ABSTRACT
This entry consists of seven sections: 1. Types of microphone. 2. First microphones: 1870s to 1920s. 3. The radio age. 4. The postwar boom. 5. The rock era: 1960s. 6. The golden era: 1970s. 7. Modern microphones. 1. Types of Microphone. A microphone converts sound energy into electrical energy, and the earliest microphones were of the pressure type. Over the years, there have been several basic microphone designs put to use. Carbon microphones use pellets, rods,or granules of carbon sandwiched between two electrodes. A DC current passes through the carbon, while a diaphragm vibrates, compressing the carbon, which causes a change in the electrical resistance of the carbon,creating an AC voltage component across the electrodes. Piezoelectric microphones rely on the properties of certain materials that create a voltage when they are mechanically distorted. A diaphragm vibrates, bending the piezoelectric material, creating an AC voltage. Crystal microphones use Rochelle salts,a naturally occurring piezoelectric substance. Ceramic microphones use a synthetic material which is more rugged. More recently, certain plastics, such as Kynar or PVDF, have been used. Piezoelectrics are inexpensive, but somewhat fragile. Dynamic microphones work much like an electrical generator, and essentially function as a dynamic loudspeaker in reverse. The diaphragm is attached to a coil of wire suspended between the poles of a magnet. As the diaphragm vibrates, the coil cuts the magnetic field, generating an AC voltage. Ribbon microphones work like dynamics, but use a pleated metal ribbon suspended between a horseshoe magnet’s poles, essentially functioning as a ribbon-type loudspeaker in reverse. Ribbons are more frail than dynamics and have lower output voltages. Ribbon microphones are also called velocity microphones. Condenser microphones consist of a capacitor having one fixed electrode and one movable electrode serving as the diaphragm. A DC bias voltage is applied to both electrodes. As the diaphragm electrode vibrates, the change in capacitance caused by its movement creates an AC voltage. Modern electric microphones are permanently charged, but still require a supply of electricity, usually a penlight battery, to power a circuit used to convert their very high impedance to a lower impedance to minimize noise and loss of high frequencies. “Condenser” is an older name for a capacitor; most newer designs use the term capacitor microphone. Some microphones, called “omni” models, are sensitive to sound equally in all directions, while others exhibit directionality, especially at higher frequencies. Cardioid microphones are more sensitive to sound arriving from the front, less so from the sides, and lesser still from the rear. Cardioid means “heart shaped,” and the term refers to the shape of a two-dimensional graph of its directional sensitivity. Supercardioid microphones are more directional cardioids.
