Considerations for a new high-accuracy transfer-coupler reciprocity system for absolute electro-acoustic calibration J F Zalesak IntroductionPrimary calibrations of hydrophones at frequencies less than about 1 kHz are typically performed in a coupler reciprocity chamber ('coupler'). The closed and controlled environment in the coupler allows for the performance of primary calibrations over the temperature and hydrostatic pressure range found in the ocean. The coupler reciprocity chamber is designed together with a primary reference standard hydrophone and two other reciprocal transducers because the volume and stiffness of the closed coupler system must be known precisely.The Underwater Sound Reference Division (USRD), a laboratory within the US Navy's (USN) research and development enterprise, provides metrology services related to underwater sound in the United States, including device calibration and leasing of underwater transducers, hydrophones, and projectors. Primary standards are maintained in-house. A coupler reciprocity system provides the primary standard calibration for frequencies less than 2 kHz [1]. The primary reference standard hydrophone is the USRD type H48 [2].To completely reproduce the acoustic waveform measured with a hydrophone, magnitude and phase information are required. Coupler reciprocity calibrations are historically performed measuring only magnitude [1, 3], and [4]. Phase response is more difficult to measure accurately and can be assumed constant when hydrophones are operating well below their resonance frequencies and, for hydrophones with preamplifiers, above their low-frequency cutoff. The type H48 reference is used well below it first resonance, but the low end of the frequency range can be affected by the low-frequency cutoff, problematic when secondary references are needed to measure frequencies below 5 Hz. Extending the coupler reciprocity calibration measurement to include phase is important to provide traceability for secondary references and to reduce AbstractA primary calibration method is demonstrated to obtain the magnitude and phase of the complex sensitivity for a hydrophone at frequencies between 1 Hz and 2 kHz. The measurement is performed in a coupler reciprocity chamber ('coupler'); a closed test chamber where time harmonic oscillations in pressure can be achieved and the reciprocity conditions required for a primary calibration can be realized. Relevant theory is reviewed and the reciprocity parameter updated for the complex measurement. Systematic errors and corrections for magnitude are reviewed and more added for phase. The combined expanded uncertainties of the magnitude and phase of the complex sensitivity at 1 Hz were 0.1 dB re 1 V µPa −1 and ±1• , respectively. Complex sensitivity, sensitivity magnitude, and phase measurements are presented on an example primary reference hydrophone.

Primary calibrations of hydrophones at frequencies less than about 1 kHz are typically performed in a coupler reciprocity chamber (“coupler“); a closed test chamber where time harmonic oscillations in pressure can be achieved and the reciprocity conditions required for a primary calibration can be realized. The closed and controlled environment in the coupler allows for the performance of primary calibrations over the temperature and hydrostatic pressure range found in the ocean. The coupler reciprocity system employed by the United States, in service since the 1960s, provides only the magnitude of the pressure sensitivity and not the phase. Recent work has demonstrated a method for the primary calibration of both the magnitude and phase of the complex sensitivity for a hydrophone at frequencies ranging from 1 Hz to 2 kHz. The combined expanded uncertainties of the magnitude and phase of the complex sensitivity at 1 Hz were 0.1 dB re 1V/μPa and ±1°, respectively.