Development of a Compound Acoustic Array Using Directional Hydrophones.

A six-element line array of directional hydrophones has been designed and constructed as outlined in reports PPD-66-E-10159 and 10160. Each hydrophone consists of a block of syntactic foam with holes machined in it to provide four different acoustic path lengths, corresponding to one-quarter, one-half, three-quarter, and one wavelength at 15 kHz. Thus, each hydrophone is equivalent to a four-element end-fire line array with element spacings of one-quarter wavelength. A ceramic disc in each hydrophone converts the acoustic signals to electrical signals which are then brought out through a length of coaxial cable. The output of each hydrophone cable is fed to a separate amplifier, and the gain of each amplifier is adjusted to a value proportional to one of the Tchebyscheff shading coefficients calculated in report 10160. The six amplifier outputs are then summed and the output of the summing amplifier is fed to the receiver of the directivity pattern measuring system. The directivity pattern of the array was measured using the above measuring system and the acoustic calibration tank. A depth of five feet and a separation of six feet between the reference projector and the array were used. The settings of the amplifier gains were found to be critical. Initial directivity pattern measurements showed excessively high side lobes or wide beam widths. By carefully adjusting the gains, improvements in both side lobe levels and beam widths were achieved.