Microphone

From Freepedia

Image:Oktava319-internal.jpg A microphone, sometimes called a mic (pronounced "mike"), is a transducer 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.

Contents

Microphone principles

Image:Oktava319.jpg In a capacitor microphone, also known as a condenser 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.):

<math>C \propto \frac{A}{d}</math>

Capacitor microphones can be expensive and require a power supply, but give a high-quality sound signal and are used in laboratory and studio recording applications.

A foil electret microphone is a relatively new type of condenser microphone invented at Bell laboratories in 1962, and often simply called an electret microphone. An electret is a dielectric material that has been permanently electrically charged or polarised. Electret microphones have existed since the 1920s but were considered impractical, but have now become the most common type of all, used in many applications from high-quality public address to built-in microphones in small sound recording devices. Unlike other condenser microphones they require no polarising voltage, but normally contain an integrated preamplifier which does require power (often incorrectly called polarizing power or bias). They are frequently phantom powered in sound reinforcement applications.

In the dynamic microphone a small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to the diaphragm. When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying current in the coil (See electromagnetic induction). Dynamic microphones are robust and relatively inexpensive, and are used in a wide variety of applications.

In ribbon microphones a thin, usually corrugated metal ribbon is suspended in a magnetic field: vibration of the ribbon in the magnetic field generates a changing current. Basic ribbon microphones detect sound in a bidirectional pattern because the ribbon, which is open to sound both front and back, responds to the pressure gradient rather than the sound pressure. This characteristic is useful in such applications as radio and television interviews, where it cuts out much extraneous sound. 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.

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 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 (lose contact). Since the voltage across a conductor is proportional to its resistance, the voltage across the capsule varies according to the sound pressure.

A piezo microphone uses the phenomenon of piezoelectricity — the tendency of some materials to produce a voltage when subjected to pressure — to convert vibrations into an electrical signal. This type of microphone is often used to mic acoustic instruments for live performance, or to record sounds in unusual environments (underwater, for instance.)

A contact microphone uses a moving-coil-type electroacoustic transducer to pick up vibrations via a physical medium, as opposed to sound vibrations carried through air. To this end, it is capable of detecting sounds of a very low level (when carried through air), such as those from small objects or insects. The microphone consists of a magnetic circuit, coil, contact plate and contact pin. The contact pin is attached to the coil via the contact plate and is the mechanism that responds to vibration. 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 laser microphone is an exotic application of laser technology. It consists of a laser beam that must be reflected off a glass window or any rigid surface that vibrates in sympathy with nearby sounds. The mic essentially measures the distance between itself and the surface very accurately in order to turn any resonant surface into a microphone. Laser microphones are new, very rare and expensive, and are most commonly portrayed in the movies as spying devices.

Other microphone terms

A lavalier (or lav) is a small mono condenser microphone used for television, theatre, and public speaking applications, in order to allow hands-free operation. They are most commonly provided with small clips for attaching 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 work), or directly to the mixer (for stationary applications).

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.

Directionality

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 polar pattern represents 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.

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 you the best omni characteristics at high frequencies.

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 top of the diagram is the front of the mic. The sound intensity for a particular frequency is plotted for angles radially from 0–360°. (Professional diagrams show these scales and would include multiple plots at different frequencies. These diagrams just provide an overview of the typical shapes and their names.)

The most common unidirectional mic 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 mics, since they are good at rejecting sounds from other directions.

Figure 8 or bi-directional mics 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 directional microphones are (partially) pressure gradient transducers, their sensitivity 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.

Measurements and specifications

Image:Oktava319vsshuresm58.png

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.

A frequency response diagram plots the sound intensity 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: "20 Hz–20 kHz ±3 dB". This is interpreted as a (mostly) linear plot between the stated frequencies, with variations in amplitude of no more than 3 dB plus or minus. 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 dBA, which is a decibel scale frequency-weighted for how the ear hears. It may also state the air pressure at which the measure was taken, for example: "15 dBA at 20 µPa". The lower the number the better.

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 dynamic range of a mic 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 mic 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. Unfortunately there are two common measures. The international standard is made in mV per pascal at 1 kHz. A higher value indicates greater sensitivity. The older American method is made at 1 V/Pa and measured in dB, resulting in a negative value. Again, a higher value indicates greater sensitivity, so −60 dB is more sensitive than −70 dB.

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 is kept 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.

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.

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 Δ L, 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 for earphones".

Conventional stereo recording for loudspeakers

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+ 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 AM 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 a = 17 cm apart at an angle of α = 110 degrees. In the NOS stereo technique of the Nederlandse Omroep Stichting = Holland Radio, the angle is 90 degrees and the distance is a = 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 a = 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 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.

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 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. (Δ t) = 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.

See also

Microphone manufacturers

External links

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