltage is applied across the metal
plates, causing a small current to flow through the carbon. One of
the plates, the diaphragm, vibrates in sympathy with incident sound
waves, applying a varying pressure to the carbon. The changing
pressure deforms the granules, causing the contact area between
each pair of adjacent granules to change, and this causes the
electrical resistance of the mass of granules to change. The
changes in resistance cause a corresponding change in the current
flowing through the microphone, producing the electrical signal.
Carbon microphones were once commonly used in telephones; they have
extremely low-quality sound reproduction and a very limited
frequency response range, but are very robust devices.
Unlike other microphone types, the carbon microphone can also be
used as a type of amplifier, using a small amount of sound energy
to produce a larger amount of electrical energy. Carbon microphones
found use as early
telephone repeaters,
making long distance phone calls possible in the era before vacuum
tubes. These repeaters worked by mechanically coupling a magnetic
telephone receiver to a carbon microphone: the faint signal from
the receiver was transferred to the microphone, with a resulting
stronger electrical signal to send down the line. (One illustration
of this amplifier effect was the oscillation caused by feedback,
resulting in an audible squeal from the old "candlestick" telephone
if its earphone was placed near the carbon microphone.
Piezoelectric microphone
A crystal microphone uses the phenomenon of
piezoelectricity — the ability of some
materials to produce a voltage when subjected to pressure — to
convert vibrations into an electrical signal. An example of this is
Rochelle salt (potassium sodium
tartrate), which is a piezoelectric crystal that works as a
transducer, both as a microphone and as a slimline loudspeaker
component. Crystal microphones were once commonly supplied with
vacuum tube (valve) equipment, such as
domestic tape recorders. Their high output impedance matched the
high input impedance (typically about 10
megohms) of the vacuum tube input stage well. They were
difficult to match to early
transistor
equipment, and were quickly supplanted by dynamic microphones for a
time, and later small electret condenser devices. The high
impedance of the crystal microphone made it very susceptible to
handling noise, both from the microphone itself and from the
connecting cable.
Piezoelectric transducers are often used as
contact microphones to amplify sound from
acoustic musical instruments, to sense drum hits, for triggering
electronic samples, and to record sound in challenging
environments, such as underwater under high pressure.
Saddle-mounted
pickups on
acoustic guitars are
generally piezoelectric devices that contact the strings passing
over the saddle. This type of microphone is different from
magnetic coil
pickups commonly visible on typical
electric guitars, which use magnetic
induction, rather than mechanical coupling, to pick up
vibration.
Fiber optic microphone
A fiber optic microphone converts acoustic waves into electrical
signals by sensing changes in light intensity, instead of sensing
changes in capacitance or magnetic fields as with conventional
microphones.
During operation, light from a laser source travels through an
optical fiber to illuminate the surface of a tiny, sound-sensitive
reflective diaphragm. Sound causes the diaphragm to vibrate,
thereby minutely changing the intensity of the light it reflects.
The modulated light is then transmitted over a second optical fiber
to a photo detector, which transforms the intensity-modulated light
into analog or digital audio for transmission or recording. Fiber
optic microphones possess high dynamic and frequency range, similar
to the best high fidelity conventional microphones.
Fiber optic microphones do not react to or influence any
electrical, magnetic, electrostatic or radioactive fields (this is
called
EMI/RFI
immunity). The fiber optic microphone design is therefore ideal for
use in areas where conventional microphones are ineffective or
dangerous, such as inside
industrial
turbines or in
magnetic
resonance imaging (MRI) equipment environments.
Fiber optic microphones are robust, resistant to environmental
changes in heat and moisture, and can be produced for any
directionality or
impedance
matching. The distance between the microphone's light source
and its photo detector may be up to several kilometers without need
for any preamplifier and/or other electrical device, making fiber
optic microphones suitable for industrial and surveillance acoustic
monitoring.
Fiber optic microphones are used in very specific application areas
such as for
infrasound monitoring and
noise-canceling. They
have proven especially useful in medical applications, such as
allowing radiologists, staff and patients within the powerful and
noisy magnetic field to converse normally, inside the MRI suites as
well as in remote control rooms.) Other uses include industrial
equipment monitoring and sensing, audio calibration and
measurement, high-fidelity recording and law enforcement.
Laser microphone
Laser microphones are often
portrayed in movies as spy gadgets. A laser beam is aimed at the
surface of a window or other plane surface that is affected by
sound. The slight vibrations of this surface displace the returned
beam, causing it to trace the sound wave. The vibrating laser spot
is then converted back to sound. In a more robust and expensive
implementation, the returned light is split and fed to an
interferometer, which detects frequency
changes due to the
Doppler effect.
The former implementation is a tabletop experiment; the latter
requires an extremely stable laser and precise optics.
A new type of laser microphone is a device that uses a laser beam
and smoke or vapor to detect
sound vibrations in free air. On 25 August 2009, U.S.
patent 7,580,533 issued for a Particulate Flow Detection Microphone
based on a laser-photocell pair with a moving stream of smoke or
vapor in the laser beam's path. Sound pressure waves cause
disturbances in the smoke that in turn cause variations in the
amount of laser light reaching the photo detector. A prototype of
the device was demonstrated at the 127th Audio Engineering Society
convention in New York City from 9 through 12 October 2009.
Liquid microphone
Early microphones did not produce intelligible speech, until
Alexander Graham Bell made improvements including a variable
resistance microphone/transmitter. Bell's liquid transmitter
consisted of a metal cup filled with water with a small amount of
sulfuric acid added. A sound wave caused the diaphragm to move,
forcing a needle to move up and down in the water. The electrical
resistance between the wire and the cup was then inversely
proportional to the size of the water meniscus around the submerged
needle. Elisha Gray filed a
caveat for
a version using a brass rod instead of the needle. Other minor
variations and improvements were made to the liquid microphone by
Majoranna, Chambers, Vanni, Sykes, and Elisha Gray, and one version
was patented by Reginald Fessenden in 1903. These were the first
working microphones, but they were not practical for commercial
application. The famous first phone conversation between Bell and
Watson took place using a liquid microphone.
MEMS microphone
The
MEMS (MicroElectrical-Mechanical System)
microphone is also called a microphone chip or silicon microphone.
The pressure-sensitive diaphragm is etched directly into a silicon
chip by MEMS techniques, and is usually accompanied with integrated
preamplifier. Most MEMS microphones are variants of the condenser
microphone design. Often MEMS microphones have built in
analog-to-digital converter (ADC) circuits on the same CMOS chip
making the chip a digital microphone and so more readily integrated
with modern digital products. Major manufacturers producing MEMS
silicon microphones are Wolfson Microelectronics (WM7xxx), Analog
Devices, Akustica (AKU200x), Infineon (SMM310 product), Knowles
Electronics, Memstech (MSMx), Sonion MEMS, AAC Acoustic
Technologies, and Omron.
Speakers as microphones
A
loudspeaker, a transducer that turns
an electrical signal into sound waves, is the functional opposite
of a microphone. Since a conventional speaker is constructed much
like a dynamic microphone (with a diaphragm, coil and magnet),
speakers can actually work "in reverse" as microphones. The result,
though, is a microphone with poor quality, limited frequency
response (particularly at the high end), and poor
sensitivity. In practical use,
speakers are sometimes used as microphones in applications where
high quality and sensitivity are not needed such as
intercoms,
walkie-talkies or
Xbox
Live chat peripherals.
However, there is at least one other practical application of this
principle: Using a medium-size
woofer placed
closely in front of a "kick" (
bass drum)
in a
drum set to act as a microphone. The
use of relatively large speakers to transduce low frequency sound
sources, especially in music production, is becoming fairly common.
Since a relatively massive membrane is unable to transduce high
frequencies, placing a speaker in front of a kick drum is often
ideal for reducing cymbal and snare bleed into the kick drum sound.
Less commonly, microphones themselves can be used as speakers,
almost always as
tweeters. This is less
common, since microphones are not designed to handle the power that
speaker components are routinely required to cope with. One
instance of such an application was the
STC microphone-derived 4001
super-tweeter, which was successfully used in a number of high
quality loudspeaker systems from the late 1960s to the mid-70s. A
well-known example of this use was the
Bowers & Wilkins DM2a model.
Capsule design and directivity
The inner elements of a microphone are the primary source of
differences in directivity. A pressure microphone uses a
diaphragm between a fixed
internal volume of air and the environment, and responds uniformly
to pressure from all directions, so it is said to be
omnidirectional. A pressure-gradient microphone uses a diaphragm
which is at least partially open on both sides; the pressure
difference between the two sides produces its directional
characteristics. Other elements such as the external shape of the
microphone and external devices such as interference tubes can also
alter a microphone's directional response. A pure pressure-gradient
microphone is equally sensitive to sounds arriving from front or
back, but insensitive to sounds arriving from the side because
sound arriving at the front and back at the same time creates no
gradient between the two. The characteristic directional pattern of
a pure pressure-gradient microphone is like a figure-8. Other polar
patterns are derived by creating a capsule that combines these two
effects in different ways. The cardioid, for instance, features a
partially closed backside, so its response is a combination of
pressure and pressure-gradient characteristics.
Microphone polar patterns
(Microphone facing top of page in diagram, parallel to
page):Image:Polar pattern omnidirectional.png|
Omnidirectional
Image:Polar pattern subcardioid.png|
Subcardioid
Image:Polar pattern cardioid.png|
Cardioid
Image:Polar pattern supercardioid.png|
Supercardioid
Image:Polar pattern hypercardioid.png|
Hypercardioid
Image:Polar pattern figure eight.png|
Bi-directional or Figure of 8
Image:Polar pattern directional.png|
Shotgun
A microphone's directionality or polar pattern indicates how
sensitive it is to sounds arriving at different angles about its
central axis. The above polar patterns represent the
locus of points that produce the same
signal level output in the microphone if a given
sound pressure level is generated from
that point. How the physical body of the microphone is oriented
relative to the diagrams depends on the microphone design. For
large-membrane microphones such as in the Oktava (pictured above),
the upward direction in the polar diagram is usually
perpendicular to the microphone body, commonly
known as "side fire" or "side address". For small diaphragm
microphones such as the Shure (also pictured above), it usually
extends from the axis of the microphone commonly known as "end
fire" or "top/end address".
Some microphone designs combine several principles in creating the
desired polar pattern. This ranges from shielding (meaning
diffraction/dissipation/absorption) by the housing itself to
electronically combining dual membranes.
Omnidirectional
An omnidirectional (or nondirectional) microphone's response is
generally considered to be a perfect sphere in three dimensions. In
the real world, this is not the case. As with directional
microphones, the polar pattern for an "omnidirectional" microphone
is a function of frequency. The body of the microphone is not
infinitely small and, as a consequence, it tends to get in its own
way with respect to sounds arriving from the rear, causing a slight
flattening of the polar response. This flattening increases as the
diameter of the microphone (assuming it's cylindrical) reaches the
wavelength of the frequency in question. Therefore, the smallest
diameter microphone will give the best omnidirectional
characteristics at high frequencies.
The wavelength of sound at 10 kHz is little over an inch
(3.4 cm) so the smallest measuring microphones are often 1/4"
(6 mm) in diameter, which practically eliminates
directionality even up to the highest frequencies. Omnidirectional
microphones, unlike cardioids, do not employ resonant cavities as
delays, and so can be considered the "purest" microphones in terms
of low coloration; they add very little to the original sound.
Being pressure-sensitive they can also have a very flat
low-frequency response down to 20 Hz or below.
Pressure-sensitive microphones also respond much less to wind noise
than directional (velocity sensitive) microphones.
An example of a nondirectional microphone is the round black
eight ball.
Unidirectional
A unidirectional microphone is sensitive to sounds from only one
direction. The diagram above illustrates a number of these
patterns. The microphone faces upwards in each diagram. The sound
intensity for a particular frequency is plotted for angles radially
from 0 to 360°. (Professional diagrams show these scales and
include multiple plots at different frequencies. The diagrams given
here provide only an overview of typical pattern shapes, and their
names.)
Cardioids

US664A University Sound Dynamic
Supercardioid Microphone
The most common unidirectional microphone is a
cardioid microphone, so named because the
sensitivity pattern is heart-shaped. A hyper-cardioid microphone is
similar but with a tighter area of front sensitivity and a smaller
lobe of rear sensitivity. A super-cardioid microphone is similar to
a hyper-cardioid, except there is more front pickup and less rear
pickup. These three patterns are commonly used as vocal or speech
microphones, since they are good at rejecting sounds from other
directions.
A cardioid microphone is effectively a superposition of an
omnidirectional and a figure-8 microphone; for sound waves coming
from the back, the negative signal from the figure-8 cancels the
positive signal from the omnidirectional element, whereas for sound
waves coming from the front, the two add to each other. A
hypercardioid microphone is similar, but with a slightly larger
figure-8 contribution. Since pressure gradient
transducer microphones are directional, putting
them very close to the sound source (at distances of a few
centimeters) results in a bass boost. This is known as the
proximity effect
Bi-directional
"Figure 8" or bi-directional microphones receive sound from both
the front and back of the element. Most ribbon microphones are of
this pattern.
Shotgun

An Audio-Technica shotgun
microphone
"Shotgun" microphones are the most highly directional. They have
small lobes of sensitivity to the left, right, and rear but are
significantly less sensitive to the side and rear than other
directional microphones are. This results from placing the element
at the end of a tube with slots cut along the side; wave
cancellation eliminates much of the off-axis sound. Due to the
narrowness of their sensitivity area, shotgun microphones are
commonly used on television and film sets, in stadiums, and for
field recording of wildlife.
Boundary or "PZM"
Several approaches have been developed for effectively using a
microphone in less-than-ideal acoustic spaces, which often suffer
from excessive reflections from one or more of the surfaces
(boundaries) that make up the space. If the microphone is placed
in, or in very close proximity to, one of these boundaries, the
reflections from that surface are not sensed by the microphone.
Initially this was done by placing an ordinary microphone adjacent
to the surface, sometimes in a block of acoustically transparent
foam. Sound engineers Ed Long and Ron Wickersham developed the
concept of placing the diaphgram parallel to and facing the
boundary. While the patent has expired, "Pressure Zone Microphone"
and "PZM" are still active trademarks of
Crown International, and the generic
term "boundary microphone" is preferred. While a boundary
microphone was initially implemented using an omnidirectional
element, it is also possible to mount a directional microphone
close enough to the surface to gain some of the benefits of this
technique while retaining the directional properties of the
element. Crown's trademark on this approach is "Phase Coherent
Cardioid" or "PCC," but there are other makers who employ this
technique as well.
Application-specific designs
A
lavalier microphone is made
for hands-free operation. These small microphones are worn on the
body. Originally, they were held in place with a lanyard worn
around the neck, but more often they are fastened to clothing with
a clip, pin, tape or magnet. The lavalier cord may be hidden by
clothes and either run to an RF transmitter in a pocket or clipped
to a belt (for mobile use), or run directly to the mixer (for
stationary applications).
A
wireless microphone is one in
which the artist is not limited by a cable. It usually sends its
signal using a small FM radio transmitter to a nearby receiver
connected to the sound system, but it can also use infrared light
if the transmitter and receiver are within sight of each
other.
A
contact microphone is designed
to pick up vibrations directly from a solid surface or object, as
opposed to sound vibrations carried through air. One use for this
is to detect sounds of a very low level, such as those from small
objects or
insects. The microphone commonly
consists of a magnetic (moving coil) transducer, contact plate and
contact pin. The contact plate is placed against the object from
which vibrations are to be picked up; the contact pin transfers
these vibrations to the coil of the transducer. Contact microphones
have been used to pick up the sound of a snail's heartbeat and the
footsteps of ants. A portable version of this microphone has
recently been developed. A
throat
microphone is a variant of the contact microphone, used to pick
up speech directly from the throat, around which it is strapped.
This allows the device to be used in areas with ambient sounds that
would otherwise make the speaker inaudible.
A
parabolic microphone uses a
parabolic reflector to collect
and focus sound waves onto a microphone receiver, in much the same
way that a
parabolic antenna (e.g.
satellite dish) does with radio
waves. Typical uses of this microphone, which has unusually focused
front sensitivity and can pick up sounds from many meters away,
include nature recording, outdoor sporting events,
eavesdropping,
law
enforcement, and even
espionage.
Parabolic microphones are not typically used for standard recording
applications, because they tend to have poor low-frequency response
as a side effect of their design.
A stereo microphone integrates two microphones in one unit to
produce a stereophonic signal. A stereo microphone is often used
for
broadcast applications or
field recording where it would be
impractical to configure two separate condenser microphones in a
classic X-Y configuration (see
microphone practice) for stereophonic
recording. Some such microphones have an adjustable angle of
coverage between the two channels.
A
noise-canceling
microphone is a highly directional design intended for noisy
environments. One such use is in
aircraft
cockpits where they are normally installed as boom microphones on
headsets. Another use is on loud concert stages for vocalists. Many
noise-canceling microphones combine signals received from two
diaphragms that are in opposite electrical polarity or are
processed electronically. In dual diaphragm designs, the main
diaphragm is mounted closest to the intended source and the second
is positioned farther away from the source so that it can pick up
environmental sounds to be subtracted from the main diaphragm's
signal. After the two signals have been combined, sounds other than
the intended source are greatly reduced, substantially increasing
intelligibility. Other noise-canceling designs use one diaphragm
that is affected by ports open to the sides and rear of the
microphone, with the sum being a 16 dB rejection of sounds that are
farther away. One noise-canceling headset design using a single
diaphragm has been used prominently by vocal artists such as
Garth Brooks and
Janet Jackson. A few noise-canceling
microphones are throat microphones.
Connectors
The most common connectors used by microphones are:
- Male XLR connector on professional
microphones
- ¼ inch (sometimes referred to as 6.5 mm) jack plug also known as 1/4 inch TRS connector on less expensive consumer
microphones. Many consumer microphones use an unbalanced
1/4 inch phone jack. Harmonica
microphones commonly use a high impedance 1/4 inch TS
connection to be run through guitar amplifiers.
- 3.5 mm (sometimes referred to as 1/8 inch mini)
stereo (wired as mono) mini phone plug on very inexpensive and
computer microphones
Some microphones use other connectors, such as a 5-pin XLR, or mini
XLR for connection to portable equipment. Some lavalier (or
'lapel', from the days of attaching the microphone to the news
reporters suit lapel) microphones use a proprietary connector for
connection to a wireless transmitter. Since 2005,
professional-quality microphones with
USB
connections have begun to appear, designed for direct recording
into computer-based software.
Impedance-matching
Microphones have an electrical characteristic called
impedance, measured in
ohms (Ω), that depends on the design. Typically,
the
rated impedance is stated. Low impedance is considered
under 600 Ω. Medium impedance is considered between 600 Ω
and 10 kΩ. High impedance is above 10 kΩ. Condenser
microphones typically have an output impedance between 50 and
200 ohms.
The output of a given microphone delivers the same
power whether it is low or high impedance.
If a microphone is made in high and low impedance versions, the
high impedance version will have a higher output voltage for a
given sound pressure input, and is suitable for use with
vacuum-tube guitar amplifiers, for instance, which have a high
input impedance and require a relatively high signal input voltage
to overcome the tubes' inherent noise. Most professional
microphones are low impedance, about 200 Ω or lower.
Professional vacuum-tube sound equipment incorporates a
transformer that steps up the impedance of the
microphone circuit to the high impedance and voltage needed to
drive the input tube; the impedance conversion inherently creates
voltage gain as well. External matching transformers are also
available that can be used in-line between a low impedance
microphone and a high impedance input.
Low-impedance microphones are preferred over high impedance for two
reasons: one is that using a high-impedance microphone with a long
cable will result in loss of high frequency signal due to the
capacitance of the cable which forms a low-pass filter with the
microphone output impedance; the other is that long high-impedance
cables tend to pick up more
hum (and
possibly
radio-frequency
interference (RFI) as well). Nothing will be damaged if the
impedance between microphone and other equipment is mismatched; the
worst that will happen is a reduction in signal or change in
frequency response.
Most microphones are designed
not to have their impedance
matched by the load to which they are connected; doing so can alter
their frequency response and cause distortion, especially at high
sound pressure levels. Certain ribbon and dynamic microphones are
exceptions, due to the designers' assumption of a certain load
impedance being part of the internal electro-acoustical damping
circuit of the microphone.
Digital microphone interface
The AES 42 standard, published by the
Audio Engineering Society, defines
a digital interface for microphones. Microphones conforming to this
standard directly output a digital audio stream through an XLR male
connector, rather than producing an analog output. Digital
microphones may be used either with new equipment which has the
appropriate input connections conforming to the AES 42 standard, or
else by use of a suitable interface box. Studio-quality microphones
which operate in accordance with the AES 42 standard are now
appearing from a number of microphone manufacturers.
Measurements and specifications
Because of differences in their construction, microphones have
their own characteristic responses to sound. This difference in
response produces non-uniform
phase
and
frequency responses. In addition,
microphones are not uniformly sensitive to sound pressure, and can
accept differing levels without distorting. Although for scientific
applications microphones with a more uniform response are
desirable, this is often not the case for music recording, as the
non-uniform response of a microphone can produce a desirable
coloration of the sound. There is an international standard for
microphone specifications, but few manufacturers adhere to it. As a
result, comparison of published data from different manufacturers
is difficult because different measurement techniques are used. The
Microphone Data Website has collated the technical specifications
complete with pictures, response curves and technical data from the
microphone manufacturers for every currently listed microphone, and
even a few obsolete models, and shows the data for them all in one
common format for ease of comparison.
[10341].
Caution should be used in drawing any solid conclusions from this
or any other published data, however, unless it is known that the
manufacturer has supplied specifications in accordance with
IEC 60268-4.
A
frequency response diagram
plots the microphone sensitivity in
decibels
over a range of frequencies (typically at least 0–20 kHz),
generally for perfectly on-axis sound (sound arriving at 0° to the
capsule). Frequency response may be less informatively stated
textually like so: "30 Hz–16 kHz ±3 dB". This
is interpreted as a (mostly) linear plot between the stated
frequencies, with variations in amplitude of no more than plus or
minus 3 dB. However, one cannot determine from this information how
smooth the variations are, nor in what parts of the
spectrum they occur. Note that commonly-made statements such as
"20 Hz–20 kHz" are meaningless without a decibel measure
of tolerance. Directional microphones' frequency response varies
greatly with distance from the sound source, and with the geometry
of the sound source. IEC 60268-4 specifies that frequency
response should be measured in
plane progressive wave
conditions (very far away from the source) but this is seldom
practical.
Close talking microphones may be measured with
different sound sources and distances, but there is no standard and
therefore no way to compare data from different models unless the
measurement technique is described.
The self-noise or equivalent noise level is the sound level that
creates the same output voltage as the microphone does in the
absence of sound. This represents the lowest point of the
microphone's dynamic range, and is particularly important should
you wish to record sounds that are quiet. The measure is often
stated in
dB, which is the equivalent loudness
of the noise on a decibel scale frequency-weighted for how the ear
hears, for example: "15 dBA SPL" (SPL means
sound pressure level relative to
20
micropascals). The lower the
number the better. Some microphone manufacturers state the noise
level using
ITU-R 468 noise
weighting, which more accurately represents the way we hear
noise, but gives a figure some 11–14 dB higher. A quiet
microphone will measure typically 20 dBA SPL or 32 dB SPL
468-weighted. Very quiet microphones have existed for years for
special applications, such the Brüel & Kjaer 4179, with a noise
level around 0 dB SPL. Recently some microphones with low
noise specifications have been introduced in the
studio/entertainment market, such as models from
Neumann and
Røde
that advertise noise levels between 5–7 dBA. Typically this is
achieved by altering the frequency response of the capsule and
electronics to result in lower noise within the A-weighting curve
while broadband noise may be increased.
The maximum SPL (
sound pressure
level) the microphone can accept is measured for particular
values of
total harmonic
distortion (THD), typically 0.5%. This is generally inaudible,
so one can safely use the microphone at this level without harming
the recording. Example: "142
dB SPL peak
(at 0.5% THD)". The higher the value, the better, although
microphones with a very high maximum SPL also have a higher
self-noise.
The clipping level is perhaps a better indicator of maximum usable
level, as the 1% THD figure usually quoted under max SPL is
really a very mild level of distortion, quite inaudible especially
on brief high peaks. Harmonic distortion from microphones is
usually of low-order (mostly third harmonic) type, and hence not
very audible even at 3-5%. Clipping, on the other hand, usually
caused by the diaphragm reaching its absolute displacement limit
(or by the preamplifier), will produce a very harsh sound on peaks,
and should be avoided if at all possible. For some microphones the
clipping level may be much higher than the max SPL.
The dynamic range of a microphone is the difference in SPL between
the noise floor and the maximum SPL. If stated on its own, for
example "120 dB", it conveys significantly less information
than having the self-noise and maximum SPL figures
individually.
Sensitivity indicates how
well the microphone converts acoustic pressure to output voltage. A
high sensitivity microphone creates more voltage and so will need
less amplification at the mixer or recording device. This is a
practical concern but is not directly an indication of the mic's
quality, and in fact the term sensitivity is something of a
misnomer, 'transduction gain' being perhaps more meaningful, (or
just "output level") because true sensitivity will generally be set
by the noise floor, and too much "sensitivity" in terms of output
level will compromise the clipping level. There are two common
measures. The (preferred) international standard is made in
millivolts per pascal at 1 kHz. A higher value indicates
greater sensitivity. The older American method is referred to a
1 V/Pa standard and measured in plain decibels, resulting in a
negative value. Again, a higher value indicates greater
sensitivity, so −60 dB is more sensitive than
−70 dB.
Measurement microphones
Some microphones are intended for testing speakers, measuring noise
levels and otherwise quantifying an acoustic experience. These are
calibrated transducers and will usually be supplied with a
calibration certificate stating absolute sensitivity against
frequency. The quality of measurement microphones is often referred
to using the designations "Class 1," "Type 2" etc., which are
references not to microphone specifications but to sound level
meters. A more comprehensive standard for the description of
measurement microphone performance was recently adopted.
Measurement microphones are generally scalar sensors of
pressure; they exhibit an omnidirectional response,
limited only by the scattering profile of their physical
dimensions.
Sound intensity or sound
power measurements require pressure-gradient measurements, which
are typically made using arrays of at least two microphones, or
with
hot-wire
anemometers.
Microphone calibration techniques
Like most manufactured products there can be variations, which may
change over the lifetime of the device. Accordingly, it is
regularly necessary to test the test microphones. This service is
offered by some microphone manufacturers and by independent
certified testing labs.
Microphone calibration is ultimately
traceable to primary standards at one of the national laboratories
such as PTB
in Germany and NIST in the
USA. Some test enough microphones to justify an in-house
calibration lab. Depending on the application, measurement
microphones must be tested periodically (every year or several
months, typically) and after any potentially damaging event, such
as being dropped (most such mikes come in foam-padded cases to
reduce this risk) or exposed to sounds beyond the acceptable
level.
Pistonphone apparatus
A pistonphone is an acoustical calibrator (sound source) using a
closed coupler to generate a precise sound pressure for the
calibration of instrumentation microphones. The principle relies on
a piston mechanically driven to move at a specified cyclic rate, on
a fixed volume of air to which the microphone under test is
exposed. The air is assumed to be compressed
adiabatically and the
sound pressure level in the chamber can be
calculated from internal physical dimensions of the device and the
adiabatic gas law, which requires that the product of the pressure
P with V raised to the power gamma be constant; here gamma is the
ratio of the specific heat of air at constant pressure to its
specific heat at constant volume.The pistonphone method only works
at low frequencies, but it can be accurate and yields an easily
calculable sound pressure level. The standard test frequency is
usually around 250 Hz.
Reciprocal method
This method relies on the reciprocity of one or more microphones in
a group of 3 to be calibrated. It can be performed in a closed
coupler or in the free field. Only one of the microphones need be
reciprocal (exhibits equal response when used as a microphone or as
a loudspeaker).
Microphone array and array microphones
A microphone array is any number of microphones operating in
tandem. There are many applications:
Typically, an array is made up of omnidirectional microphones
distributed about the
perimeter of a
space, linked to a
computer that records
and interprets the results into a coherent form.
Microphone windscreens
Windscreens are used to protect microphones that would otherwise be
buffeted by wind or vocal
plosives
from consonants such as "P", "B", etc. Most microphones have an
integral windscreen built around the microphone diaphragm. A screen
of plastic, wire mesh or a metal cage is held at a distance from
the microphone diaphragm, to shield it. This cage provides a first
line of defense against the mechanical impact of objects or wind.
Some microphones, such as the
Shure SM58,
may have an additional layer of foam inside the cage to further
enhance the protective properties of the shield. Beyond integral
microphone windscreens, there are three broad classes of additional
wind protection.
One disadvantage of all windscreen types is that the microphone's
high frequency response is attenuated by a small amount, depending
on the density of the protective layer.
Microphone covers
Microphone covers are often made of soft open-cell polyester or
polyurethane foam because of the inexpensive, disposable nature of
the foam. Optional windscreens are often available from the
manufacturer and third parties. A very visible example of an
optional accessory windscreen is the A2WS from Shure, one of which
is fitted over each of the two
Shure SM57
microphones used on the United States president's lectern. One
disadvantage of polyurethane foam microphone covers is that they
can deteriorate over time. Windscreens also tend to collect dirt
and moisture in their open cells and must be cleaned to prevent
high frequency loss, bad odor and unhealthy conditions for the
person using the microphone. On the other hand, a major advantage
of concert vocalist windscreens is that one can quickly change to a
clean windscreen between users, reducing the chance of transferring
germs. Windscreens of various colors can be used to distinguish one
microphone from another on a busy, active stage.
Pop filters
Pop filters or pop screens are used in
controlled studio environments to minimize
plosive when recording. A typical pop filter
is composed of one or more layers of acoustically transparent
gauze-like material, such as woven nylon
stretched over a circular frame and a clamp and a flexible mounting
bracket to attach to the microphone stand. The pop shield is placed
between the vocalist and the microphone. The need for a pop filter
increases the closer a vocalist brings his lips the microphone.
Singers can be trained either to soften their plosives or direct
the air blast away from the microphone, in which cases they don't
need a pop filter.
Pop filters also keep spittle off the microphone. Most condenser
microphones can be damaged by spittle.
Blimps
Blimps (also known as Zeppelins) are large, hollow windscreens used
to surround microphones for outdoor location audio, such as nature
recording,
electronic news
gathering, and for film and video shoots. They can cut wind
noise by as much as 25 dB, especially low-frequency noise. The
blimp is essentially a hollow cage or basket with acoustically
transparent material stretched over the outer frame. The blimp
works by creating a volume of still air around the microphone. The
microphone is often further isolated from the blimp by an elastic
suspension inside the basket. This reduces wind vibrations and
handling noise transmitted from the cage. To extend the range of
wind speed conditions in which the blimp will remain effective,
many have the option of fitting a secondary cover over the outer
shell. This is usually an acoustically transparent, synthetic fur
material with long, soft hairs. The hairs act as shock absorbers to
any wind turbulence hitting the blimp. A synthetic fur cover can
reduce wind noise by an additional 12 dB.
See also
References
- http://www.national.com/nationaledge/dec02/article.html
- "AKG D 112 - Large-diaphragm dynamic microphone for
bass instruments"
- History & Development of Microphone. Lloyd
Microphone Classics.
- Proximity Effect. Geoff Martin,
Introduction to Sound Recording.
- ( )
- Crown Audio. Tech Made Simple. The Crown Differoid
Microphone
- International Standard IEC 60268-4
- http://www.shure.com/ProAudio/Products/us_pro_ea_imepdance
- Robertson, A. E.: "Microphones" Illiffe Press for BBC,
1951-1963
- IEC Standard 61672 and/or ANSI S1.4
- IEC 61094
- Shure - Accessories - A2WS Microphone
Windscreens
- Full Windshield System. Rycote
Microphones.
External links