Microphone

A microphone, sometimes called a mic (pronounced "mike"), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, tape recorders, hearing aids, motion picture production and in radio and television broadcasting.

Microphone principles

In a capacitor microphone, also known as a condenser microphone, the diaphragm acts as one plate of a capacitor, and the vibrations produce changes in the distance between the plates. Since the plates are biased with a fixed charge, the voltage maintained across the capacitor plates changes with the vibrations in the air. (Q = C·V, where Q = charge in coloumbs, C = capacitance in farads and V = potential difference in volts. The capacitance of the plates is inversely proportional to the distance between them. See capacitance for details.) Capacitor microphones can be expensive and require a power supply, but give a high-quality sound signal and are used in laboratory and studio recording applications.

Related Topics:
Capacitor - Bias - Voltage - Capacitance - Power supply

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A foil electret microphone is a relatively new type of condenser microphone invented at Bell laboratories in 1962, and often simply called an electret microphone. An electret is a dielectric material that has been permanently electrically charged or polarised. Electret microphones have existed since the 1920s but were considered impractical, but have now become the most common type of all, used in many applications from high-quality public address to built-in microphones in small sound recording devices. Unlike other condenser microphones they require no polarising voltage, but normally contain an integrated preamplifier which does require power (often incorrectly called polarizing power or bias). They are frequently phantom powered in sound reinforcement applications.

Related Topics:
Bell laboratories - 1962 - Electret - Dielectric - Electrically charged - Public address - Sound recording - Preamplifier - Phantom power

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In the dynamic microphone a small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to the diaphragm. When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying current in the coil (See electromagnetic induction). Dynamic microphones are robust and relatively inexpensive, and are used in a wide variety of applications.

Related Topics:
Dynamic microphone - Induction coil - Magnetic field - Permanent magnet - Diaphragm - Current - Electromagnetic induction

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In ribbon microphones a thin, usually corrugated metal ribbon is suspended in a magnetic field: vibration of the ribbon in the magnetic field generates a changing current. Basic ribbon microphones detect sound in a bidirectional pattern because the ribbon, which is open to sound both front and back, responds to the pressure gradient rather than the sound pressure. This characteristic is useful in such applications as radio and television interviews, where it cuts out much extraneous sound. Other directional patterns are produced by enclosing one side of the ribbon in an acoustic trap or baffle, allowing sound to reach only one side.

Related Topics:
Ribbon microphone - Current - Interview

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A carbon microphone, formerly used in telephone handsets, is a capsule containing carbon granules pressed between two metal plates. A voltage is applied across the metal plates, causing a current to flow through the carbon. One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon. The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules to change (lose contact). Since the voltage across a conductor is proportional to its resistance, the voltage across the capsule varies according to the sound pressure.

Related Topics:
Telephone - Capsule - Carbon

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A piezo microphone uses the phenomenon of piezoelectricity ? the tendency of some materials to produce a voltage when subjected to pressure ? to convert vibrations into an electrical signal. This type of microphone is often used to mic acoustic instruments for live performance, or to record sounds in unusual environments (underwater, for instance.)

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A contact microphone uses a moving-coil-type electroacoustic transducer to pick up vibrations via a physical medium, as opposed to sound vibrations carried through air. To this end, it is capable of detecting sounds of a very low level (when carried through air), such as those from small objects or insects. The microphone consists of a magnetic circuit, coil, contact plate and contact pin. The contact pin is attached to the coil via the contact plate and is the mechanism that responds to vibration. Contact microphones have been used to pick up the sound of a snail's heartbeat and the footsteps of ants. A portable version of this microphone has recently been developed.

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A laser microphone is an exotic application of laser technology. It consists of a laser beam that must be reflected off a glass window or any rigid surface that vibrates in sympathy with nearby sounds. The mic essentially measures the distance between itself and the surface very accurately in order to turn any resonant surface into a microphone.

Related Topics:
Laser microphone - Laser

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Laser microphones are new, very rare and expensive, and are most commonly portrayed in the movies as spying devices.

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