Spartan 821/721 Manual
Glossary
C-10
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 to prevent interference:
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 is applied on one-half the diaphragm and
an equal positive pressure is applied to the other half, the net force is zero and
essentially no motion of the diaphragm occurs. This occurs at high frequencies and for
specific orientations of the microphone.
Rules to obtain the most accurate measurement:
1. Do not use a microphone at frequencies higher than specified by the manufacturer; to
increase the frequency response choose smaller microphones.
2. Choose a microphone for free field or random incidence to minimize the influence of
orientation.
A microphone influences the sound being measured. The microphone measures very
small forces, low level sound can run about one-billionth of a PSI! Every measurement
instrument changes the thing being measured, and for very small forces that effect can
be significant. When sound impinges directly on a microphone the incident wave must
be reflected since it cannot pass through the microphone. This results in the extra force
required to reflect the sound and a microphone output that is higher than would exist if
the microphone were not there. This is more important at high frequencies and when
the microphone is facing the sound source.
Rules:
1. Do not use a microphone at frequencies higher than specified by the manufacturer; to
increase the frequency response choose smaller microphones.
2. Choose a microphone for free field or random incidence to minimize the influence of
orientation.
A microphone measures what is there from any direction: Most measurements are
intended to measure the sound level of a specific source, but most microphones are not
directional so they measure whatever is there, regardless of source.
