On the construction of linear-response hydrophones

Abstract

Most hydrophone elements in use nowadays can be classified either as volume-mode elements or as piezoceramic elements. Although volume-mode elements are usually quite linear, nearly all have low capacitance, and so, in practice, are used in conjunction with preamplifiers. Preamplifiers, however, can be nonlinear and are not usually suitable for close-range parametric source measurements. Piezoceramic elements have high capacitances and therefore can be used without preamplifiers. However, they do not work well in the volume mode, and so they are usually configured so as to expose only one element surface to the acoustic pressure. Such configurations invariably require bonds of some sort, e.g., two ceramic hemispheres are glued together to form a spherical hydrophone. Nonlinearity of an epoxy bond appears to account for the nonlinearity of F42D spherical hydrophones. Linear-response hydrophones suitable for use with parametric sources at close range can be constructed with volume-mode elements. If cable lengths can be kept short (⩽ 10 m), no preamplifier should be used. Otherwise a passive low-pass filter should precede the preamplifier. [Work supported by Naval Material Command and Naval Sea Systems command.]