Microphones

A microphone, sometimes called a mic (pronounced "mike"), is a transducer that converts sound into an electrical signal. more...

Microphones are used in many applications such as telephones, tape recorders, hearing aids, motion picture production, live and recorded audio engineering, and in radio and television broadcasting.

The invention of a practical microphone was crucial to the early development of the telephone system. Emile Berliner invented the first microphone on March 4, 1877, but the first useful microphone was invented by Alexander Graham Bell. Many early developments in microphone design took place in Bell Laboratories.

In all microphones, sound waves (sound pressure) are translated into mechanical vibrations in a thin, flexible diaphragm. These sound vibrations are then converted by various methods into an electrical signal which varies in voltage amplitude and frequency in an analog of the original sound. For this reason, a microphone is an acoustic wave to voltage modulation transducer.

Microphone varieties

Capacitor or Condenser microphones

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 (Q), the voltage maintained across the capacitor plates changes with the vibrations in the air., according to the capacitance equation:

where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor. (See capacitance for details.):

Capacitor microphones can be expensive and require a power supply, commonly provided from mic inputs as phantom power, but give a high-quality sound signal and are now the preferred choice in laboratory and studio recording applications.

Electret condenser microphones

A foil electret microphone is a relatively new type of condenser microphone invented at Bell laboratories in 1962 by Gerhard Sessler and Jim West , 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 they have now become the most common type of all, used in many applications from high-quality recording and lavalier use to built-in microphones in small sound recording devices. Though electret mics were once considered low-cost and low quality, the best ones can now rival capacitor mics in every respect apart from low noise and even have the long-term stability and ultra-flat response needed for a measuring microphone. 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). The integral preamp usually forms part of the mic capsule in modern designs, sensing the electric field from the diaphragm stretched across in front of it directly, and this sets the limit of noise performance, which though it can be as low as 26 dBA SPL (38 dB SPL 468-wtd) cannot currently rival the best capacitor mics. Electrets are frequently phantom powered in sound reinforcement applications.

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