The scan transducers must be free-field or pressure microphones with mutual phase
deviations not exceeding ±3° within the frequency range of interest. In addition, their
amplitude linearity must be good and they must have a high stability. Array Microphone
Type 4935 is recommended as a best-value solution.
Before a measurement can begin, the noise source and its surroundings must be checked
and, if necessary, modified so that either free-field or mirror-ground conditions exist
over the entire measurement region and frequency range of interest. In addition, the
noise radiated by the source must be kept stationary throughout the measurement.
Imperfect stationarity may lead to both variation in amplitude and shift in spectral
components. As a consequence, the narrower the bandwidth used, the steadier the
sound source must be.
During a measurement, the software monitors the reference signals for deviations from
stationarity of sound source.
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