Microphones (80048)

The word "microphone" (Greek mikros "small" and phone "voice" or "sound") originally referred to a mechanical hearing aid for small sounds. more...

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The invention of a practical microphone was crucial to the early development of the telephone system. Several early inventors built primitive microphones (then called transmitters) prior to Alexander Bell, but the first commercially practical microphone was the carbon microphone conceived in October, 1876 by Thomas Edison. Many early developments in microphone design took place at Bell Laboratories. See also Timeline of the telephone. The main basic designs still popular are of American, British or Russian origin.

Principle of operation

A microphone is a device made to capture waves in air, water or hard material and translate it to an electrical signal. The most common method is via a thin membrane producing some proportional electrical signal. Most microphones in use today for audio use electromagnetic generation (dynamic microphones), capacitance change (condenser microphones) or piezoelectric generation to produce the signal from mechanical vibration.

Microphone varieties

Condenser or capacitor microphones

Technology
In a condenser microphone, also known as a capacitor 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.)

The charge across the capacitor is not maintained perfectly constant. As the capacitance changes, the charge across the capacitor changes to make the voltage drop across the capacitor equal to the bias voltage. However, the rate of this change is kept slow by using a series resistor of a very high value (of the order of 10 MΩ). Note that the time constant of a RC circuit equals the product of the resistance and capacitance.

Within the time-frame of the capacitance change (on the order of 100 μs), the charge thus appears practically constant and the voltage across the capacitor adjusts itself instantaneously to reflect the change in capacitance. Within the time-frame of the capacitance change, the voltage across the capacitor really up to the bias voltage. The voltage difference between the bias and the capacitor is seen across the series resistor. The voltage across the resistor is amplified and reproduced to audio or recording. Condenser microphones are elongation receivers.

Read more at Wikipedia.org