The acoustic characteristics of underwater cylindrical Helmholtz resonator are analyzed theoretically. Based on the theories of electro-acoustic analogy, a low frequency lumped-parameter model of the Helmholtz resonator is constructed with due consideration of the effects of the elasticity and the radiation impedance of the resonator. To our knowledge, this is the first time such a complete model is constructed. The input impedance and the transfer function of the system are given by circuit analysis. The effects of parameter values of the resonator on the acoustic characteristics are studied by numerical method. Some useful conclusions are drawn. A small aluminum cylindrical Helmholtz resonator is measured in a standing-wave tube filled with water. Error analysis is made in detail. The experimental results are in agreement with the simulation results considering the effect of the piezoelectric hydrophone. The validity of the theoretical analysis is testified. This paper supplies a theoretical and experimental basis for the design of underwater cylindrical Helmholtz resonators, and is useful for the estimation of underwater acoustic performance of Helmholtz resonators of other shapes.

Influence of cavity wall elasticity on resonant frequency of small underwater cylindrical Helmholtz resonator is studied theoretically and experimental. Based on the theory of electroacoustic analogy, the simplified low frequency lumpedparameter model of the Helmholtz resonator is constructed. A general, convenient for calculation expression of the resonant frequency is given by circuit analysis. The influence of the thickness and the material of the resonator on the resonant frequency is investigated by numerical method. And the approximate rigid conditions for small underwater cylindrical Helmholtz resonators of different sizes are given. Small cylindrical Helmholtz resonators of different wall thickness and material were tested in a standingwave tube filled with water. Experimental results well testified the theoreticl results and the approximate rigid condition. This paper is useful for the design and application of the underwater cylindrical Helmholtz resonators.

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Response properties of the fourth-order acoustic low-pass filtering fiber-optic hydrophones are investigated theoretically and experimentally. A mechanical acoustic resistance， which is used to describe the system’s mechanical loss， is introduced into the previous lumped parameters model of the hydrophone， and an improved acoustic equivalent circuit is given. Phase frequency response is an important parameter for the hydrophone array applications. It has great effect on beamforming， which affects the abilities of locating， discerning， and tracking targets. Therefore， the phase response peroperties is studied with the amplitude response peroperties. Some results， which are instructive for designing of acoustic low-pass filtering hydrophone， are obtained by numerical simulation. The measured rsponse curves are in good agreement with the simulation results， which verifies the correctness of the theory and the model. The fourth-order acoustic low-pass filtering fiber-optic hydrophones are useful for improvement of the anti-aliasing ability of modern sonar arrays.

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