A STUDY OF THE OUTPUT OF A HYDROPHONE FIXED ON A VIBRATING STRUCTURE

Abstract
In the design of sonar systems, the hydrophones, which are the basic acoustic pressure sensors for the receiving arrays, are generally fixed on a supporting structure. The vibrations of the support generate self noise on the hydrophones, that reduces the performances of the array. In fact, the output signal on a hydrophone due to the vibrations of the supporting structure comes from two contributions : - the signal related to the accelerometric sensitivity of the hydrophone, caused by the mechanical vibrations of the piezoelectric element ; - the signal related to the acoustic pressure sensitivity of the hydrophone, caused by the radiation of the vibrating support in the nearfield. The purpose of this paper is to put forward these phenomena. After a brief presentation of the theory, some numerical examples of the output of hydrophones fixed on a vibrating support are first given. Then, a specific experiment is described : two experimental large area hydrophones (different with respect to their sensitivities) have been built. First, the pressure and accelerometric sensitivities are measured using conventional techniques. Then, a global experiment has been done in an acoustic facility, where the hydrophones were fixed on the head of a "Tonpilz" type projector. It is verified that the outputs of the hydrophones are formed by the two contributions mentionned above. This result may have consequences in the design of hydrophones for sonar arrays.