Learn more about Microphone

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[edit] History

The word "microphone" (Greek mikros "small" and phone "voice" or "sound") originally referred to a mechanical hearing aid for small sounds.<ref>[1]</ref>

The invention of a practical microphone was crucial to the early development of the telephone system. 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. See also Timeline of the telephone. The main basic designs still popular are of American, British or Russian origin.

[edit] Principle of operation

An Oktava condenser microphone.

A microphone is a device made to capture waves in air, water or hard material and translate it to an electrical signal. The most common method is via a thin membrane producing some proportional 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.

[edit] Microphone varieties

[edit] Condenser or capacitor microphones

Inside the Oktava 319 condenser microphone.

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. Since 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:

<math>Q = C \cdot V</math>

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 charge across the capacitor is not maintained perfectly constant. As the capacitance changes, the charge across the capacitor changes to make the voltage drop across the capacitor equal to the bias voltage. However, the rate of this change is kept slow by using a series resistor of a very high value (of the order of 10 MΩ). Note that the time constant of a RC circuit equals the product of the resistance and capacitance.

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 adjusts itself instantaneously to reflect the change in capacitance. Within the time-frame of the capacitance change, the voltage across the capacitor never really catches up to 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 and reproduced to audio or recording. Condenser microphones are elongation receivers.

Condenser microphones range from cheap to expensive. 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. Professional microphones often sport an external power supply for reasons of quality perception. 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.<ref>http://www.rodemic.com/?pagename=Products&product=NT2000</ref>

[edit] Electret condenser microphones

Main article: Electret microphone

An electret microphone is a relatively new type of capacitor microphone invented at Bell laboratories in 1962 by Gerhard Sessler and Jim West<ref>"Electret Microphone Turns 40"</ref>, and often simply called an electret microphone. An electret is a dielectric 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 mics were once low-cost and considered low quality, the best ones can now rival capacitor mics in every respect (apart from low noise) and can even have 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.

[edit] Dynamic microphones

Dynamic microphone works via electromagnetic induction. Two types can be distinguished: the plunger coil microphone and the ribbon microphone. Dynamic microphones are velocity receivers.

[edit] Plunger coil microphones

The Shure SM57 and Beta 57A dynamic microphones

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. The frequency content of the generated signal is proportional to the perceived frequency. So a 1KHz sine wave would generate an identical frequency output from the microphone. 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. AKG D112 is for example designed for bass content rather than treble. In audio engineering several kinds of microphones are often used at the same time to get the best result.

The dynamic principle is exactly the same as in a loudspeaker, only reversed.

Dynamic microphones are robust, relatively inexpensive, and resistant to moisture, and for this reason they are widely used on-stage by singers.

[edit] Ribbon microphones

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 dynamic 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 mic horizontally, for example above cymbals, so that the rear lobe picks up only sound from the cymbals. 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. Ribbon mics give very high quality sound reproduction, and were once valued for this reason, but a good low-frequency response can be obtained only if the ribbon is suspended very loosely, and this makes them fragile. Protective wind screens can reduce the danger of damaging the ribbon, but will somewhat reduce the bass response at large miking distances.

Ribbon microphones don't require phantom power; in fact, this voltage can damage these microphones.

[edit] 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.

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.)

[edit] Piezo microphones

A 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 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.

Piezo transducers are often used as contact microphones to amplify sound from musical instruments for live performance, or to record sounds in unusual environments (underwater, for instance). Saddle mounted pickups in acoustic guitars are piezos and are mechanically connected to the strings through the saddle. This is not to be confused with magnetic coil pickups commonly visible on typical electric guitars that use steel strings. Instruments that use non-metallic strings cannot use magnetic pickups, but can use piezo transducers which pickup sound vibrations mechanically. Small acoustic microphones have also been used, but are less practical because they also pickup ambient noise and feedback. Some instruments feature hybrid combinations of the piezo and magnetic technologies. John Petrucci of Dream Theater has a signature series of typical electric guitars which also sports a piezo pickup in the saddle. A saddle is where the strings are mounted to the guitar on the opposite end of where the head and tuning pegs are.

[edit] Laser microphones

A laser microphone is an exotic application of laser technology. It consists of a laser beam that must be reflected off a glass window or another rigid surface that vibrates in sympathy with nearby sounds. This device essentially turns any vibrating surface near the source of sound into a microphone. It does this by measuring the distance between itself and the surface extremely accurately; the tiny fluctuations in this distance become the electrical signal of the sounds picked up. Usage
Laser microphones are new, very rare and expensive, and are most commonly portrayed in the movies as spying devices.

[edit] Liquid Microphones

Early versions did not produce intelligible speech, Alexander Graham Bell innovated a set of improvements in acidity and diaphram movement. Bell’s liquid transmitter consisted of a metal cup filled with a mixture of sulfuric acid and water. A sound wave causes the diaphragm to move thereby forcing the a brass tube to move up and down in the liquid. The electrical resistance between the wire and the cup is inversely proportional to the amount of wire submerged. Elisha Gray patented a version using a needle instead of the brass tube. Other minor variantions and/or improvements were made to the liquid microphone by Majoranna,Chambers,Vanni and Sykes Elisha Gray and one version was even patented by Reginald Fessenden in 1903.

These were the first microphones and are utterly obsolete now. It was with this arrangement that the famous first phone conversation between Bell and Watson took place. The liquid variable resistance was not practical as a commercial application. Other inventors soon devised superior devices.

[edit] Speakers as microphones

A loudspeaker is the exact opposite of a microphone, since it's a transducer that turns an electrical signal into sound waves. However, because 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 such applications as intercoms or walkie-talkies, where high quality and sensitivity are not needed. However, there is at least one other novel 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. This has been commercialized with the Yamaha "Subkick".[2]

If you were to connect a pair of any kind of ordinary headphones to a mic or line input, you would be able to record a stereo track. For example to a Minidisc or iPod. It might not be high fidelity but it does work to a certain degree. With the new accessiorie mic input modules for iPod 5th generation you could fold a regular small sized pair of earphones over your earlobes and get a proper stereo recording. Since the membranes are tiny in small headphones sensitivity would be usable.

[edit] 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.

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.

[edit] Microphone Polar Patterns

Common polar patterns for microphones
(microphone facing up in diagram)
Image:Omnipattern.svg Image:Cardioidpattern.svg Image:Hypercardioidpattern.svg Image:Bidirectionalpattern.svg Image:Shotgunpattern.svg
Omnidirectional Cardioid Hypercardioid Bi-directional Shotgun

A microphone's directionality or polar pattern indicates how sensitive it is to sounds arriving at different angles about its central axis. The above polar patterns represent the locus of points that produce the same signal level output in the microphone if a given sound pressure level is generated from that point. How the physical body of the microphone is oriented relative to the diagrams depends on the microphone design. For large-membrane microphones such as in the Oktava (pictured above), the upward direction in the polar diagram is perpendicular to the microphone body. For dynamic microphones such as the Shure (also pictured above), it extends from the axis of the microphone. 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.

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 about an inch (2.5 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" mikes 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 mikes also respond much less to wind noise than directional (velocity sensitive) mikes.

A unidirectional microphone is sensitive to sounds from only one direction. The diagram above illustrates a number of these patterns, with the microphone capsule being represented as a red dot. The mike 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.)

The most common unidirectional mike 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. These two patterns are commonly used as vocal or speech mikes, since they are good at rejecting sounds from other directions. Because they employ internal cavities to provide front-back delay, directional mikes tend to have more coloration than omnis, and they also suffer from low-frequency roll-off. These problems are overcome to a large extent by careful design, but only the best cardioids can begin to approach the performance of a tiny low-cost omni in terms of absolute accuracy. This is not always recognised, but is the price paid for directionality, often needed to exclude ambient reverberation wherever very close placement is impossible.

Figure 8 or bi-directional mikes receive sound from both the front and back of the element. Most ribbon microphones are of this pattern.

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 location recording of wildlife.

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; the output voltage is proportional to the difference in pressure on the front and on the back side. 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.

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(partially), their frequency response is dependent on the distance to the sound source. This is known as the proximity effect, a bass boost at distances of a few centimeters. A phenomenon related to the physics of wave and particle propagation. Low frequency audio does not exhibit the same properties as high frequency audio.

[edit] Design concerning practical application

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 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 contact microphone is designed to pick up vibrations directly from a solid surface or object, as opposed to sound vibrations carried through air. One use for this is to detect sounds of a very low level, such as those from small objects or insects. The microphone commonly consists of a magnetic (moving coil) transducer, contact plate and contact pin. The contact plate is placed against the object from which vibrations are to be picked up; the contact pin transfers these vibrations to the coil of the transducer. Contact microphones have been used to pick up the sound of a snail's heartbeat and the footsteps of ants. A portable version of this microphone has recently been developed.

A throat microphone is a variant of the contact microphone, used to pick up speech directly from the throat, around which it is strapped. This allows the device to be used in areas with ambient sounds that would otherwise make the speaker inaudible.

A parabolic microphone uses a parabolic reflector to collect and focus sound waves onto a microphone receiver, in much the same way that a parabolic antenna (e.g. satellite dish) does with radio waves. Typical uses of this microphone, which has unusually focused front sensitivity and can pick up sounds from many meters away, include nature recording, outdoor sporting events, eavesdropping, law enforcement, and even espionage. Parabolic microphones are not typically used for standard recording applications, because they tend to have poor low-frequency response as a side effect of their design.

[edit] Connectivity

[edit] Connectors

The most common connectors used by microphones are:

  • Male XLR connector on professional microphones
  • ¼ inch mono phone plug (UK "jack plug") on less expensive consumer microphones
  • 3.5 mm mono mini phone plug on very inexpensive and computer microphones

Some microphones use other connectors, such as TRS, 5-pin XLR, or stereo mini phone plug on some stereo microphones. Some lavaliers have a proprietary connector to connect them to their transmitter. Since 2005, professional-quality microphones with USB connections have begun to appear, designed for direct recording into computer-based software studios.

[edit] Impedance matching

Microphones have an electrical characteristic called impedance, measured in ohms (Ω) that depends on the design. 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. Less expensive models have an impedance of at least 600 Ω. Low-impedance microphones are preferred over high impedance on long-run cables for two reasons: one is that using a high-impedance mike with a long cable is likely to result in loss of high frequency signal; the other is that long high-impedance cables tend to pick up more hum (and possibly radio-frequency interference (RFI) as well).

To get the best sound, the impedances of the microphone and the equipment to which it is connected must match. There are transformers (called matching transformers) that adapt impedances, such as DI units. In general, any XLR microphone can be connected to any mixer with XLR inputs, and any plug microphone can be connected to any jack plug that is marked as a microphone input, but it can't be connected to a line input.

[edit] 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, mics 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 (IEC 60268-4), but very few manufacturers adhere to it.

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.

The self-noise or equivalent noise level is the sound level that creates the same output voltage as the inherent noise of the microphone. 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.

The maximum SPL (sound pressure level) the microphone can accept is measured for particular values of total harmonic distortion (THD), typically 1%. This is generally inaudible, so one can safely use the mic at this level without harming the recording. Example: "142 dB SPL peak (<1% THD)". The higher the value, the better.

The clipping level is perhaps a better indicator of maximum useable 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 mikes the clipping level may be much higher than the max SPL.

The dynamic range of a mike 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 mike converts acoustic pressure to output voltage. A high sensitivity mic creates more voltage and so will need less amplification at the mixer or recording device. This is a practical concern but 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.

[edit] 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.

[edit] Microphone calibration techniques

[edit] 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 PV = const. 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.

[edit] 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).

[edit] Microphone techniques

There exist a number of well-developed microphone techniques used for miking musical, film, or voice sources. Choice of technique depends on a number of factors, including:

  • The collection of extraneous noise. This can be a concern, especially in amplified performances, where audio feedback can be a significant problem. Alternatively, it can be a desired outcome, in situations where ambient noise is useful (hall reverberation, audience reaction).
  • Choice of a signal type: Mono, stereo or multi-channel.
  • Type of sound-source: Acoustic instruments produce a very different sound than electric instruments, which are again different from the human voice.
  • Situational circumstances: Sometimes a microphone should not be visible, or having a microphone nearby is not appropriate. In scenes for a movie the microphone may be held above the pictureframe, just out of sight. In this way there is always a certain distance between the actor and the microphone.
  • Processing: If the signal is destined to be heavily processed, or "mixed down", a different type of input may be required.

[edit] Basic techniques

There are several classes of microphone placement for recording and amplification.

  • In close miking, a directional microphone is placed relatively close to an instrument or sound source. This serves to eliminate extraneous noise, including room reverberation, and is commonly used when attempting to record a number of separate instruments while keeping the signals separate, or when trying to avoid feedback in an amplified performance. This technique was first used by Les Paul on his version of "The Tennessee Waltz" with Mary Ford.[citation needed]
  • In ambient or distant miking, a microphone — typically a sensitive one — is placed at some distance from the sound source. The goal of this technique is to get a broader, natural mix of the sound source or sources, along with ambient sound, including reverberation from the room or hall.

[edit] Stereo recording techniques

There are two features of sound that the human brain uses to place objects in the stereo sound-field between the loudspeakers. These are the relative level (or loudness) difference between the two channels Δ L, and the time-delay difference in arrival times for the same sound in each channel Δ t. The "interaural" signals (binaural ILD and ITD) at the ears are not the stereo microphone signals which are coming from the loudspeakers, and are called "interchannel" signals (Δ L and Δ t). These signals are normally not mixed. Loudspeaker signals are different from the sound arriving at the ear. See the section "Binaural recording for earphones".

[edit] Conventional stereo recording for loudspeakers

The following microphone techniques can be used to capture the live "soundstage":

  • The X-Y technique involves the coincident placement of two directional (cardioid) microphones. When two directional microphones are placed coincidentally, typically at a 90° angle (or greater) to each other (typically with each microphone pointing to a side of the soundstage), a stereo effect is achieved simply through intensity differences between the sound entering each microphone. Due to the lack of time-of-arrival stereo information, the stereo effect in X-Y recordings has less ambience. The main advantage is that the signal is mono-compatible, i.e., the signal is suitable for playback on non-stereo devices such as AM radio. If two bi-directional (figure 8) microphones are used instead of cardioid microphones, this technique is known as a Blumlein pair . Angles for microphones are: 90° - bidirectional, 131° - cardioid, 105° - hypercardioid, 115° - supercardioid.
  • The Middle-Side (M-S) array technique is a special case of X-Y and uses a directional cardioid or an omnidirectional pressure microphone (M - middle microphone) and a bidirectional (figure-8) microphone (S - side microphone), placed at a 90° angle to each other with the directional microphone facing the soundstage. The outputs of these microphones are mixed in such a way as to generate sum and difference signals between the outputs. The S signal is added to the M for one channel, and is subtracted (by reversing phase and adding) to generate the other channel. M-S has two advantages: when the stereo signal is combined into mono, the signal from the S microphone cancels out entirely, leaving only the mono recording from the directional M microphone; additionally, M-S recordings can be "remixed" after recording to alter or even remove the stereo spread. The M-S technique with an omnidirectional M microphone is equivalent to X-Y with two cardioids at a 180° angle.
  • Near-coincident recording is a variant of the X-Y technique and incorporates interchannel time delay by placing the microphones several inches apart. The ORTF stereo technique of the Office de Radiodiffusion Télévision Française (Radio France), calls for a pair of cardioid microphones placed 17 cm apart at an angle of 110°. In the NOS stereo technique of the Nederlandse Omroep Stichting (Holland Radio), the angle is 90° and the distance is 30 cm. The choice between one and the other depends on the recording angle of the microphone system, not on the distance to and the width of the sound source. This technique leads to a realistic stereo effect and has reasonable mono-compatibility. These interchannel signals have nothing to do with interaural signals which come only from artificial head recordings. Even the spacing of 17 cm has nothing to do with human ear distance. The ORTF and NOS engineers did not think in those terms, because this microphone system was developed for a set of stereo loudspeakers, not for earphones.
  • The A-B technique uses two omnidirectional microphones at a moderate distance from each other (20 centimeters up to a few meters). Stereo information consists of large time-of-arrival distances and some sound level differences. With excessively large distances, the stereo image can be perceived as somewhat unnatural, as if the left and right channel are independent sound sources without an even spread from left to right. A-B recordings are not so good for mono playback because the time-of-arrival differences can lead to certain frequency components being canceled out and others being amplified, the so-called comb-filtering effect, but the stereo sound can be really convincing. If wide A-B is used for large orchestras, the center can be filled with another microphone. Then one gets the famous "Decca tree", which has brought us many good sounding recordings.
  • The Blumlein shuffler technique uses two microphones spaced around 20 cm (head width), and these are usually, but not necessarily, omnidirectional. A special "Blumlein shuffler" circuit integrates the difference signal, before matrixing it to produce an output in which phase (time delay) information has been converted to amplitude difference. This is a purist technique for providing true stereo from binaural capture, permitting omnidirectional microphones to be used (with their low coloration and flat low-frequency response) for true stereo. It has been little used, probably because of the lack of commercial shufflers. While offering very realistic stereo, it can emphasise low frequencies picked up from the sides unless the shuffler incorporates rolloff in the difference path. A central baffle, in the form of a foam disc suspended between the microphones, provides level separation above 2 kHz where the shuffling has to be phased out.
  • The Baffled Omnidirectional technique uses a pair of near-coincident omnidirectional microphones with an absorptive baffle between them and is closely related to binaural technique. Stereo information consists primarily of time-of-arrival differences between the microphones and intensity differences from the baffle. The Jecklin Disk, described by the Swiss radio technician Juerg Jecklin, uses of a 30 cm flat circular sound absorbing baffle arranged vertically with the faces perpendicular to the sound source. Pressure microphones are placed 16.5 cm apart, directly left and right of the disk's center. The KFM Sphere, described by Guenther Theile, consists of two pressure microphones mounted on opposite sides of a 20 cm sphere. The microphones are mounted flush with the surface and arranged with the 0-axis perpendicular to the sound source.

[edit] Binaural recording for earphones

Binaural recording is a highly specific attempt to recreate the conditions of human hearing, reproducing the full three-dimensional sound-field with earphones. Most binaural recordings use a model of a human head, with microphones placed where the ear canal would be. A sound source is then recorded with all of the stereo and spatial cues produced by the head and human pinnae with frequency dependent ILD (interaural level difference) and ITD (interaural time difference, max. (Δt) = 630 µs = 0.63 ms) ear signals. A binaural recording is usually only somewhat successful, in addition to being highly inconvenient. For one thing, it tends to work well only when played back directly into the ear canal, via headphones (no speakers), as other methods of playback add additional spatial cues. Furthermore, as all heads and pinnae are different, a recording from one "pair of ears" will not always sound correct to another person. Also, headphones have a frequency response that compensates for the fact that the reflections from the pinnae, head and shoulders strongly affect the frequency spectrum, with the assumption that a recording is taken with a flat frequency spectrum. Introducing the spectral distortion already in the binaural recording results in an unnatural frequency spectrum, even when played through headphones. Finally, as visual cues are generally much more powerful than auditory cues when determining the source of a sound, binaural recordings are not always convincing to listeners.

[edit] Surround microphone techniques

  • The Double MS Technique was developed by Chris Wittig and Neil Muncy, and uses a front-facing mid-side microphone pair of direct sound pickup and a rear MS pair facing away from the front. The rear pair is placed at or just beyond the critical distance of the room where the reverberant sound level equals the direct sound level. The matrixed outputs feed front-left, front-right, rear-left, and rear-right speakers.
  • The Surround Ambience Microphone Array was developed by Gunther Theile of the Institute for Rundfunktechnik (IRT). Four cardioid microphones are placed 90 degrees to each other and 21 to 25cm apart. No center channel is described.
  • The Spider Microphone Array uses a special mike mount with five arms that radiate out from a center point, like a star. At the end of each arm is a condenser microphone aiming outward from the center. Two examples: The Microtech Gefell INA 5 uses five M930 mics in shock mounts. In the SPL Atmos 5.1/ASM 5 Surround Recording System, five Brauner condenser mikes feed a five channel mixing console, which adjusts the mic polar patterns and offers panning, bass management, and surround monitoring. SPL's Web site is www.spl-usa.com Both systems use the Ideal Cardioid Arrangement (ICA 5, ITU-775 specification), developed by Volker Henkels and Ulf Herrmann.

[edit] See also

[edit] Microphone manufacturers

[edit] External links

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[edit] References

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