In order to speak to larger groups of people, there was a desire to
increase the volume of the spoken word. The earliest known device to
achieve this dates to 600 BC with the invention of masks with specially
designed mouth openings that acoustically augmented the voice in amphitheatres.[2] In 1665, the English physicist Robert Hooke was the first to experiment with a medium other than air with the invention of the "lovers' telephone" made of stretched wire with a cup attached at each end.[3] In 1874, Ernst von Siemens described the "dynamic" or "moving-coil" transducer, though the first result of this invention was not the microphone, but its adaptation in 1920 to make a loudspeaker.[4]
During the mid-19th century a number of inventors came up with devices
that led to the invention of the first practical electrical telephone
patented by Alexander Graham Bell in 1876.[5]
Inventors Emile Berliner and Thomas Edison were inspired to improve this and both went on to design and build the first carbon microphone (then called transmitter) in mid-1877, within a month of each other. After a long legal dispute, Edison was awarded the patent.[6]
Edison continued to refine the carbon microphone, which was employed
at the first ever radio broadcast, a performance at the New York Metropolitan Opera House in 1910.[7] In 1916, C. Wente of Bell Labs developed the next breakthrough with the first condenser microphone.[8]
In 1923 the first practical moving coil microphone was built. "The Marconi Skykes" or "magnetophon", developed by Captain H. J. Round, was the standard for BBC studios in London.[9] This was improved in 1930 by Blumlein and Holman who released the HB1A and was the best standard of the day.[10]
In the same year, the ribbon microphone was introduced, another electromagnetic type, believed to have been developed by Harry F. Olson, who essentially reverse-engineered a ribbon speaker.[11]
Over the years these microphones were developed by several companies,
most notably RCA that made large advancements in pattern control, to
give the microphone directionality. With television and film technology
booming there was demand for high fidelity microphones and greater
directionality. Electro-Voice responded with their Academy Award-winning shotgun microphone in 1963.
During the second half of 20th century development advanced quickly with the Shure Brothers bringing out the SM58 and SM57. Digital was pioneered by Milab in 1999 with the DM-1001.[12] The latest research developments include the use of fibre optics, lasers and interferometers.
Components
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. A wireless microphone contains a radio transmitter.
Varieties
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 microphone
Inside the Oktava 319 condenser microphone
The condenser microphone, invented at Bell Labs in 1916 by E. C. Wente[13] is also called a capacitor microphone or electrostatic microphone—capacitors were historically called condensers. Here, the diaphragm acts as one plate of a capacitor,
and the vibrations produce changes in the distance between the plates.
There are two types, depending on the method of extracting the audio signal from the transducer: DC-biased microphones, and radio frequency (RF) or high frequency (HF) condenser microphones. With a DC-biased microphone, the plates are biased 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 to 100 pF) and the value of the bias resistor (100 MΩ
to tens of GΩ) form a filter that is high-pass for the audio signal,
and low-pass 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. In most cases, the electronics
in the microphone itself contribute no voltage gain as the voltage
differential is quite significant, up to several volts for high sound
levels. Since this is a very high impedance circuit, current gain only
is usually needed, with the voltage remaining constant.
RF condenser microphones use a comparatively low RF voltage,
generated by a low-noise oscillator. The signal from 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 by-product of
which is that RF condenser microphones can be operated in damp weather
conditions that could create problems in DC-biased microphones with
contaminated insulating surfaces. 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 recording studio
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 that require the sound wave to do more work. They
require a power source, provided either via microphone inputs on
equipment 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 that can be electrically connected to provide a range of
polar patterns (see below), such as cardioid, omnidirectional, and
figure-eight. It is also possible to vary the pattern continuously with
some microphones, for example the Røde NT2000 or CAD M179.
First patent on foil electret microphone by G. M. Sessler et al. (pages 1 to 3)
An electret microphone is a type of capacitor microphone invented by Gerhard Sessler and Jim West at Bell laboratories in 1962.[14]
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 magnet;
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[15]
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 that does require power (often incorrectly called polarizing power or bias). This preamplifier is frequently phantom powered in sound reinforcement and studio applications. Monophonic microphones designed for personal computer
(PC) use, sometimes called multimedia microphones, use a 3.5 mm plug as
usually used, without power, for stereo; the ring, instead of carrying
the signal for a second channel, carries power via a resistor from
(normally) a 5 V supply in the computer. Stereophonic microphones use
the same connector; there is no obvious way to determine which standard
is used by equipment and microphones.
Only the best electret microphones rival good DC-polarized units in
terms of noise level and quality; electret microphones lend themselves
to inexpensive mass-production, while inherently expensive non-electret
condenser microphones are made to higher quality.
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 does 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.[16] 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 bi-directional (also called figure-eight, as in the diagram below) 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 provide high quality sound reproduction, were once valued
for this reason, but a good low-frequency response could only be
obtained when the ribbon was suspended very loosely, which made them
relatively fragile. Modern ribbon materials, including new nanomaterials[17]
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.
A carbon microphone,
also known as a carbon button microphone (or sometimes just a button
microphone), use a capsule or button containing carbon granules pressed
between two metal plates like the Berliner and Edison
microphones. 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. The Boudet microphone, which used relatively large carbon
balls, was similar to the granule carbon button microphones.[18]
Unlike other microphone types, the carbon microphone can also be used
as a type of amplifier, using a small amount of sound energy to control
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 or piezo 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 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.
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.[19][20]
During operation, light from a laser source travels through an
optical fiber to illuminate the surface of a reflective diaphragm. Sound
vibrations of the diaphragm modulate the intensity of light reflecting
off the diaphragm in a specific direction. 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 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.[21])
Other uses include industrial equipment monitoring and sensing, audio
calibration and measurement, high-fidelity recording and law
enforcement.
Laser microphones
are often portrayed in movies as spy gadgets, because they can be used
to pick up sound at a distance from the microphone equipment. A laser
beam is aimed at the surface of a window or other plane surface that is
affected by sound. The vibrations of this surface change the angle at
which the beam is reflected, and the motion of the laser spot from the
returning beam is detected and converted to an audio signal.
In a more robust and expensive implementation, the returned light is split and fed to an interferometer, which detects movement of the surface by changes in the optical path length
of the reflected beam. 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 soundvibrations
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.
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.
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), NXP Semiconductors, Sonion MEMS, AAC
Acoustic Technologies,[22] and Omron.[23]
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 video game voice chat peripherals, or when conventional microphones are in short supply.
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. A product example of this type of device is
the Yamaha Subkick,
a 6.5-inch (170 mm) woofer shock-mounted into a 10" drum shell used in
front of kick drums. 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.
Microphones, however, 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.
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 that 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.[24]
Microphone polar patterns
(Microphone facing top of page in diagram, parallel to page):
A microphone's directionality or polar pattern indicates how
sensitive it is to sounds arriving at different angles about its central
axis. The polar patterns illustrated above represent the locus of points that produce the same signal level output in the microphone if a given sound pressure level
(SPL) 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 gives the best
omnidirectional characteristics at high frequencies.
The wavelength of sound at 10 kHz is little over an inch (3.4 cm).
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 and plosives than
directional (velocity sensitive) microphones.
An example of a nondirectional microphone is the round black eight ball.[25]
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.)
Cardioid
University Sound US664A dynamic supercardioid microphone
The most common unidirectional microphone is a cardioid microphone, so named because the sensitivity pattern is a cardioid.
The cardioid family of microphones are commonly used as vocal or speech
microphones, since they are good at rejecting sounds from other
directions. In three dimensions, the cardioid is shaped like an apple
centred around the microphone which is the "stalk" of the apple. The
cardioid response reduces pickup from the side and rear, helping to
avoid feedback from the monitors. 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.[26] The SM58 has been the most commonly used microphone for live vocals for more than 40 years[27] demonstrating the importance and popularity of cardioid mikes.
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 hyper-cardioid
microphone is similar, but with a slightly larger figure-8 contribution
leading to 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.
While any pattern between omni and figure 8 is possible by adjusting
their mix, common definitions state that a hypercardioid is produced by
combining them at a 3:1 ratio, while supercardioid is produced with a
5:3 ratio.[28][29]
Bi-directional
"Figure 8" or bi-directional microphones receive sound equally from
both the front and back of the element. Most ribbon microphones are of
this pattern. In principle they do not respond to sound pressure at all,
only to the change in pressure between front and back; since
sound arriving from the side reaches front and back equally there is no
difference in pressure and therefore no sensitivity to sound from that
direction. In more mathematical terms, while omnidirectional microphones
are scalar transducers responding to pressure from any direction, bi-directional microphones are vector
transducers responding to the gradient along an axis normal to the
plane of the diaphragm. This also has the effect of inverting the output
polarity for sounds arriving from the back side.
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. This results from placing the element at the back 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 very close 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.[30] 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
transmits the audio as a radio or optical signal rather than via 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 waves if the transmitter and receiver are within sight of each
other.
A contact microphone
picks 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
directly on the vibrating part of a musical instrument or other surface,
and the contact pin transfers vibrations to the coil. 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 that picks up speech directly
from a person's throat, which it is strapped to. This lets the device 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 in live event support on loud concert stages for vocalists involved with live performances.
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.[31] A few noise-canceling microphones are throat microphones.
¼ inch (sometimes referred to as 6.3 mm) phone connector
on less expensive consumer microphones, using an unbalanced 1/4 inch TS
phone connector. 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, such as a radio pack. 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.[32]
Low impedance is considered under 600 Ω. Medium impedance is considered
between 600 Ω and 10 kΩ. High impedance is above 10 kΩ. Owing to their
built-in amplifier, condenser microphones typically have an output impedance between 50 and 200 Ω.[33]
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 has 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
results in high frequency signal loss due to cable capacitance, 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 is damaged if the impedance between microphone
and other equipment is mismatched; the worst that happens is a reduction
in signal or change in frequency response.
Most microphones are designed not to have their impedance matched by the load they are connected to.[34]
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.[35][dubious– discuss]
Digital microphone interface
Neumann D-01 digital microphone and Neumann DMI-8 8-channel USB 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 or XLD
male connector, rather than producing an analog output. Digital
microphones may be used either with new equipment with appropriate input
connections that conform to the AES 42 standard, or else via a suitable
interface box. Studio-quality microphones that operate in accordance
with the AES 42 standard are now available from a number of microphone
manufacturers.
Measurements and specifications
A comparison of the far field on-axis frequency response of the Oktava 319 and the Shure SM58
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,[32]
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.[3].
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 20 Hz to 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 meaning a nearly flat,
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(A),
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 typically measures
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 the microphone can accept is measured for particular values of total harmonic distortion (THD), typically 0.5%. This amount of distortion is generally inaudible[citation needed], so one can safely use the microphone at this SPL 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 an important 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.
Clipping is much more audible. 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 needs
less amplification at the mixer or recording device. This is a
practical concern but is not directly an indication of the microphone'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 is generally set by the noise floor,
and too much "sensitivity" in terms of output level compromises 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 are usually supplied with a calibration
certificate that states 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.[36] A more comprehensive standard[37] 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.
To take a scientific measurement with a microphone, its precise sensitivity must be known (in volts per pascal). Since this may change over the lifetime of the device, it is necessary to regularly calibrate
measurement microphones. This service is offered by some microphone
manufacturers and by independent certified testing labs. All microphone calibration is ultimately traceable to primary standards at a national measurement institute such as NPL in the UK, PTB in Germany and NIST
in the United States, which most commonly calibrate using the
reciprocity primary standard. Measurement microphones calibrated using
this method can then be used to calibrate other microphones using
comparison calibration techniques.
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 microphones
come in foam-padded cases to reduce this risk) or exposed to sounds
beyond the acceptable level.
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[note 1] 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. 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.
Beyond integral microphone windscreens, there are three broad classes of additional wind protection.
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 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.[38]
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 plosives when recording. A typical pop filter is composed of one or more layers of acoustically transparent gauze-like material, such as woven nylon (e.g., pantyhose) 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
closer a vocalist brings his or her lips to the microphone, the greater
the requirement for a pop filter. Singers can be trained either to
soften their plosives or direct the air blast away from the microphone,
in which cases they do not need a pop filter.
Pop filters also keep spittle off the microphone. Most[who?] 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 remains effective,
many have the option of a secondary cover over the outer shell. This is
usually an acoustically transparent, synthetic fur material with long,
soft hairs. Common and slang names for this include "dead cat" or
"windmuff". The hairs deaden the noise caused by the shock of wind
hitting the blimp. A synthetic fur cover can reduce wind noise by an
additional 10 dB.[39]
Two recordings being made—a blimp is being used on the left. An open-cell foam windscreen is being used on the right.
"Dead cat" and a "dead kitten" windscreens. The dead kitten covers a
stereo microphone for a DSLR camera. The difference in name is due to
the size of the fur.
Types of Computer Microphones
The stand alone computer microphone is long and thin.
Some
computer users find it easier to participate in voice chats or videos
than to type a note to those with whom they are communicating, while
others may use their computers to perform calls instead of traditional
telephone service. These users, along with professionals who
telecommute, will find a computer microphone useful or even essential to
their activities. One of several types of computer microphones may suit
their needs.
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Computer headsets include headphones through which the user
can hear audio output from the computer as well as a microphone that the
user can use to create audio input. Typical analog headsets contain two
3.5 mm plugs, the same type of plug that regular headphones use, that
the user insert into audio input and output ports on his computer.
Because the plugs are separate, the user is able to use the microphone
without the headphones and vice versa. Consumers also may purchase USB
headsets that connect via the computer's USB port and transmit the audio
data digitally.
Stand Alone
Stand alone microphones rest on a base that sits on the
user's desk. These microphones tend to have a long and narrow
appearance. This style of microphone tends to produce more white noise
than headset microphones and the light designs can easily lose balance
if someone or something bumps the device. Some stand alone microphones
may include a mechanism to mount the microphone to the computer so it
does not sit on the desk and some have adjustable height. Generally,
stand alone microphones connect to the computer with the 3.5 mm plug.
Many companies that make Web cams for computers also
manufacture models that include microphone capability. Consumers can
purchase this style of accessory instead of purchasing both products
separately. These combination devices save room in the user's workspace
and are especially useful for video chats and calls. Many of them attach
to the top or side of the computer monitor with a clip so that the
Webcam faces the user, capturing audio and video. These are also digital
devices that rely on the USB port to connect with the user's computer.
Integrated Microphones
Some computers and laptops, including all Macs, have
built-in microphones that require no additional purchase or
installation. Users can consult their computer manual to determine if it
includes a built-in microphone. Computers that do have an integrated
microphone may have one or more small holes located near the top or
bottom of the monitor and, possibly, a microphone image to indicate the
presence of the device. Users who desire higher quality audio input may
still wish to purchase an external microphone for use with their
computers.
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A Sennheiser microphone
A microphone (colloquially called a mic or mike; both pronounced /ˈmaɪk/)[1] is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. Microphones are used in many applications such as telephones, tape recorders, karaoke systems, hearing aids, motion picture production, live and recorded audio engineering, FRS radios, megaphones, in radio and television broadcasting and in computers for recording voice, speech recognition, VoIP, and for non-acoustic purposes such as ultrasonic checking or knock sensors.
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.
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