Microphone, device used to transform
sound energy into electrical energy .
Microphones are important in many kinds of communications systems
and in instruments that measure sound and noise. The American
inventor Alexander Graham Bell built the first microphone in 1876
when he constructed his telephone transmitter.
The simplest type of modern
microphone is the carbon microphone, used in telephones. This
microphone consists of a metallic cup filled with carbon granules;
a movable metallic diaphragm mounted in contact with the granules
covers the open end of the cup. Wires attached to the cup and
diaphragm are connected to an electrical circuit so that a current
flows through the carbon granules. Sound waves vibrate the
diaphragm, varying the pressure on the carbon granules. The
electrical resistance of the carbon granules changes with the
varying pressure, causing the current in the circuit to change
according to the vibrations of the diaphragm. The varying current
may either actuate a nearby telephone receiver or may be amplified
and transmitted to a distant receiver. If the current variation is
suitably amplified, it may also be used to modulate a radio
Another common type, the crystal
microphone, utilizes piezoelectric crystals, in which a voltage
develops between two faces of the crystal when pressure is applied
to the crystal . In this microphone sound waves vibrate a
diaphragm, which in turn varies the pressure on a piezoelectric
crystal. This generates a small voltage, which is then
Examples of dynamic microphones include ribbon microphones and
In ribbon microphones, a thin metallic ribbon is attached to the
diaphragm and placed in a magnetic field. When sound waves strike
the diaphragm and vibrate the ribbon, a small voltage is generated
in the ribbon by electromagnetic induction. A moving-coil
microphone operates on essentially the same principle but has a
coil of light wire, instead of a ribbon, attached to the diaphragm.
Some modern microphones, designed to pick up sound from one
direction only, combine both ribbon and coil elements.
Another type of microphone is the condenser microphone. The
condenser microphone has two thin metallic plates placed close to
each other that serve as a capacitor. The back plate of the
capacitor is fixed, and the front plate serves as the diaphragm.
Sound waves alter the spacing between the plates, changing the
electrical capacitance between them. By placing such a microphone
in a suitable circuit, these variations may be amplified, producing
an electrical signal. Condenser microphones can be very small.
A common type of condenser microphone, the electret condenser
microphone, is used in hearing aids.
Among the important characteristics of microphones are their
frequency response, directionality, sensitivity, and immunity to
outside disturbances such as shock or vibration.