When two phase‐matched microphones are used together to form a sound intensity probe, then the two‐microphone intensity probe can be calibrated by fitting an intensity coupler attached to the pistonphone [7]. The coupler normally consists of two chambers (upper and lower) connected by a coupling element. When the pistonphone is connected to the coupler, a phase difference is created between the two sound pressures generated in the upper and lower chambers. The sound pressure amplitudes are the same, however, in both chambers. Thus the propagation of a plane progressive sound wave in a free field is simulated. If one microphone of the intensity pair is fitted to the upper chamber and the other is fitted to the lower chamber, then the simulated sound wave produced by the sound in the two chambers can be used for the calibration of the intensity probe for the measurement of both sound intensity and particle velocity. See Chapter 8 and text discussing Figures 8.21 and 8.25.
The intensity coupler and pistonphone can also be used for the sound pressure sensitivity calibration of the two microphones. For this measurement, the two microphones are connected to the upper chamber of the coupler. They are then automatically exposed to the same sound pressure, and the amplitude and phase difference between the two microphones can be measured and checked to see that it is within acceptable limits. If a sound source, which can generate broadband sound, is attached to the coupler, then the pressure residual‐intensity index spectrum can be measured as well. This index is used to determine the accuracy of the sound intensity measurements made with the probe [3, 7, 22]. See Chapter 8.
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