A Method and an Experimental Setup for Measuring the Self-Noise of Piezoelectric Hydrophones

Self-noise of hydrophone is one of the important parameters for critical sonar applications, as it finally decides the lowest detectable signal from a target. To measure the ambient noise of the ocean, the self-noise of the hydrophone should be much less than the sea state zero (SS0) noise. Hence, an experimental setup is required to measure the self-noise generated by the hydrophones. An experimental setup has been designed, developed, and demonstrated. The facility consists of a vibration-isolated and EMI-shielded vacuum chamber which encloses the sample holder assembly and effectively isolates the test hydrophones from external environmental noise sources, such as ground vibrations, electromagnetic interference, and airborne acoustic noises. This enables the measurement of self-noise that is originating purely within the hydrophone. A procedure is developed to remove the noise contribution of the preamplifier including its resistors from the measured output signal of the preamplifier. Uncertainty in measurement using this setup is estimated using the propagation of distribution method through Monte Carlo simulation. This article describes the factors contributing to the self-noise in piezoelectric (PE) transducers, the noise measurement principle, the constructional details of the experimental setup, and the calibration procedure. The technique is demonstrated by measuring the self-noise of a few different types of hydrophones. Thermal noise as low as 0.5 nV / √{Hz} with an estimated uncertainty of 0.5 nV / √{Hz} with a confidence level of 95% is achieved.