Sound Recording Solutions
Sound Card Recorder
Powerful voice activated microphone recorder for Windows. Click here to learn more.Microphone
A microphone, sometimes referred to as a mike or mic (pronounced "mike"), is an acoustic to electric transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, tape recorders, hearing aids, motion picture production, live and recorded audio engineering, in radio and television broadcasting and in computers for recording voice, VoIP and numerous other computer applications. All microphones capture sound waves with a thin, flexible diaphragm (or ribbon in the case of ribbon microphones). The vibrations of this diaphragm are then converted by various methods into an electrical signal that is an analog of the original sound. Most microphones in use today use electromagnetic generation (dynamic microphones), capacitance change (condenser microphones) or piezoelectric generation to produce the signal from mechanical vibration. In a dynamic microphone 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 vibrations move the diaphragm, When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying current in the coil (See electromagnetic induction). The 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. They tend to have a poor low-frequency response, which is advantageous for reducing handling noise as a vocal mic, but tends to exclude them from other uses. 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 (also called a figure-of-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 ceiling. 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 can give very high quality, and were once valued for this reason, but a good low-frequency response can only be obtained if the ribbon is suspended very loosely, and this makes them fragile. Protective wind screens can however reduce the danger of damaging the ribbon, but will somewhat reduce the bass response at large miking distances. 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. A piezo (pronounced "pee-ay-zo" or "pie-ee-zo") 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. Piezo transducers are often used as contact microphones to amplify acoustic instruments for live performance, or to record sounds in unusual environments (underwater, for instance.) An example of this is Rochelle salt (potassium sodium tartrate), which is a piezoelectric crystal that works as a transducer both ways; it is also commonly used as a slimline loudspeaker component. 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. Laser microphones are new, very rare and expensive, and are most commonly portrayed in the movies as spying devices. 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. They are also called "velocity microphones", since the output voltage is proportional to the air particle velocity. A lavalier (or lav or lapel mike) is a small electret or dynamic microphone used for television, theatre, and public speaking applications, in order to allow hands-free operation. They are most commonly provided with small clips for attaching to collars, ties, or other 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). These miniature mics are often supplied with a choice of push-on grilles of differing lengths which provide gentle high-frequency boost by forming a resonant cavity. A peak of around 6 dB at 6-8 kHz is considered beneficial for compensating loss of clarity when chest mounted, and a peak of a few decibels at 10-15 kHz when mounted in the hair above the forehead. This method of boosting high frequencies does not worsen noise performance, as electronic equalization would do. A contact microphone is designed to pick up vibrations via a physical medium, as opposed to sound vibrations carried through air. One use for this is to detect sounds of a very low level (when carried through air), 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 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 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. Microphones have an electrical characteristic called impedance, measured in ohms (?) that depends on the design. Low impedance is considered under 600 ?. Medium impedance is considered between 600 ? and 10 k?. High impedance is above 10 k?. Most professional microphones are low impedance, about 200 ?. Less expensive models have an impedance of at least 600 ?. 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. 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 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 top of the diagram is the front of the mic. The sound intensity for a particular frequency is plotted for angles radially from 0 to 360ø. (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 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. 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 90ø 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 betwen 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 . 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 90ø 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 180ø 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 Television Francaise (Radio France), calls for a pair of cardioid microphones placed 17 cm apart at an angle of 110ø. In the NOS stereo technique of the Nederlandse Omroep Stichting (Holland Radio), the angle is 90ø 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. 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. (?t) = 630 çs = 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.
Phone Call Recorder
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