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Microphones

From Audio Recording

A microphone, sometimes called a mic (pronounced "mike"), is a device that converts sound into an electrical signal. Microphones are used in many applications such as telephones, tape recorders, hearing aids, motion picture production and in radio and television broadcasting.

The invention of a practical microphone was crucial to the early development of the telephone system. Emile Berliner invented the first microphone on March 4, 1877, but the first useful microphone was invented by Alexander Graham Bell. Many early developments in microphone design took place in Bell Laboratories.

In all microphones, sound waves (sound pressure) are translated into mechanical vibrations in a thin, flexible diaphragm. These sound vibrations are then converted by various methods into an electrical signal which varies in voltage amplitude and frequency in an analog of the original sound. For this reason, a microphone is an acoustic wave to voltage modulation transducer.

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[edit] Kinds of microphones

The different types of microphones include Condensor Microphones, Foil Electret microphones, Dynamic Microphones, Ribbon Microphones, Carbon Microphones, Piezo Microphones, and Lav Microphones.

[edit] Directionality

Microphones are often classified according to their directionality. Depending on various aspects of a microphone's construction, it may be nearly equally sensitive to sound coming in all directions (an omnidirectional microphone), or it may be more sensitive to sound coming from a particular direction (a unidirectional microphone). Between the omnidirectional microphone and the microphone with a cardioid characteristic there should be a "wide" cardioid" (not printed here). The most common of the unidirectional type is called a cardioid microphone, because the sensitivity pattern somewhat resembles the shape of a heart; most vocal mikes are cardioid or hyper-cardioid (similar to cardioid but with a tighter area of front sensitivity and a tiny lobe of rear sensitivity.) Some microphones have more complex sensitivity patterns. Most ribbon microphones are bi-directional, receiving sound from both in front and back of the element. This type of response is also known as a figure-8 pattern, because of its shape.

Shotgun microphones, the most directional form of studio microphone, reserve most of their sensitivity for sounds directly in front of, and to a lesser extent, the rear of the microphone. Shotgun microphones also have small lobes of sensitivity to the left and right. This effect is a result of the microphone design, which generally involves placing the element inside of a tube with slots cut along the side; wave-cancellation eliminates most of the off-axis noise. Shotgun microphones are used most commonly on TV and film sets.

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 microphone with an omnidirectional characteristic 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. The result of this is that a sound wave coming from the back will lead to a signal with a sign 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 microphone with a cardioid directional characteristic 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 directional microphones are (partially) pressure gradient transducers, their sensitivity is dependent from the distance to the sound source. This effect is known as proximity effect, a bass-boost at distances of a few centimeters.

[edit] Response

Because of differences in their construction, all microphones will have their own characteristic responses to sound. This difference in response produces a non-uniform phase and frequency response. Non-omnidirectional microphones usually have a frequency response which also varies with the angle of the sound source because the directionality mechanism's effectiveness is frequency-dependent.

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.


[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.
  • 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 in order to avoid feedback in an amplified performance.
  • In ambient or distant miking, a sensitive microphone or microphone 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 reverberation from the room or hall.

[edit] Stereo recording techniques

There are two essential components that the stereo loudspeakers need to place objects (phantom sources) in the stereo sound-field between the loudspeakers. These are level difference ΔL, the relative loudness, and time-delay difference Δ t, the difference in arrival time. The "interaural" signals (binaural ILD and ITD) at the ears are not the stereo microphone technique signals which are coming from the loudspeakers, and are called "interchannel" signals (Δ L and Δ t). Do not mix these sort of signals. Loudspeaker signals are never ear signals and vice versa. Read the header "Binaural recording".

[edit] Conventional stereo recording

In most recordings on CDs, the stereo effect is a level difference that is created during the mixing process. The following techniques can be used to capture the live soundstage.

  • The X-Y technique involves the coincident placement of two directional microphones. When two directional microphones are placed coincidentally, typically at a 90 - 130 degree angle to each other (typically with each microphone pointing to a side of the sound-stage), a stereo effect is achieved simply through intensity differences of 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 radio.
  • The Mid-Side (M-S) technique is a special case of X-Y and uses a directional cardioid or an omnidirectional pressure microphone (M) and a bidirectional (figure-8) microphone (S), placed at a 90 degree angle to each other with the directional microphone facing the sound-stage. 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-degree 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 degrees. In the NOS stereo technique of the Nederlandse Omroep Stichting = Holland Radio, the angle is 90 degrees and the distance is 30 cm. The choice between one and the other depends on the recording angle of the microphone system and not on the distance to and the width of the sound source. This technique leads to a realistic stereo effect and has a 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 want to think of that, because a useful microphone system for a set of stereo loudspeakers should be developed and not for ear phones.
  • The A-B technique uses two omnidirectional microphones at an especial distance to each other (20 centimeters up to some meters). Stereo information consists in large time-of-arrival distances and some sound level differences. On playback, with too large A-B 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 other being amplified, the so-called comb-filtering effect, but the stereo sound can be really convincing. If you use wide A-B for big orchestras, you can fill the center with another microphone. Then you get the famous "Decca tree", which has brought us many good sounding recordings.
  • Baffled Omnidirectional technique uses a pair of near coincident omnidirectional microphones with an absorbtive 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 Juerg Jecklin, uses of a 30 cm flat circular 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 Gunther 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 stereo recording

Binaural recording is a highly specific attempt to recreate the conditions of human hearing, reproducing the full three-dimensional sound-field. Most binaural recordings use model of a human head, with microphones placed where the ear canal could 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. 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] Microphone Reviews

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