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The open circuit free-field receiving voltage sensitivity of an underwater hydrophone is defined as a ratio of the open circuit voltage of the hydrophone measured at its terminals to the sound pressure in the undisturbed free field in the position of the reference center of the hydrophone if the hydrophone were removed. This definition itself demands that the measurement is to be conducted in free field environment where the direct signal should not interfere with reflections. It is impractical to make anechoic chambers which can be used for measurements over a broad range of frequencies. Hence special methods such as time gating in transmit and receive pulses, and time delay spectrometry are used to make quasi free field measurements. The time gating method is relatively simple to perform after the arrival of high speed data acquisition and processing devises. SNR issues and availability of broadband transducers are the limitation of this method. The second method of time delay spectrometry is difficult to implement due to the complex instrumentation and algorithm involved. In this paper, a new method for hydrophone evaluation is proposed based on cross-correlation technique. This method helps to simultaneously achieve a quasi-free-field condition, SNR improvement and broadband measurement. In the proposed method, the frequency response function(FRF) between the projector and hydrophone signals will be measured twice with a reference hydrophone and the test hydrophone using filtering in the correlation domain. Ratio of FRF gives the receiving sensitivity of the test hydrophone. This method works well for in-situ calibration of hydrophones in harbour where the noise levels are high and the projector output is low due to cavitation limit for low depth operation or non-availability of suitable power amplifiers. The measurement procedure, its validation using the method of reciprocity calibration, and uncertainties are explained in the paper.
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