Low-Frequency Hydrophone Calibration Using Elastic Waveguides

Abstract

Modern hydrophone calibration techniques typically require sensors to be placed in a free field or within a hydrostatically varying chamber. Between 1 kHz and 10 kHz, the wavelength in water is too long for free-field conditions to be used in common tank sizes. A novel technique to calibrate hydrophones is investigated to address the measurement gap between very low-frequency and high-frequency techniques. Calibrations are performed in a 12-meter copper tube coiled into a 1-meter diameter helix. This elastic waveguide decreases the speed of sound within the apparatus, providing a longer reverb-free time for calibrations using a small volume. The propagation within the waveguide is studied with the deviations from the predicted theory discussed. Calibrations performed on Ocean Sonics icListen hydrophones are shown to be within 1 dB of independently calibrated reference measurements. This research demonstrates that using elastic waveguides is an effective and simple low-frequency hydrophone calibration method.