Larson Davis SoundAdvisor 831C Reference Manual page 211

Microphone
Guidelines
SoundAdvisor Model 831C
Standard: ANSI S12.19
Microphone - Types: A device for detecting the presence of sound. Most often it
converts the changing pressure associated with sound into an electrical voltage
that duplicates the changes. It can be composed of one of the following types:
Capacitor (Condenser): A microphone that uses the motion of a thin diaphragm
caused by the sound to change the capacitance of an electrical circuit and
thereby to create a signal. For high sensitivity, this device has a voltage applied
across the diaphragm from an internal source.
Electret: A microphone that uses the motion of a thin diaphragm caused by the
sound to change the capacitance of an electrical circuit and thereby to create a
signal. The voltage across the diaphragm is caused by the charge embedded in
the electret material so no internal source is needed.
Microphone - Uses: The frequency response of microphones can be adjusted to
be used in specific applications. Among those used are:
Frontal incidence (Free Field): The microphone has been adjusted to have an
essentially flat frequency response when in a space relatively free of reflections
and when pointed at the source of the sound.
Random incidence: The microphone has been adjusted to have an essentially
flat frequency response for sound waves impinging on the microphone from all
directions.
Pressure: The microphone has not been adjusted to have an essentially flat
frequency response for sound waves impinging on the microphone from all
directions.
What a microphone measures: A microphone detects more than just sound. The
motion of a microphone diaphragm is in response to a force acting on it. The
force can be caused by a number of sources only one of which are we interested:
sound. Non-sound forces are: (1) direct physical contact such as that with a finger
or a raindrop; (2) those caused by the movement of air over the diaphragm such
as environmental wind or blowing; (3) those caused by vibration of the
microphone housing; and (4) those caused by strong electrostatic fields.
Rules:
1. Do not permit any solid or liquid to touch the microphone diaphragm. Keep a
protective grid over the diaphragm.
2. Do not blow on a microphone and use a wind screen over the microphone to
reduce the effect of wind noise.
3. Mount microphones so their body is not subject to vibration, particularly in
direction at right angles to the plane of the diaphragm.
4. Keep microphones away from strong electrical fields.
A microphone measures forces not pressures. We would like the microphone to
measure sound pressure (force per unit area) instead of sound force. If the
pressure is applied uniformly over the microphone diaphragm a simple constant
(the diaphragm area) relates the two, but if the pressure varies across the
diaphragm the relationship is more complex. For example, if a negative pressure
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