Transfer coupler reciprocity: A new low-frequency coupler-reciprocity technique for the absolute calibration of field hydrophones under full environmental conditions

Abstract

The reciprocity coupler system in the Low-Frequency Facility (LOFAC) at the Underwater Sound Reference Divsion (USRD) of the Naval Undersea Warfare Center is a closed-chamber environmentally controlled system 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. The transfer reciprocity method described here is an answer to this need. 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 which can be eliminated by correcting the sensitivities obtained using the transfer reciprocity method. This correction has a frequency independent component of 0.31 dB and a frequency dependent component of 0.1 dB at 700 Hz and 1.0 dB at 2000 Hz. After correction for the systematic error, the uncertainty in sensitivity is ±0.25 dB for frequencies below 1000 Hz, growing to ±0.35 dB for frequencies near 2000 Hz.