A microphone actually is a transducer which converts acoustic energy into electrical energy by allowing the acoustic energy to vibrate a diaphragm or membrane, with the vibration thereof being converted to an electrical signal indicative of the acoustic energy. Microphones are used everywhere in A/V applications. Microphones for voice input have become a necessary device. For example, a voice mail, net conference, KTV, need a microphone for inputting voices. Since multimedia computers become a popular trend in the computer development, many computer peripheral devices include a speaker, a microphone, a scanner, a digital camera, etc. Outdoor microphones are widely used in various applications by various people including television reporters, public addressers, movie makers, and motorcycle riders. Popular types of built-in type microphones to be mounted on a disk-top system personal computer or a mobile system personal computer include a non-directional condenser microphone and a boundary microphone. Miniature microphones, such as those used in hearing aids, convert acoustical sound waves into an audio signal, which is processed and sent to a receiver of the hearing aid. The receiver then converts the processed signal to acoustical sound waves that are broadcast towards the eardrum. Beam-steered microphone arrays are in common usage, as in telephone conferencing systems.

Various types of microphones are known which vary in sophistication and ability to accurately detect acoustic energy. Microphones are evaluated by various performance criteria including frequency range and response, dynamic range, sensitivity, and polar pattern or the directional response capability of the microphone. There're many types of microphones, including capacitor or condenser microphones, electret capacitor microphones, dynamic microphones, ribbon microphones, carbon microphones, piezo microphones, laser microphones, pressure gradient microphone, lavalier microphone, wireless microphone, contact microphone, throat microphone, and parabolic microphone. Capacitor/condenser and dynamic microphones are the two main types in the microphone family. The electric output of a condenser microphone is obtained on the basis of the relative displacement of an oscillator and a stator. On the other hand, a typical microphone for obtaining a signal output by the relative speed is a dynamic type microphone. Microphones can also be classified into different types according to the operational principle. The microphone types most commonly used in acoustics are based on an electrostatic or electromagnetic (a moving coil or magnet) principle, or to the piezoelectric phenomenon. A capacitor microphone is a type of electroacoustic converters which catch a mechanical displacement of a vibration plate vibrated by a sound wave as the variation of electrostatic capacitance and convert the variation of the electrostatic capacitance to an electric signal. Various kinds of capacitor microphones each capable of being miniaturized easily have been utilized as speech microphones for portable telephones. A so-called compact microphone is practically a capacitor microphone. A capacitor microphone is provided with an electrically conductive diaphragm, which vibrates with the sound. An electrically conductive back plate is typically placed parallel to the diaphragm. The capacitor microphone(s) require addition of a power supply for the drive.

Generally, a condenser microphone or capacitor microphone comprises a condenser microphone unit, a microphone signal output transmission line, a load resistor, and a power source. Condenser microphones include a flexible diaphragm or membrane and a rigid backplate that may contain one or more openings. A condenser microphone has a condenser where a thin conductive film of diaphragm and a fixed electrode are arranged in parallel, for converting the vibration of sound into an electrical signal to obtain an electrical output. Condenser or capacitor microphones are widely used in the audio, electronics and instrumentation industries.

Electret capacitor microphones are often provided with a thin diaphragm consisting of a thin metal film facing toward an opening in a metal casing and a fixed electrode. These microphones utilize the principal of a change in capacity between the diaphragm and the fixed electrode dependently on the vibration of the diaphragm due to a sound wave. An electret capacitor microphone has an electret capacitor portion including a diaphragm, and a back electrode plate disposed opposite to the diaphragm through a spacer, and an impedance conversion device for converting change of capacitance of the electret capacitor portion into change of electrical impedance. An electret is a dielectric that produces a permanent external electric field which results from permanent ordering of molecular dipoles or from stable uncompensated surface or space charge. Electrets have been the subject of study for their charge storage characteristics as well as for their application in a wide variety of devices such as acoustic transducers, electrographic devices, and photocopy machines. An electret microphone contains a preamplifier, typically a field effect transistor (FET), and voice is converted to electrical signal by capacitance. Electret microphones are used because of their low cost and good performance. In many electronic appliances (telephones, answering machines, audio cassette and mini-disc recorders, etc.) integrated microphones are provided to capture acoustic signals. Assisted-listening systems and devices, such as hearing aids, may utilize an electret microphone for converting sound energy to an electrical signal, signal processing circuitry for processing the electrical signal, and a receiver for converting the electrical signal to acoustical energy.

A microphone's directionality or polar pattern indicates how sensitive it is to sounds arriving at different angles about its central axis. Two kinds of directional microphones are in use today. Onetype of directional microphone is called a shotgun microphone which is also known as a line plus gradient microphone. Shotgun microphones typically comprise an acoustic tube that by its mechanical structure reduces noises that arrive from directions other than directly in front of the microphone along the axis of the tube. Another type of directional microphone is a parabolic dish that concentrates the acoustic signal from one direction by reflecting away other noise sources that are in a direction away from the desired direction. Directional microphones are widely used in the professional market for various applications such as news gathering, sporting events, outdoor film recording, and outdoor video recording. A condenser microphone is a non-directional microphone which is not much affected by a breath of a speaker or wind.

Microphones typically connect electrically to the electrical equipment through a conductive wire. Today a wide variety of wireless communications systems use a wireless microphone transmission apparatus for inputting a voice, converting the voice into a voice signal, modulating the voice signal with a carrier signal to generate a radio microphone signal, and transmitting the radio microphone signal carrying the voice signal. A wireless microphone utilizes a radio wave as a communication medium. Wireless microphones and microphone-receivers are generally used as a pair of one microphone and one microphone-receiver. A wireless microphone transmits electric wave containing an audio signal to a receiver unit. The receiver unit includes a tone signal detection circuit for detecting the tone signal, a noise detection circuit for detecting the noise signal, and a signal level detection circuit for detecting the signal level. Wireless microphone systems used in a concert halls, live stages or wayside recording are roughly classified into two types: hand-held microphone transmitter systems and belt pack microphone transmitter systems.

A microphone receives various kinds of noises and sounds depending on use environments. Accordingly, microphone noise rejection systems have been developed to minimize the level of background noise relative to the level of the desired information or voice signal. Noise canceling microphones are gaining more importance nowadays, especially with the development of multimedia applications and wireless communication technologies.