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Microphones
in the sound field
Distance effect As most microphones respond to pressure
waves,
their output decreases with distance in accord with
sound pressure. Thus for a point source in a far field, doubling
the distance from the source results in a 6 dB drop or half the
output. In a diffuse field the output may vary little with
distance.
Proximity effect A velocity or pressure gradient microphone
responds in part to particle velocity. This decreases according
to the square of the distance in the near field, but in proportion
to distance in the far field. The division between near and far
fields depends on frequency: the near field i