An Analysis of the Effect of Vibration Sensitivity on Hydrophone Design

Hydrophones used in the ocean produce spurious outputs due to vibration sensitivity which can severely degrade measurement accuracy. Sources of these vibration inputs are ocean surface waves, flow turbulence, and induced mechanical vibration. The hydrophone response to these vibrations is a noise voltage output. This can leave to a signal-to-noise problem particularly when measurements of small sound pressure levels are to be made. The objective of this thesis is to analyze the vibration response of three typical piezoelectric hydrophone sensor elements configurations and give design methods and constraints for reducing the problem of vibration sensitivity to an acceptable level. The sensor element configurations analyzed are the radially polarized cylindrical shell, radially polarized spherical shell, and axially polarized cylindrical shell. The analysis is carried out due to two causes. An electromechanical analysis is given of the voltage sensitivity of each of the three sensor configuration to the inertial effect of acceleration inputs. The second effect analyzed is the voltage sensitivity of a pressure sensitive sensor element to the hydrostatic pressure amplitude caused by periodic vertical displacement of a hydrophone. Results of the analyses show that the radially polarized cylindrical and spherical shell configurations have zero acceleration sensitivity to inputs on the axes analyzed. An equation is derived for the axial acceleration sensitivity of the axially polarized cylindrical shell in terms of the equivalent sound pressure. The analysis of hydrophone sensor response to periodic vertical displacements shows high voltage sensitivity to very small displacement amplitudes. Data is given for the maximum permissible vertical displacement amplitude to produce a 20dB signal-to-noise ratio. Based on these analyses, design considerations are given to minimize hydrophone vibration sensitivity