Larson Davis SoundAdvisor 831C Reference Manual page 207

Frontal incidence (Free Field): The microphone has been adjusted to have an essentially flat fre-
quency 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 micro-
phone 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 pres-
sure 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:
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 orienta-
tion.
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 micro-
phone 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 orienta-
tion.
SoundAdvisor Model 831C C-10