Page 45 - HydrophoneHandbook_eReader_Ed2_20151208
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Calibration

2. The reference hydrophone is replaced by the hydrophone to
    be calibrated, which is then aligned the same way at the
    same position as the reference hydrophone. The pressure
    waveform is recorded with the absence of the high-pass
    filter. An FFT of the pressure waveform is then calculated,
    and the voltage at each harmonic is determined. 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).

8.5 Calibration Uncertainty
The uncertainties in the calibration are predominantly
systematic in origin, the random error is negligible in
comparison. The dominant source of uncertainty is the
uncertainty in the calibration of the reference hydrophone (as
provided by NPL). The other sources of uncertainty are
temperature variation (± 1 °C, resulting in calibration variations
of ± 1%) source stability (± 2%) digitizer error (± 2%) and
positional repeatability (this effect varies with frequency, from ±
2.5% at 1 MHz to ± 5% at 20 MHz). The combined effect of these
error sources is calculated according to [2-3], with a coverage
factor of k = 2 to calculate the uncertainty with a 95%
confidence level. This yields uncertainties that are typically
within ± 1.5 dB from 0.5 to 1 MHz, ± 1 dB from 1 to 15 MHz, ±
1.5 dB from 15 to 20 MHz, ± 2.2 dB from 20 to 40 MHz and ± 3
dB from 40 to 60 MHz.

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