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microphone,device for converting sound into electrical energy, used in radio broadcasting, recording, and sound amplifying systems. Its basic component is a diaphragm that responds to the pressure or particle velocity of sound waves. The microphone, various forms of which were developed independently c.1877 by inventors Emile Berliner, David E. Hughes, and Thomas A. Edison, was first used as a telephone transmitter. The carbon microphone, which was used in the first telephones and was very popular in telephones until about 1970, contains loosely packed carbon grains. Sound makes the diaphragm vibrate, causing the grains to be compressed and released, thus changing the resistance of the microphone. That can be exploited by an associated electric circuit. Electrostatic microphones, also called condenser microphones, consist of a fixed electrode (the backplate) and a movable electrode (the diaphragm), with an air gap between them. Sound waves impinge on the diaphragm, making it vibrate, and changing the capacitance formed by the two electrodes. Electretelectret,
solid electrically insulating, or dielectric, material that has acquired a long-lasting electrostatic polarization. Electrets are produced by heating certain dielectric materials to a high temperature and then letting them cool while immersed in a strong electric field.
..... Click the link for more information. microphones, which are the most widely used microphones, have a permanently charged dielectric between the two electrodes and thus generate voltages when the electrodes vibrate. Crystal microphones generate minute voltages by the piezoelectric effectpiezoelectric effect
, voltage produced between surfaces of a solid dielectric (nonconducting substance) when a mechanical stress is applied to it. A small current may be produced as well. The effect, discovered by Pierre Curie in 1883, is exhibited by certain crystals, e.g.
..... Click the link for more information. . Both the dynamic microphone and the rarely used ribbon microphone generate voltages by electromagnetic inductioninduction,
in electricity and magnetism, common name for three distinct phenomena. Electromagnetic induction is the production of an electromotive force (emf) in a conductor as a result of a changing magnetic field about the conductor and is the most important of the
..... Click the link for more information. . For example, in the dynamic microphone, the diaphragm is attached to a light movable coil that generates a voltage as it moves back and forth between the poles of a permanent magnet.
See G. M. Ballou, Handbook for Sound Engineers (1991).
an electroacoustic instrument for converting sound vibrations into electrical oscillations. It is used in telephony, radio broadcasting, television, public-address systems, and sound recording. A distinction is made among carbon, electrodynamic, condenser, electret, piezoelectric, and electromagnetic microphones, according to principle of operation. According to their directivity they are classified as nondirectional, unidirectional (cardioid), and bidirectional.
In the powdered-carbon microphone, which was first built by the Russian inventor M. Makhal’skii in 1878 and independently by P. M. Golubitskii in 1883, a carbon or metal diaphragm is made to vibrate by the action of sound waves, thus changing the density and, consequently, the electrical resistance of the carbon powder adjoining the diaphragm in a case. As a result, the strength of the current flowing through the microphone is also changed. A pulsating current is generated and then, in the simplest case, flows along a wire to a telephone receiver and produces vibrations of the diaphragm in the latter that correspond to the vibrations of the microphone’s diaphragm. After many years of improvements in the design and electrical parameters of the powdered-carbon microphone, the carbon-cup type, which now is widely used in telephony, was developed.
In the coil electrodynamic microphone, which was invented by the American scientists E. C. Wente and A. L. Thuras in 1931, the diaphragm is made of thin polystyrene film or aluminum foil and is rigidly fastened to a coil made of fine wire positioned in an annular gap in a magnetic system. When the diaphragm vibrates because of the action of a sound wave, the turns of the coil cut the magnetic lines of force, and an electromotive force (emf) is induced in the coil, thus generating an alternating voltage on its terminals. Such a microphone is simple in design, has small dimensions, and is reliable in service.
In the ribbon electrodynamic microphone, which was invented by the German scientists E. Gerlach and W. Schottky in 1924, the coil is replaced in the magnetic field by an embossed ribbon made of very thin aluminum foil (of the order of 2 microns). This type of microphone is used mainly for the transmission of music from a studio.
In the condenser microphone, which was invented by the American scientist E. C. Wente in 1917, sound waves act on a thin metal diaphragm, thus changing the spacing—and, consequently, the capacitance—between the diaphragm and the fixed metal casing that make up the plates of a capacitor. When a DC voltage is applied to the plates, the change in capacitance produces a current through the capacitor that varies in magnitude with the sound frequencies. Such microphones are widely used in high-quality sound-recording and sound-transmission systems.
In the electret microphone, which was invented by the Japanese scientist Yoguchi in the early 1920’s and is similar to the condenser microphone in its design and principle of operation, the role of the fixed capacitor plate and the DC voltage source is played by an electret plate.
In the piezoelectric microphone, which was first built by the Soviet scientists S. N. Rzhevkin and A. I. lakovlev in 1925, the sound waves act on a plate made from a substance that has piezoelectric properties (for example, Rochelle salt) and cause electric charges to appear on its surface.
In the electromagnetic microphone the sound waves act on a diaphragm that is rigidly fastened to a steel armature that vibrates in the gap of a permanent magnet. An emf is thereby
|Table 1. Average values of main parameters of microphones|
|Type of microphone||Frequency|
at frequency of
|*Numbers and letters in parentheses refer to classes of quality (see text)|
|Carbon ........................||300–3,400 (3)*||20||1,000|
|Coil electrodynamic .................||100–10,000 (1)||12||0.5|
|Ribbon electrodynamic ...............||50–10,000 (1)||10||1|
|Condenser .........................||30–15,000 (H)||5||5|
|Piezoelectric .....................||100–5,000 (2)||15||50|
produced at the output leads of a fixed coil that is wound over the armature. Piezoelectric and electromagnetic microphones are used mainly in amateur radio apparatus and hearing aids.
A system of two identical unidirectional microphones (frequently condenser or electrodynamic microphones), with both microphones in a common housing close to each other, so that their directions of maximum sensitivity are at an angle of 90° to one another (stereophonic microphone), is used in stereophonic radio broadcasting and sound recording.
The average values of the basic parameters of microphones are shown in Table 1. (Classes of quality are indicated in parentheses: H is the highest class, 1 is the first class, 2 is the second class, and 3 is the third class.)
REFERENCESFurduev, V. V. Akusticheskie osnovy veshchaniia. Moscow, 1960.
Dol’nik, A. G., and M. M. Efrussi. Mikrofony, 2nd ed. Moscow, 1967.
A. V. NIKONOV
A microphone converts an acoustic waveform consisting of alternating high and low air pressure travelling through the air into a voltage. To do this it uses some kind of pressure or movement sensor. The simplest kind of microphone is actually very similar in construction to a loudspeaker.
The analogue electrical signal can be fed into a computer's sound card where it is amplified and sampled to convert it into a digital waveform for storage or transmission.
microphoneA device that converts sound waves into analogous electrical waves. Usually called a "mic" or "mike," it contains a flexible diaphragm composed of film or foil that vibrates as it makes contact with the sound. The diaphragm movement modulates an electrical current by various methods. In a carbon mic, used in telephones for more than a hundred years, the diaphragm alters the pressure in carbon grains, changing its resistance.
In a condenser mic, also called an "electrostatic mic" or "capacitor mic," the diaphragm changes the capacitance between itself and a metal plate, both acting as electrodes. The widely used electret mic has a charged dielectric between the electrodes that generates voltage.
Crystal and Dynamic Microphones
Crystal microphones use a piezoelectric diaphragm that produces voltage when subjected to the sound waves (mechanical pressure).
Dynamic mics, which are like speakers in reverse, use a diaphragm attached to a movable coil that generates voltage as air moves the coil between the poles of a magnet.
Unidirectional shotgun and cardioid mics aimed at a sound source eliminate much of the ambient noise, whereas omnidirectional microphones capture everything in the surrounding environment. The cardioid name comes from its heart-shaped pickup pattern. In the past, bidirectional mics were used for interviews; however, two unidirectional mics are commonly used instead.