Transfer coupler reciprocity method for the absolute low-frequency calibration of field hydrophones under full environmental conditions

Abstract

The reciprocity coupler is a closed chamber used for the absolute calibration of standard hydrophones at low frequencies. There is a need to extend this capability to field hydrophones that are not specifically designed for use in a reciprocity coupler. This requires that one determine the reciprocity parameter for the coupler with the field hydrophone installed. Uncertainties in the effective volume and compliance of the coupler with the field hydrophone installed preclude calculation of the reciprocity parameter from first principles. The transfer reciprocity method described here provides a way to measure the reciprocity parameter of an ill-characterized coupler by first making measurements in a well-characterized coupler without the field hydrophone. An error analysis is presented indicating the current state of the existing reciprocity coupler including the transfer reciprocity method. The analysis shows that there is a systematic error that can be eliminated by correcting the sensitivities obtained using the transfer reciprocity method. This error has a frequency-independent component of 0.31 dB and a frequency-dependent component ranging from 0.1 dB at 700 Hz to 1.0 dB at 2000 Hz. After correcting for the systematic error, the uncertainty in the sensitivity is ±0.25 dB for frequencies below 1000 Hz, rising to ±0.35 dB for frequencies near 2000 Hz.