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What the Calibration Procedure Includes
Calibration of an intensity-measuring instrument includes:
•
sound pressure calibration of the individual microphone channels
•
measurement of the pressure-residual intensity index spectrum of the sound intensity
measurement system
Pressure-Residual Intensity Index Measurement
Fig.1.2 shows an arrangement for measuring the pressure-residual intensity index. The probe is
placed in the Type 4297. The sound source produces the same sound pressure level at each
microphone, so both microphones are exposed to the same sound pressure and same phase, and
therefore any intensity detected is residual intensity. The microphone channels are calibrated
against this known sound pressure level (nominal value 94 dB).
Fig.1.4 shows how small differences in the phase responses of the microphones and input
channels result in the detection of 'residual intensity'. Residual intensity is a parameter that
should be taken into account when interpreting measured intensity data. The pressure residual
intensity spectrum is not fixed; it is 'tied to', and rises and falls with, the measured sound pressure
level.
It can be shown that, for a given measurement system and frequency, the difference between
measured sound pressure level and detected residual intensity level will be a constant. This
constant difference is called the pressure-residual intensity index.
The pressure-residual intensity index spectrum can be measured with the arrangement shown in
Fig.1.2 by subtracting the detected intensity spectrum from the sound pressure spectrum. An
example is shown in Fig.1.4.
CHAPTER 1
Introduction and Description
3
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