A device that converts sound energy into electrical energy. It forms the first element in a telephone, a broadcast transmitter, and all forms of electrical sound recording, sound reinforcement, and public address systems. It is the converse of the loudspeaker.
There are many different types of microphone: the most common types are the carbon, capacitor, crystal, electret, moving-coil, and ribbon microphones. Most types of microphone operate by converting the sound waves into mechanical vibrations that in turn produce electrical energy. The most common method is to use a thin diaphragm mechanically coupled to a suitable device. The force exerted is usually proportional to the sound pressure but in the case of the ribbon microphone it is proportional to the particle velocity.
In the moving-coil microphone a small coil is attached to the centre of the diaphragm; when the diaphragm is caused to move by sound waves in a steady magnetic flux, an e.m.f. is produced in the coil by electromagnetic induction. The e.m.f. is a function of the incident sound pressure. The moving-iron microphone operates in a similar manner but a small piece of iron is moved by the diaphragm. This induces an e.m.f. in a current-carrying coil surrounding it (compare induction microphone). Microphones in which the electrical energy is produced by the motion of a coil or conductor in a magnetic flux density are described as magnetic microphones.
In the magnetostriction and crystal microphones the sound pressure is converted into electrical pressure by direct deformation of suitable magnetostrictive or piezoelectric crystals. In the glow-discharge microphone the current of the glow discharge is modulated by the sound waves.
Specially shaped microphones have been designed. The ear microphone is specially shaped to fit into the human ear; the lip microphone is designed to be held close to the lips and thus cut down on extraneous external sounds. The throat microphone is shaped to be worn on the throat and responds directly to vibrations of the larynx, thus cutting out background noise.
Most microphones have strong directional properties that provide improved rejection of extraneous noise (see cardioid microphone; figure-eight microphone; hypercardioid microphone; omnidirectional microphone). Their usual lack of sensitivity is not a great disadvantage since the output from the microphone is usually amplified and the directional properties tend to minimize background noise. Good quality sound reproduction is usually achieved but resonance in the mechanical system must be avoided. This is usually done by making the resonant frequency of the moving parts either much higher or much lower than the required operating sound-frequency range.