Acceleration canceling pressure gradient hydrophone

An acceleration canceling pressure gradient hydrophone is described. The unique difference between the conventional pressure gradient hydrophone and the current phone is a plumbing and counterweight arrangement whereby acceleration induced forces from the enclosed fluid are balanced by counterweight forces. Cancelation ratios of 35 dB flat from 10 Hz up are reported from tank measurements. One axis decoupling techniques are described to enhance this ratio at 12 dB/octave from decoupler resonance. The effects of some hydrophone outer housings on the decoupler are shown. Three field tests are described: two using a new radial arm device in a quiet lake, and one using a bottom mounted cable in a tidal estuary. The first radial arm test indicated a cross-over between structure borne and turbulent boundary layer noise as flow rates rise above about 12knot. The tidal estuary test showed large enhancements of gradient acoustic output near irregular boundaries. This is attributed to the excess pressure gradients in curved wavefronts. Tests of a cardioid version of the hydrophone are included. [Work supported by NAVELEXSYSCOM.]