Larson Davis System 824, Reference Manual

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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 uni-
formly over the microphone diaphragm a simple constant
(the diaphragm area) relates the two, but if the pressure var-
ies 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:
1. Do not use a microphone at frequencies higher than speci-
fied 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 instru-
ment 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 speci-
fied 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 direc-
tional so they measure whatever is there, regardless of
source.
Rules:
1. When making hand-held measurements, keep your body
at right angles to the direction of the sound you are inter-