Voltage to pressure conversion: are you getting `phased' by the problem?

Abstract

The process of calibration provides the user with the sensitivity data necessary to convert thevoltage measured at the end of the hydrophone cable to an acoustic pressure. This paper addressesthe precise nature of how this data is used. Most regulatory standards simply require the user to usethe sensitivity figure at the acoustic working frequency of the source in conjunction with a known(or pre-specified) `flatness' criterion for the overall frequency response. This paper questions thevalidity of this approach, and looks at the differences that can be obtained when the voltage topressure conversion is accomplished by means of a deconvolution process that utilises all of thefrequency response curve (as opposed a single data point). Theoretical and experimentalcomparisons between single frequency and full waveform deconvolution are presented.Currently, hydrophone calibration is a magnitude only measurement, but metrologicalimprovements offer the possibility for measurement of hydrophone phase. As a logical extension towaveform deconvolution, the influence of phase response and the impact that this has on measureddata is also presented. Clearly the proposed methods require greater computation during themeasurement process, but this does not necessarily require longer measurement. This paperdiscusses practical methods of implementing these enhanced voltage to pressure conversiontechniques in a commercial environment.