Design and evaluation of noise suppressing tonpilz hydrophone structures

This paper describes the design and evaluation of a noise suppressing hydrophone that is robust to external noise without sacrificing its performance as a receiver. To increase robustness of the receiver to the external noise, first, effects of location of the external noise on its performance are analyzed with the finite element method (FEM). Based on the results, geometrical variations are implemented on the structure with additional air pockets and damping layers that work as acoustic shields or scatterers of the noise, and fourteen trial models are developed for the noise suppressing hydrophone structures. Also acoustic impedance and damping coefficients of the material inside the acoustic walls are varied and the effects of the material property on the hydrophone response to the external noise are analyzed. The results show that the effect of the external noise is most significant when it is applied to near the mid-side surface of the hydrophone housing. The external noise is isolated most efficiently when two thin damping layers combined with five air pockets are inserted to the circumference of the hydrophone housing. The damping coefficients are not so influential in de-coupling the structural vibration from the sensing element. The optimum acoustic impedance of the damping layer is estimated to be smaller than 1 Mrayl or larger than 4 Mrayl. These properties can not be readily obtained from conventional polymers alone, and hence development of new composite materials, i.e. ceramic-polymer composite or metal-ceramic composites, is required for realization of effective damping layers. Overall, of the fourteen structural variations of the hydrophone, the best one shows about 87% reduction in the response of the original structure to external noise.