Cooling, M.P., Humphrey, V.F. and Wilkens, V.
Hydrophone area-averaging correction factors in nonlinearly generated ultrasonic beams.
Journal of Physics: Conference Series, 279, (1), . (doi:10.1088/1742-6596/279/1/012002).
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The nonlinear propagation of an ultrasonic wave can be used to produce a wavefield
rich in higher frequency components that is ideally suited to the calibration, or intercalibration,
of hydrophones. These techniques usually use a tone-burst signal, limiting the
measurements to harmonics of the fundamental calibration frequency. Alternatively, using a
short pulse enables calibration at a continuous spectrum of frequencies. Such a technique is
used at PTB in conjunction with an optical measurement technique to calibrate devices.
Experimental findings indicate that the area-averaging correction factor for a hydrophone in
such a field demonstrates a complex behaviour, most notably varying periodically between
frequencies that are harmonics of the centre frequency of the original pulse and frequencies
that lie midway between these harmonics. The beam characteristics of such nonlinearly
generated fields have been investigated using a finite difference solution to the nonlinear
Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation for a focused field. The simulation results
are used to calculate the hydrophone area-averaging correction factors for 0.2 mm and 0.5 mm
devices. The results clearly demonstrate a number of significant features observed in the
experimental investigations, including the variation with frequency, drive level and
hydrophone element size. An explanation for these effects is also proposed.
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