Technically speaking, a microphone is a device that changes sound energy into electrical energy and, though relatively simple, forms the basis of all voice communication, enabling sound to be transmitted and recorded by electronic means. There are four basic types of microphones in use.
The carbon microphone, the oldest and most widely used type, is familiar as the telephone transmitter. Credit for the invention of the microphone can therefore be attributed to Alexander Graham Bell as an indirect achievement of his pioneer work on the telephone. Though restricted in sound quality, the carbon microphone provides high intelligibility for accurate voice communication, can be manufactured at low cost, and is rugged in nature. It consists of a metal diaphragm placed against an insulated cup that contains loosely packed carbon granules. Sound vibrations striking the diaphragm alternately increase and decrease pressure on the granules, producing a variation in the current because of a proportional change in resistance of the mass of granules. This variation in current corresponds to variations in sound applied to the diaphragm, and the resulting alternating voltage may be amplified and used to provide the signal necessary for the desired communication.
The crystal microphone, which is widely used in public-address systems and home recording work, depends for its action on the piezoelectric effect of certain crystals, most commonly Rochelle salts. The term "piezoelectric" refers to the fact that when pressure (in this instance, sound waves) is applied to the crystal in the proper direction, a proportionately varying voltage is produced between opposite faces of the crystal. The advantages of the crystal microphone are its relatively high output voltage, good sound quality, and low cost. Crystal microphones are quite sensitive to extremes of heat and humidity, however, and they do not tolerate rough handling or severe mechanical shock.
The third basic type of microphone, which depends on magnetism for the translation of sound energy into electrical energy, includes the dynamic microphone and the velocity or ribbon microphone. The dynamic microphone closely resembles the dynamic loudspeaker, found in all radio and television receivers, in principle. In fact, a two- or three-inch dynamic speaker will make a satisfactory microphone for such limited-quality use as intercommunication systems and is frequently so employed.
The dynamic microphone consists of a number of turns of wire wound in what is called a voice coil and rigidly attached to a diaphragm. This coil is suspended between the poles of a permanent magnet. Sound causes the diaphragm to vibrate, moving the coil back and forth between the poles and producing an alternating voltage proportional to the applied sound. The velocity microphone, a high-quality device widely used in commercial broadcasting and recording, is similar in principle. A thin, corrugated metallic ribbon is suspended between the poles of a permanent magnet. When vibrated by sound, the ribbon cuts the lines of force between the poles in alternating directions, generating an alternating voltage across the length of the ribbon. Since the voltage generated by both dynamic and velocity microphones is very small, much greater amplification is necessary for practical use than in the case of carbon and crystal microphones.
The fourth general variety of microphone in use is the condenser microphone, in which a tightly stretched metal disk or diaphragm is very closely positioned to a heavy, fixed metal disk. A direct current potential is applied between the disks (which are actually the plates of the condenser) through a high resistance. Any changes in capacitance, such as are caused by applied sound energy, develop charging and discharging currents through the series resistance, generating the alternating voltage output of the microphone. Condenser microphones, particularly those of European manufacture, are among the highest-quality units available, and are almost universally used in the most exacting broadcasting and recording work.