Page 44 - HydrophoneHandbook_eReader_Ed2_20151208
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Hydrophone Handbook

                     Calibration

The reference hydrophone is then replaced by the hydrophone
to be calibrated, and re-aligned. The voltage driving the source
is again swept across the same frequency settings, and the
output of the hydrophone at the fundamental of the driving
signal is determined and recorded. This voltage is then divided
by the pressure amplitude (measured in step 1) to determine
the output of the hydrophone in Volts per Pascal (V/Pa).

Harmonic Comparison Technique:
1. The reference hydrophone is placed at a position in the non-

    linear acoustic field produced by a plane circular source
    transducer driven by a 2.25 MHz sinusoidal tone-burst
    signal. The position is chosen where many strong harmonics
    of the fundamental frequency are generated in the distorted
    pressure waveform due to non-linear propagation. Using a
    50 MHz high-pass filter, the reference hydrophone is aligned
    by maximizing the hydrophone’s high frequency components
    in the plane at that position to an optimal orientation. The
    pressure waveform at that position is recorded after the
    high-pass filter is removed. An FFT of the waveform is
    calculated, and the voltage amplitude at each harmonic is
    determined. The reference hydrophone has previously been
    calibrated by NPL, so that the voltage it detects can be
    translated into a pressure value, hence calibrating the
    pressure output of the source at each harmonic.

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