ASA 124th Meeting New Orleans 1992 October

4aEA13. An inexpensive air-compensated parabolic dish for underwater acoustics.

Gerald M. Santos

Richard K. Menoche

Naval Undersea Warfare Ctr., Code 382, Newport, RI 02841

Peter J. Stein

Atlantic Appl. Res. Corp., 4 ``A'' Street, Burlington, MA 01803

The benefits of using highly directive parabolic reflectors for acoustics and electromagnetics work is well known. The primary difficulty for underwater acoustics work is making a highly reflective surface, one that is acoustically very hard or very soft, in the shape of a parabola. Very hard would require a great deal of machined metal and would most likely be cost and weight prohibitive. Therefore, for underwater acoustics work, parabolas are usually made to be acoustically soft. Although soft foam is a good choice, it will crush at even shallow depths. Other solutions include compliant tube designs with and without pressurized air compensation. These tend to be very expensive. Here, an inexpensive and rugged parabolic receiver/transmitter with virtually no depth limitation is described. It consists of a fiberglass parabola backed by a fiberglass dome. The air cavity between them results in a soft reflector. Vibration isolation is provided by steel rings and rubber gaskets. To prevent crushing, the air chamber can be pressurized either by compressed air fed by hose from the surface or an attached tank and regulator. Both transmit and receive directivity patterns will be shown. The device is currently being used for target strength measurements at the NUWC Lake Seneca Test Facility.