Electroacoustic Devices:
Microphones and
Loudspeakers
Microphones 5
Microphones are electroacoustic devices that convert acoustical energy into
electrical energy. All microphones have a diaphragm or moving surface that is
excited by the acoustical wave. The corresponding output is an electrical signal
that represents the acoustical input.
Microphones fall into two classes: pressure and velocity. In a pressure
microphone the diaphragm has only one surface exposed to the sound source
so the output corresponds to the instantaneous sound pressure of the impressed
sound waves. A pressure microphone is a zero-order gradient microphone, and
is associated with omnidirectional characteristics.
The second class of microphone is the velocity microphone, also called a
first-order gradient microphone, where the effect of the sound wave is the difference
or gradient between the sound wave that hits the front and the rear of
the diaphragm. The electrical output corresponds to the instantaneous particle
velocity in the impressed sound wave. Ribbon microphones as well as pressure
microphones that are altered to produce front-to-back discrimination are of the
velocity type.
Microphones are also classified by their pickup pattern or how they discriminate
between the various directions the sound source comes from, Fig. 1.1.
These classifications are:
● Omnidirectional—pickup is equal in all directions.
● Bidirectional—pickup is equal from the two opposite directions (180°) apart
and zero from the two directions that are 90° from the first.
● Unidirectional—pickup is from one direction only, the pickup appearing
cardioid or heart-shaped.
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