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4
Configuration and operation
4.1
Random-incidence microphone
Random-incidence microphones are intended to measure sound in a
diffuse field where the sound is arriving from undefined directions,
for example in an area with many reflections or numerous sources.
The random-incidence microphone in S51 Sound Level Meter
measures the sound pressure which existed before it was placed in a
diffuse field. At higher frequencies it compensates for the effects of
reflection and diffractions because of its presence in the sound field.
The most ideal frequencies response is reached when the microphone
is pointed 90 degrees off the direction of the sound source. At
normal diffuse field measurements, the S51 shall be mounted with its
microphone pointing up-right. A little overcompensation will occur
at higher frequencies, but this is within the class 1 tolerances of the
standards. The figure below shows the microphone free field
responses for various angles of incidence and also, based on these,
random incidence. The definitions used here are described in the
standards IEC 61183 and ANSI S1.10.
Free-field frequency characteristics of the measured sound pressure level
for various angles of incidence. The calculated random incidence is also illustrated.
