Low-frequency calibration of hydrophones based on laser interferometry using a zoom-averaging method

Acoustic reflection superposition in confined underwater environments results in waveform distortion, thereby limiting laser interferometry use in hydrophone calibrations at low frequencies (below 10 kHz). To address this challenge, we propose a low-frequency extension method named zoom-averaging. Instead of conventional single-frequency measurements, this approach employs multi-frequency measurements and waveform alignment, enabling coherent superposition of direct-path signals through precise acoustic time-of-flight estimation and non-coherent superposition of reflections via resampling-based waveform zooming. Theoretical analyses confirm considerable improvement in the signal-to-reverberation ratio. Calibration experiments using a laser heterodyne interferometry system were conducted in a 3 × 2 × 2 m water tank over a 2 to 10 kHz range, confirming effective recovery of direct-path waveforms. Results of the proposed method demonstrate metrological equivalence with reference values obtained using the reciprocity method, with a measurement uncertainty of 0.7 dB (k = 2). This study extends the lower frequency applicability of free-field calibration systems using laser interferometry and bridges the gap of frequency ranges with pressure-field calibration methods.