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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 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.
Kinds of microphone
In a capacitor microphone, also known as a condenser microphone, the diaphragm acts as one plate of a capacitor, and the distance changing vibrations produce changes in a voltage maintained across the capacitor plates. 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 polarising power). 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, corrugated metal ribbon is suspended in a magnetic field: vibration of the ribbon in the magnetic field generates a changing voltage. Ribbon microphones detect sound in a bidirectional pattern: this characteristic is useful in such applications as radio and television interviews, where it cuts out much extraneous sound.
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.)
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 omidirectional 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.
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.
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.)
- 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.
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 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".
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+ 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 TÚlÚvision Franšaise = 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 signals have nothing to do with interaural signals which come only from artificial head recordings.
- 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.
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 Ás = 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.
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