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Encyclopedia > Microphone

A Neumann U87 capacitor microphone

Several early inventors built primitive microphones (then called transmitters) prior to Alexander Bell, but the first commercially practical microphone was the carbon microphone conceived in October 1876 by Thomas Edison. Many early developments in microphone design took place at Bell Laboratories, including the first condenser microphone, described by Wente in 1914. Carbon microphone from Western Electric telephone. ... Edison redirects here. ... Bell Telephone Laboratories or Bell Labs was originally the research and development arm of the United States Bell System, and was the premier corporate facility of its type, developing a range of revolutionary technologies from telephone switches to specialized coverings for telephone cables, to the transistor. ...

## Principle of operation

Edmund Lowe using a microphone

A microphone is a device made to capture waves in air, water (hydrophone) or hard material and translate them into an electrical signal. The most common design uses a thin membrane which vibrates in response to sound pressure, this movement being subsequently translated into an electrical signal. Most microphones in use today for audio use electromagnetic generation (dynamic microphones), capacitance change (condenser microphones) or piezoelectric generation to produce the signal from mechanical vibration. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Edmund Dantes Lowe (March 3, 1890 - April 21, 1971) was an actor. ... A hydrophone is a sound-to-electricity transducer for use in water or other liquids, analogous to a microphone for air. ...

## Microphone varieties

### Condenser, capacitor or electrostatic microphones

Inside the Oktava 319 condenser microphone.

Download high resolution version (1324x2016, 353 KB) Wikipedia does not have an article with this exact name. ... Download high resolution version (1324x2016, 353 KB) Wikipedia does not have an article with this exact name. ...

#### Technology

In a condenser microphone, also known as a capacitor microphone, the diaphragm acts as one plate of a capacitor, and the vibrations produce changes in the distance between the plates. In a loudspeaker, a diaphragm is the thin, semi-rigid membrane attached to the central magnet. ... See Capacitor (component) for a discussion of specific types. ...

There are two methods of extracting an audio output from the transducer thus formed. They are known as DC biased and RF (or HF) condenser microphones.

##### DC-biased microphone operating principle

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: In electronics, voltage bias (sometimes simply called bias) is a steady-state voltage It is used to activate the active component like transistor transistor, coil or other source of load. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ...

$Q = C cdot V$

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 coulomb (symbol: C) is the SI unit of electric charge. ... Examples of various types of capacitors. ... Josephson junction array chip developed by NIST as a standard volt. ... Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential. ...

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 and the value of the bias resistor form a filter which is highpass for the audio signal, and lowpass for the bias voltage. Note that the time constant of a RC circuit equals the product of the resistance and capacitance. resistor-capacitor circuit (RC circuit), or RC filter or RC network, is one of the simplest analogue electronic filters. ...

Within the time-frame of the capacitance change (on the order of 100 μs), the charge thus appears 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 microphone operating principle

In a DC-biased condenser microphone, a high capsule polarization voltage is necessary. In contrast, RF condenser microphones use a comparatively low RF voltage, generated by a low-noise oscillator. The oscillator may either be frequency 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 amplitude of the fixed-frequency oscillator signal. Demodulation yields a low-noise audio frequency signal with a very low source impedance. This technique permits the use of a diaphragm with looser tension, which may be used to achieve better low-frequency response. Categories: Technology stubs ...

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 would effectively short out a DC-biased microphone. The Sennheiser "MKH" series of microphones use the RF biasing technique. Sennheiser electronic GmbH & Co. ...

#### Usage

Patti Smith singing into a Shure SM58 microphone

Condenser microphones span the range from cheap throw-aways to high-fidelity quality instruments. They generally produce a high-quality audio signal and are now the popular choice in laboratory and studio recording applications. They require a power source, provided either from microphone inputs as phantom power or from a small battery. Power is necessary for establishing the capacitor plate voltage, and is also needed for internal amplification of the signal to a useful output level. 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. Patricia Lee (Patti) Smith (born December 30, 1946) is an American musician, singer, and poet. ... Phantom power (labeled as +48 V on some audio equipment) is a method that sends a DC electrical voltage through microphone cables. ... RÃ¸de Microphones, based in Silverwater, New South Wales, Australia, is a company that designs and manufactures Microphones and related accessories. ...

#### Electret condenser microphones

Main article: Electret microphone
First patent on foil electret microphone by G. M. 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[1]. The externally-applied charge described above under condenser microphones is replaced by a fixed 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. 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 low-cost and considered low quality, the best ones can now rival capacitor microphones in every respect and can even offer the long-term stability and ultra-flat response needed for a measuring microphone. Unlike other capacitor microphones, they require no polarizing voltage, but normally contain an integrated preamplifier which does require power (often incorrectly called polarizing power or bias). This preamp is frequently phantom powered in sound reinforcement and studio applications. 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 electrets cheaply don't lend themselves to the precision needed to produce the highest quality microphones. Electret condenser microphone capsules A typical electret microphone preamp circuit uses an FET in a common source configuration. ... Image File history File links Size of this preview: 800 Ã— 395 pixelsFull resolution (3964 Ã— 1958 pixel, file size: 207 KB, MIME type: image/png) United States patent 3118022 of first foil electret microphone (electroacustic transducer), invented by G. M. Sessler and J. E. West at Bell Laboratories in 1962. ... Image File history File links Size of this preview: 800 Ã— 395 pixelsFull resolution (3964 Ã— 1958 pixel, file size: 207 KB, MIME type: image/png) United States patent 3118022 of first foil electret microphone (electroacustic transducer), invented by G. M. Sessler and J. E. West at Bell Laboratories in 1962. ... Bell Telephone Laboratories or Bell Labs was originally the research and development arm of the United States Bell System, and was the premier corporate facility of its type, developing a range of revolutionary technologies from telephone switches to specialized coverings for telephone cables, to the transistor. ... Gerhard Sessler (February 15, 1931-) co-invented the foil electret microphone with James West at Bell Laboratories in work culminating in 1962. ... James Edward Maceo West (Februrary 10, 1931 - ) is an American inventor and Acoustian. ... Electret (formed of elektr- from electricity and -et from magnet) is material that has been permanently electrically charged (polarised). ... In physics, the ferroelectric effect is an electrical phenomenon whereby certain ionic crystals may exhibit a spontaneous dipole moment. ... This box:      Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... A clip-mounted lavalier A lavalier microphone or lavalier (or lav or lapel mike) is a small electret or dynamic microphone used for television, theatre, and public speaking applications, in order to allow hands-free operation. ... Methods and media for sound recording are varied and have undergone significant changes between the first time sound was actually recorded for later playback until now. ... An example of a typical high-end stereo preamplifier. ... Phantom power (labeled as +48 V on some audio equipment) is a method that sends a DC electrical voltage through microphone cables. ...

### Dynamic microphones

Dynamic microphones work via electromagnetic induction. They are robust, relatively inexpensive and resistant to moisture, and for this reason they are widely used on-stage by singers. There are two basic types: the moving coil microphone and the ribbon microphone. For magnetic induction, see Magnetic field. ...

#### Moving coil microphones

The Shure SM57 and Beta 57A dynamic microphones

##### Technology

The dynamic principle is exactly the same 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 D112, for example, is designed for bass response rather than treble[citation needed]. In audio engineering several kinds of microphones are often used at the same time to get the best result. For the Marty Friedman album, see Loudspeaker (album) An inexpensive low fidelity 3. ... An induction coil or spark coil (archaically known as a Ruhmkorff coil) is a type of disruptive discharge coil. ... For the indie-pop band, see The Magnetic Fields. ... Magnetic lines of force of a bar magnet shown by iron filings on paper A magnet is an object that has a magnetic field. ... In a loudspeaker, a diaphragm is the thin, semi-rigid membrane attached to the central magnet. ... In electricity, current refers to electric current, which is the flow of electric charge. ... For magnetic induction, see Magnetic field. ...

### Ribbon microphones

Main article: Ribbon microphone

In ribbon microphones a thin, usually corrugated metal ribbon is 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 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. 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 is suspended very loosely, and this made them fragile. Modern ribbon materials, including new nanomaterials[2] 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. The sensitivity pattern of a bidirectional microphone (red dot) viewed from above. ... A microphone with a cord A microphone, sometimes called a mic (pronounced mike), is a device that converts sound into an electrical signal. ... Pressure Gradient is the change in pressure over a distance. ... This article is about audible acoustic waves. ...

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. (There are some new modern ribbon microphone designs which incorporate a preamplifier and therefore do require phantom power, also there are new ribbon materials available that are immune to wind blasts and phantom power.) Phantom power (labeled as +48 V on some audio equipment) is a method that sends a DC electrical voltage through microphone cables. ...

### Carbon microphones

Main article: Carbon microphone

A carbon microphone, formerly used in telephone handsets, 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 voltage across the two plates, and hence 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. Carbon microphone from Western Electric telephone. ... For other uses, see Telephone (disambiguation). ... The word capsule (from the Latin capsula, a small box), has many similar meanings in English: In botany, a capsule is a type of dry fruit as in the poppy, iris, foxglove, etc. ... For other uses, see Carbon (disambiguation). ...

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.) For the British rock band of the same name, see Amplifier (band). ... For other uses, see Repeater (disambiguation). ...

### Piezoelectric microphones

#### Technology

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. Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential[1] in response to applied mechanical stress. ... Potassium sodium tartrate is a double salt first prepared (in about 1675) by an apothecary, Pierre Seignette, of La Rochelle, France. ...

#### Usage

Crystal microphones used to be 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. Structure of a vacuum tube diode Structure of a vacuum tube triode In electronics, a vacuum tube, electron tube, or (outside North America) thermionic valve or just valve, is a device used to amplify, switch or modify a signal by controlling the movement of electrons in an evacuated space. ... Assorted discrete transistors A transistor is a semiconductor device, commonly used as an amplifier or an electrically controlled switch. ...

Piezo 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 piezos 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 Contact Microphone is a form of microphone designed not for picking up airborne sound waves, but rather to act as a transducer which picks up vibrations through solid materials and converts them into audible sound. ... Three magnetic pickups on an electric guitar. ... Acoustic guitar can refer to the following musical instruments: Nylon and gut stringed guitars: Renaissance guitar Baroque guitar Romantic guitar Classical guitar, the modern version of the original guitar, with nylon strings Flamenco guitar Steel stringed guitars: Steel-string acoustic guitar, also known as western, folk or country guitar Twelve... Three magnetic pickups on an electric guitar. ... An electric guitar An electric guitar is a type of guitar that uses pickups to convert the vibration of its steel-cored strings into electrical current, which is then amplified. ...

### Laser microphones

Main article: Laser microphone

This article or section does not cite its references or sources. ...

#### Usage

Laser microphones are very rare and expensive, and are most commonly portrayed in movies as spying devices.

### Liquid microphones

Main article: Water microphone

A water microphone or water transmitter is based on Ohms Law that current in a wire varies inversely with the resistance of the circuit. ...

#### Technology

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 even patented by Reginald Fessenden in 1903. A patent caveat was a legal document filed with the United States Patent Office. ...

#### Usage

These were the first working microphones, but they were not practical for commercial application and are utterly obsolete now. It was with a liquid microphone that the famous first phone conversation between Bell and Watson took place. Other inventors, especially Thomas Edison, soon devised superior microphones

### MEMS microphones

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 mics 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 using MEMS silicon microphones are Akustica (AKU200x), Infineon (SMM310 product), Knowles Electronics, Memstech and Sonion MEMS. A mite next to a gear set produced using MEMS. Courtesy Sandia National Laboratories, SUMMiTTM Technologies, www. ...

### 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. For the Marty Friedman album, see Loudspeaker (album) An inexpensive low fidelity 3. ... The sensitivity of an electronic device, a communications system receiver, or detection device, PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria. ...

In practical use, speakers are sometimes used as microphones in such applications as intercoms or walkie-talkies, where high quality and sensitivity are not needed. 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. Intercom system in the Pittock Mansion An intercom is an electronic communications system within a building or group of buildings. ... A walkie-talkie is a portable, bi-directional radio transceiver, first developed for military use. ... A Sony 9 inch woofer Woofer is the term for a loudspeaker driver that is designed to produce low frequency sounds, typically from around 40 hertz up to a few hundred hertz. ... A bass drum is a large drum that produces a note of low definite or indefinite pitch. ... An extended 4-piece drum kit A drum kit (or drum set or trap set - the latter an old-fashioned term) is a collection of drums, cymbals and other percussion instruments arranged for convenient playing by a sole percussionist (drummer), usually for jazz, rock, or other types of contemporary music. ...

Less commonly, microphones themselves can be used as speakers, almost always as tweeters. This is less common since microphones are very rarely designed for handling the power speaker components are routinely required to cope with. One instance of this is the STC microphone capsule which was used as a supertweeter on the high quality B&W DM2a bookshelf loudspeaker about 1970. A shielded Peerless v-line dome tweeter A tweeter is a driver designed to produce high frequencies, typically from around 2,000 hertz to 20,000 hertz (20,000 Hz is generally considered to be the upper limit of the human ear). ... STC may stand for: Saudi Telecom Company Samuel Taylor Coleridge Society for Technical Communication Sound transmission class Standard Telephones and Cables Space-time code St. ... B&W may refer to: Black-and-white (photography, cinematography, etc. ...

## Capsule design and directivity

The shape of the microphone defines its directivity. Inner elements are of major importance and concerns the structural shape of the capsule, outer elements may be the interference tube. In electromagnetics, directivity is a figure of merit for an antenna. ...

A pressure gradient microphone is a microphone in which both sides of the diaphragm are exposed to the incident sound and the microphone is therefore responsive to the pressure differential (gradient) between the two sides of the membrane. Sound incident parallel to the plane of the diaphragm produces no pressure differential, giving pressure-gradient microphones their characteristic figure-eight directional patterns.

The capsule of a pressure microphone however is closed on one side, which results in an omnidirectional pattern.

## Microphone polar patterns

Regarding directionality, omnidirectional microphones are pressure transducers, whereas all others are pressure gradient transducers or a combination between the two.

Common polar patterns for microphones (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 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". 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".

### Omnidirectional

An omnidirectional 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.

### 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. These diagrams just provide an overview of the typical shapes and their names.)

### Cardioids

US664A University Sound Dymamic Supercardioid Microphone

The most common unidirectional microphone is a cardioid microphone, so named because the sensitivity pattern is heart-shaped (see cardioid). A hyper-cardioid is similar but with a tighter area of front sensitivity and a tiny 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. Image File history File links Metadata Size of this preview: 800 Ã— 600 pixelsFull resolutionâ€Ž (2,048 Ã— 1,536 pixels, file size: 1. ... Image File history File links Metadata Size of this preview: 800 Ã— 600 pixelsFull resolutionâ€Ž (2,048 Ã— 1,536 pixels, file size: 1. ... In geometry, the cardioid is an epicycloid which has one and only one cusp. ...

### 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 more sensitive to the front. This results from placing the element inside a tube with slots cut along the side; wave-cancellation eliminates most of the off-axis noise. Shotgun microphones are commonly used on TV and film sets, and for field recording of wildlife. Image File history File linksMetadata Size of this preview: 800 Ã— 398 pixel Image in higher resolution (1536 Ã— 764 pixel, file size: 102 KB, MIME type: image/jpeg) Audio-Technica AT815a shotgun microphone from the 1990s. ... Image File history File linksMetadata Size of this preview: 800 Ã— 398 pixel Image in higher resolution (1536 Ã— 764 pixel, file size: 102 KB, MIME type: image/jpeg) Audio-Technica AT815a shotgun microphone from the 1990s. ...

An omnidirectional microphone is a pressure transducer; the output voltage is proportional to the air pressure at a given time.

On the other hand, a figure-8 pattern is a pressure gradient transducer; A sound wave arriving from the back will lead to a signal with a polarity opposite to that of an identical sound wave from the front. Moreover, shorter wavelengths (higher frequencies) are picked up more effectively than lower frequencies. The polarity of an object is, in general, its physical alignment of atoms. ...

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, at distances of a few centimeters of the sound source results in a bass boost. This is known as the proximity effect[3] This article is about transducers in engineering. ... The proximity effect in audio refers to a change in the frequency response of a directional microphone as the sound source is brought in close proximity to the microphone. ...

## Application-specific microphone designs

A lavalier microphone is made for hands-free operation. These small microphones are worn on the body and held in place either with a lanyard worn around the neck or a clip fastened to clothing. The 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 clip-mounted lavalier A lavalier microphone or lavalier (or lav or lapel mike) is a small electret or dynamic microphone used for television, theatre, and public speaking applications, in order to allow hands-free operation. ...

A wireless microphone is one which does not use a cable. It usually transmits 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 wireless microphone, as the name implies, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated. ...

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 Contact Microphone is a form of microphone designed not for picking up airborne sound waves, but rather to act as a transducer which picks up vibrations through solid materials and converts them into audible sound. ... Orders Subclass Apterygota Archaeognatha (bristletails) Thysanura (silverfish) Subclass Pterygota Infraclass Paleoptera (Probably paraphyletic) Ephemeroptera (mayflies) Odonata (dragonflies and damselflies) Infraclass Neoptera Superorder Exopterygota Grylloblattodea (ice-crawlers) Mantophasmatodea (gladiators) Plecoptera (stoneflies) Embioptera (webspinners) Zoraptera (angel insects) Dermaptera (earwigs) Orthoptera (grasshoppers, etc) Phasmatodea (stick insects) Blattodea (cockroaches) Isoptera (termites) Mantodea (mantids) Psocoptera...

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 throat microphone is a type of microphone that picks up sound directly through sensors in contact with the neck. ...

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 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. ... A parabolic reflector (also known as a parabolic dish or a parabolic mirror) is a reflective device formed in the shape of a paraboloid of revolution. ... The Parabolic antenna is a high-gain, reflector antenna used for radio, television and data communications, and also for radiolocation (RADAR), on the UHF and SHF frequencies. ... Sky Digital mini-dish Astros mini-dish. Special dish for up to 16 satellite positions (Ku-band) Satellite dish antenna for C-Band Satellite Dishes installed on an apartment complex A satellite dish is a type of parabolic antenna designed with the specific purpose of transmitting signals to and... To eavesdrop is to surreptitiously overhear a private conversation. ... For the band, see The Police. ... Spy and Secret agent redirect here. ...

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. The word broadcast can refer to: Broadcasting, the transmission of audio and video signals. ... Field recording is the technique for capturing the audible illustration of an environment, produced outside of a recording studio. ... There exist a number of well-developed microphone techniques used for miking musical, film, or voice sources. ...

A noise-canceling microphone is intended for noisy environments such as aircraft cockpits. They are normally installed as boom mics on headsets. They pick up environmental noise, ideally without also picking up the intended signal, with one diaphragm and electrically combine the output with the intended signal picked up with another diaphragm. In older designs, there is no active electronics involved in the cancellation technique, unlike active noise cancellation microphones. So, in the common configuration, the intended signal is voice and one diaphragm is mounted close to the mouth. The other is, often, placed behind the first, farther away from the intended signal source and electrically out of phase with the first. After combination, signals other than the voice are greatly reduced, substantially increasing intelligibility. Some noise-canceling microphones are also throat microphones. Flying machine redirects here. ...

## Connectivity

Electronic symbol for a microphone.

Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... ...

### Connectors

The most common connectors used by microphones are:

• Male XLR connector on professional microphones
• ¼ inch mono phone plug on less expensive consumer microphones
• 3.5 mm (Commonly 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 1/4 inch TRS (tip ring sleeve), 5-pin XLR, or stereo mini phone plug (1/8 inch TRS) on some stereo microphones. Some lavalier 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 studios. XLR3 cable connectors, female on left and male on right The XLR connector is a rugged electrical connector design. ... 2. ... It has been suggested that this article or section be merged with Jack plug. ... Note: USB may also mean upper sideband in radio. ...

### Impedance matching

Microphones have an electrical characteristic called impedance, measured in ohms (Ω), that depends on the design. Typically, the rated impedance is stated.[4] Low impedance is considered under 600 Ω. Medium impedance is considered between 600 Ω and 10 kΩ. High impedance is above 10 kΩ.
Most professional microphones are low impedance, about 200 Ω or lower. 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; the other is that long high-impedance cables tend to pick up more hum (and possibly radio-frequency interference (RFI) as well). However, some devices, such as vacuum tube guitar amplifiers, have an input impedance that is inherently high, requiring the use of a high impedance microphone or a matching transformer. 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. The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ... A multimeter can be used to measure resistance in ohms. ... Spectrum of mains hum at 60 Hz Electric hum, mains hum, or power line hum is an audible oscillation at the frequency of the mains alternating current, which is usually 50 or 60 hertz depending on the local electric utility configuration (see Mains electricity). ... Radio Frequency Interference (RFI) is electromagnetic radiation which is emitted by electrical circuits carrying rapidly changing signals, as a by-product of their normal operation, and which causes unwanted signals (interference or noise) to be induced in other circuits. ...

To get the best sound in most cases, the impedance of the microphone must be distinctly lower (by a factor of at least five) than that of the equipment to which it is connected. 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. There are transformers (confusingly called matching transformers) that adapt impedances for special cases such as connecting microphones to DI units or connecting low-impedance microphones to the high-impedance inputs of certain amplifiers, but microphone connections generally follow the principle of bridging (voltage transfer), not matching (power transfer). In general, any XLR microphone can usually be connected to any mixer with XLR microphone inputs, and any plug microphone can usually be connected to any jack that is marked as a microphone input, but not to a line input. This is because the signal level of a microphone is typically 40 to 60 dB lower (a factor of 100 to 1000) than a line input. Microphone inputs include the necessary amplification to handle these very low level signals. Certain ribbon and dynamic microphones, which are most linear when operated into a load of known impedance, are exceptions[clarify].[5] A DI unit or DI box is an electronic device designed for connecting a piece of equipment with an electronic audio output to a standard microphone or line level input. ...

### 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. Established in 1948, the Audio Engineering Society (AES) draws its membership from amongst engineers, scientists, manufacturers and other organisations and individuals with an interest or involvement in the professional audio industry. ...

## Measurements and specifications

A comparison of the far field on-axis frequency response of the Oktava 319 and the Shure SM58

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. For other uses, see Decibel (disambiguation). ...

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 to 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 and 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 A-weighting curve dB(A) or dBA stands for decibels adjusted. ... The pascal (symbol: Pa) is the SI unit of pressure. ... The ITU-R 468-weighting curve (originally defined in CCIR recommendation 468) is widely used when measuring noise in audio systems, especially in the UK, Europe, and former countries of the British Empire such as Australia and South Africa. ... Neumann (German for new man, and one of the 20 most common German surnames) may refer to: Alfred Neumann, German playwright Alfred Neumann, East German politician Bernhard Neumann, German-born mathematician Bernard de Neumann, (aka Bernhard von Neumann), British, (The Mathematician in a series of portraits by John Wonnacott) - Mathematician... RÃ¸de Microphones, based in Silverwater, New South Wales, Australia, is a company that designs and manufactures Microphones and related accessories. ...

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. It has been suggested that this article or section be merged into Sound pressure. ... The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental. ... It has been suggested that this article or section be merged into sound pressure level. ...

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. The sensitivity of an electronic device, a communications system receiver, or detection device, PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria. ...

## Measurement microphones

Some microphones are intended for use as standard measuring microphones for the testing of speakers and checking noise levels etc. These are calibrated transducers and will usually be supplied with a calibration certificate stating absolute sensitivity against frequency. Image File history File links Question_book-3. ...

### Microphone calibration techniques

Measurement microphones are used in sound analysis meters, noise measurement (in public nuisance abatement contexts), acoustic laboratories, loudspeaker design and quality control work, etc. They are made with greater care than most microphones and generally come with a calibration certificate. However, 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. 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 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 SPL in the chamber can be calculated from 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. This article covers adiabatic processes in thermodynamics. ...

#### Reciprocal method

This method relies on the reciprocity of one or more microphones in a group of 3 to be calibrated. It can still be used when only one of the microphones is reciprocal (exhibits equal response when used as a microphone or as a loudspeaker).

## Microphone array and array microphones

Main article: Microphone array

A microphone array is any number of microphones operating in tandem. There are many applications: A microphone array is an set of multiple microphones operating in tandem. ... Look up Tandem in Wiktionary, the free dictionary. ...

• Systems for extracting voice input from ambient noise (notably telephones, speech recognition systems, hearing aids)
• Surround sound and related technologies
• Locating objects by sound: acoustic source localization, e.g. military use to locate the source(s) of artillery fire. Aircraft location and tracking.
• High fidelity original recordings

## 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. A stop, plosive, or occlusive is a consonant sound produced by stopping the airflow in the vocal tract. ... Shure Incorporated is a consumer and professional audio electronics corporation. ... The Shure SM58 is a professional cardioid dynamic microphone. ...

### 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 optional accessory windscreen is the A2WS from Shure, one of which is fitted over each of the two SM57s used on the United States Presidential lectern.[7]. 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. The Shure SM57 is a very common professional cardioid dynamic microphone in live and studio applications. ...

### Pop filters

Pop filters or pop screens are used in controlled studio environments to keep plosives down when recording. A typical pop filter is composed of one or more layers of acoustically semi-transparent 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 windscreen increases the closer a vocalist brings the microphone to their lips. Singers can be trained to soften their plosives, in which case they don't need a windscreen for any reason other than wind. A pop filter or pop shield is an anti-pop noise protection filter for microphones. ... A pop filter or pop shield is an anti-pop noise protection filter for microphones. ...

### 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 a furry material with long soft hairs and a weave that is as acoustically transparent as possible. The hair acts as a filter to any wind turbulence hitting the blimp. A synthetic furry cover can reduce wind noise by a further 12 dB.[8]. In 1974, Joseph Flaherty, then vice-president at CBS Inc. ...

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.

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Image File history File links Nuvola_apps_ksim. ... For the Marty Friedman album, see Loudspeaker (album) An inexpensive low fidelity 3. ... A hydrophone is a sound-to-electricity transducer for use in water or other liquids, analogous to a microphone for air. ... This article needs to be cleaned up to conform to a higher standard of quality. ... An ionophone is a device in which sound is produced by matter in the plasma state. ... There exist a number of well-developed microphone techniques used for miking musical, film, or voice sources. ... The A-weighting curve is one of a family of curves defined in IEC179 and various other standards for use in sound level meters. ... Button microphone from Western Electric telephone. ... The ITU-R 468-weighting curve (originally defined in CCIR recommendation 468) is widely used when measuring noise in audio systems, especially in the UK, Europe, and former countries of the British Empire such as Australia and South Africa. ... In electrical engineering or audio, the nominal impedance of an input or output is the equivalent impedance of all of the output or input circuitry of a device lumped into one (imaginary) component. ... Impedance matching is the practice of attempting to make the output impedance of a source equal to the input impedance of the load to which it is ultimately connected, usually in order to maximize the power transfer and minimize reflections from the load. ... It has been suggested that this article or section be merged into Sound pressure. ... A wireless microphone, as the name implies, is a microphone without a physical cable connecting it directly to the sound recording or amplifying equipment with which it is associated. ... XLR3 cable connectors, female on left and male on right The XLR connector is a rugged electrical connector design. ...

## References

1. ^ "Electret Microphone Turns 40"
2. ^ "Local firms strum the chords of real music innovation" (February 8, 2008). Mass High Tech: The Journal of New England Technology.
3. ^ http://www.tonmeister.ca/main/textbook/node473.html Introduction To Sound Recording Geoff Martin
4. ^ International Standard IEC 60268-4
5. ^ Robertson, A. E.: "Microphones" Illiffe Press for BBC, 1951-1963
6. ^ International Standard IEC 60268-4
7. ^ Shure - Accessories - A2WS Microphone Windscreens
8. ^ http://www.rycote.com/products/windshield/

is the 39th day of the year in the Gregorian calendar. ... 2008 (MMVIII) is the current year, a leap year that started on Tuesday of the Anno Domini (or common era), in accordance to the Gregorian calendar. ...

Results from FactBites:

 Microphone - Wikipedia, the free encyclopedia (5399 words) Electret microphones have existed since the 1920s but were considered impractical, but they have now become the most common type of all, used in many applications from high-quality recording and lavalier use to built-in microphones in small sound recording devices and telephones. In a dynamic microphone a small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to the diaphragm. A pressure gradient microphone is a microphone in which both sides of the diaphragm are exposed to the incident sound and the microphone is therefore responsive to the pressure differential (gradient) between the two sides of the membrane.
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