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A microphone, sometimes referred to as a '''mike''' or '''mic''' (both or Sensor that converts Sound into an Electrical Signal . U87 capacitor microphone]] Microphones are used in many applications such as Telephone s, Tape Recorder s, Hearing Aid s, Motion Picture production, live and recorded Audio Engineering , in Radio and Television broadcasting and in computers for recording voice, VoIP , and for non-acoustic purposes such as ultrasonic checking. HISTORY 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 . PRINCIPLE OF OPERATION A microphone is a device made to capture waves in air, water ( Hydrophone ) or hard material and translate them into 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. The piezoelectric microphone is now largely obsolete. However, piezoelectric pickups are still the most common device for amplifying acoustic guitars, usually placed under the guitar's saddle or embedded in the bridge. MICROPHONE VARIETIES Condenser, capacitor or electrostatic microphones Technology 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. There are two methods of extracting an audio output from the transducer thus formed. They are known as DC biased and RF (or HF) condenser microphones. = DC-biased microphone operating principle The plates are Bias ed with a fixed charge (''Q''). The Voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation: : where Q = charge in Coulomb s, C = capacitance in Farad s and V = potential difference in Volt s. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor. (See Capacitance for details.) A nearly constant charge is maintained on the capacitor. As the capacitance changes, the charge across the capacitor does change very slightly, but at audible frequencies it is sensibly constant. The capacitance of the capsule and the value of the bias resistor form a filter which is highpass for the audio signal, and lowpass for the bias voltage. 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. The voltage across the capacitor varies above and below 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 for performance or recording. = RF condenser microphone operating principle In a DC-biased condenser microphone, a high capsule polarisation voltage is necessary. In contrast, RF condenser microphones use a comparatively low RF voltage, generated by a low-noise oscillator. The oscillator is frequency modulated by the capacitance changes produced by the sound waves moving the capsule diaphragm. Demodulation yields a low-noise audio frequency signal with a very low source impedance. This technique achieves better low frequency response - in fact it will theoretically operate down to DC. The RF biasing process results in a lower electrical impedance capsule, a useful byproduct of which is that RF condenser microphones can be operated in damp weather conditions which would effectively short out a DC biased microphone. The Sennheiser "MKH" series of microphones use the RF biased technique. Usage Condenser microphones span the range from cheap throw-aways to high-fidelity quality instruments. 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. Electret condenser microphones See Also: Electret microphone An electret microphone is a relatively new type of capacitor microphone invented at is a Ferroelectric material that has been permanently Electrically Charged or ''polarized''. The name comes from ''electr''ostatic and magn''et''; 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 microphones were once low-cost and considered low quality, the best ones can now rival capacitor microphones in every respect and can even offer 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 Power ed 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. Dynamic microphones Dynamic microphones work via Electromagnetic Induction . They are robust, relatively inexpensive and resistant to moisture, and for this reason they are widely used on-stage by singers. There are two basic types: the moving coil microphone and the ribbon microphone. Moving coil microphones and Beta 57A dynamic microphones]] = Technology 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 . 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. Ribbon microphones See Also: Ribbon microphone 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 moving coil 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 microphone horizontally, for example above cymbals, so that the rear lobe picks up only sound from the cymbals. Crossed figure 8, or Blumlein stereo recording is gaining in popularity, and the figure 8 response of a ribbon microphone is ideal for that application. 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. Older ribbon microphones, some of which still give very high quality sound reproduction, and were once valued for this reason, but a good low-frequency response could only be obtained only if the ribbon is suspended very loosely, and this made them fragile. Modern ribbon materials have now been introduced that eliminate those concerns. Protective wind screens can reduce the danger of damaging a vintage ribbon, and also reduce plosive artifacts in the recording. Properly designed wind screens produce negligible treble attenuation. In common with other classes of dynamic microphone, ribbon microphones don't require Phantom Power ; in fact, this voltage can damage some older ribbon microphones. (There are some new modern ribbon microphone designs which incorporate a preamplifier and therefore do require phantom power, also there are new ribbon materials available that are immume to wind blasts and phantom power.) Carbon microphones See Also: 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.) Crystal (Piezo) microphones Technology A crystal 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. Usage Crystal microphones used to be commonly supplied with Vacuum Tube (valve) equipment such as domestic tape recorders. Their high output impedance matched well to the high input impedance of the vacuum tube input stage (10 Megohms was not uncommon). They were difficult to match to early Transistor equipment and were quickly supplanted by dynamic microphones for a short while, and later small eletret condenser devices. The high impedance of the crystal microphone made it very susceptable to handling noise, partly from the microphone itself, but also from the handling of the connecting cable. Piezo transducers are often used as Contact Microphone s to amplify sound from acoustic musical instruments, or to record sounds in unusual environments (underwater, for instance). Saddle Mounted Pickups on Acoustic Guitar s are generally piezos that are mechanically connected to the strings through the saddle. This type of microphone is not to be confused with Magnetic Coil Pickups commonly visible on typical Electric Guitar s. Laser microphones Usage Laser microphones are new, very rare and expensive, and are most commonly portrayed in movies as spying devices. Liquid microphones See Also: Water microphone Technology Early microphones did not produce intelligible speech, until Alexander Graham Bell made improvements including a variable resistance microphone/transmitter. Bell’s liquid transmitter consisted of a metal cup filled with water with a small amount of sulfuric acid added. A sound wave caused the diaphragm to move, forcing a needle to move up and down in the water. The electrical resistance between the wire and the cup was then inversely proportional to the size of the water meniscus around the submerged needle. Elisha Gray filed a Caveat for a version using a brass rod instead of the needle. Other minor variations and improvements were made to the liquid microphone by Majoranna, Chambers, Vanni, Sykes, and Elisha Gray, and one version was even patented by Reginald Fessenden in 1903. Usage These were the first working microphones, but they were not practical for commercial application and are utterly obsolete now. It was with a liquid microphone that the famous first phone conversation between Bell and Watson took place. Other inventors, especially Thomas Edison, soon devised superior microphones. MEMS microphones The MEMS microphone is also called a microphone chip or silicon microphone. The pressure-sensitive diaphragm is etched directly on a silicon chip by MEMS techniques, and is usually accompanied with integrated preamplifier. Most MEMS microphones are modern embodiments of the standard condenser microphone. Often MEMS mics have a built in ADC on the same CMOS chip making the chip a digital microphone and easily integrated into modern digital products. Major manufacturers using MEMS manufacturing for silicon microphones are Akustica (AKU200x), Infineon (SMM310 product), Knowles Electronics and Sonion MEMS. Speakers as microphones A Loudspeaker , a transducer that turns an electrical signal into sound waves, is the functional opposite of a microphone. Since 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 Intercom s or Walkie-talkie s, 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". {Link without Title} 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. MICROPHONE POLAR PATTERNS Regarding directionality, omnidirectional microphones are pressure transducers, whereas all others are '''pressure gradient''' transducers or a combination between the two. Common polar patterns for microphones (Microphone facing top of page in diagram, parallel to page): |
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