Acoustic Output and Noise Measurements

The response of hydrophones or microphones can be calibrated in terms of either sound level (dB) or acoustic pressure either of which are related to ultrasound intensity. The Fourier transform of these give a frequency-dependent signal, which is also related to sound intensity. It is possible therefore to obtain a plot of sound level (and thus theoretically ultrasound intensity) against frequency. However, as already mentioned, ultrasound power measurements using acoustic probes are not straightforward and require preliminary calibration of the probes with another 

From the response of a microphone located far away from the source, Neppiras observed that the fundamental signal increases and then decreases as the cavitation threshold is approached and then passed due to the screening effect of the bubbles at the surface of the emitter. At the same time, the subharmonic increases sharply at the threshold, almost at the same time as white noise, and passes through a maximum and then decreases due to cavitation between the source and the microphone. It is possible that this maximum could provide a test to detect the optimum volume coverage of cavitation. 

Quantitative correlations between the intensities of fundamental, subharmonic, or harmonic intensities and the overall ultrasound intensity are not clear since the signal received depends on the nature and shape of the probe used for measurement. Preliminary calibration is required and this is certainly possible. For instance, it has been shown that the erosion of a metal foil increases linearly with the total noise [149], with the white noise output and with the square of the acoustic pressure 

As has already mentioned, acoustic probes can be made very small and very sensitive however their use requires somewhat sophisticated equipment together with accurate calibration which is not easy to achieve. 

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Class 4-Methods Based on Secondary Effects of Sound Propagation and Cavitation. These include methods based on volume changes [142-144], acoustic output and noise measurements [145—150], conductance changes, electric and electrokinetic effects [151-156], sonoluminescence [157-171], and chemical probes [172-198]. 

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