A microphone is a device that translates sound vibrations in the air into electronic signals or scribes them to a recording medium. Microphones enable many types of audio recording devices for purposes including communications of many kinds, as well as music and speech recording.
Frequently, microphones are designed for a given purpose. One of the main considerations, aside from the type of device, is what is being recorded. Directionality of microphones is one such consideration in microphone design. Omnidirectional microphones are suited to recording all sounds in an area but poor for focusing on a single subject amongst background noise. Directional, bidirectional and shotgun microphones are suitable for interviews. However, the same effect is often achieved with two unidirectional devices, such as cardioid microphones.
The first electronic microphone was a liquid-based mechanism that used a diaphragm attached to a current-charged needle in a diluted sulfuric acid solution. This early microphone was not able to reproduce intelligible speech.
Since then, there have been and continue to be many types of microphone technologies, some of which are described below:
The first viable diaphragm microphones used a sheet of metal attached to a needle which scribed transmitted vibrations to a foil recording medium. When the needle moved over the foil after the scribing, the process was reversed and translated back to sounds again through the same metal diaphragm.
Carbon microphones were used in telephones for over one hundred years. These devices used the variable electrical resistance of carbon under pressure between metal plates to translate airborn sound waves into a electrical signal.
Condenser microphones use the varied electrical charge of a capacitor with a diaphragm acting as a one of the plates of a capacitor. The plate is biased by a fixed charge, often from a battery which also amplifies the signal. Capacitance varies with movement of the diaphragm in relation to the other plates providing the means to record the sound in electrical signal. Electret microphones used in cell phones and computers are a type of condenser microphone that use a polarized ferroelectric material.
Crystal microphones use a piezoelectric crystal that generates small amounts of electricity under compression of a diaphragm to provide the recording signal.
Dynamic microphones use a coil suspended in a magnetic field that may be attached to multiple membranes for extended frequency response. They use electro-magnetic induction to produce the signal. These microphones are well suited to stage performance.
Laser microphones use the vibrations of surfaces affected by sound waves to capture sound at distances. The lasers return at different angles due to the vibration and these changes are interpreted and translated into sound waves.
Due to similarities in function, microphones can operate as speakers. Generally, microphone-derived speakers are suited to high-frequency sounds like the STC company's microphone derived super-tweeter. Speakers can also operate as microphones, although they are generally suited for the capture of low frequencies.
Acmeschool provides an introduction to microphones: