Directional 420-Hz Sound Source

Abstract

A 24-ft compliant-tube parabolic reflector, with a flextensional transducer at its focus, has yielded acoustic source levels up to 122 dB/bar/yd at 420 Hz in 85 ft of water off Fowey Rocks near Miami, Florida. The projector is part of an ONR supported research program to study underwater sound propagation and related environmental parameters. Modulation signals of continuous-wave pulses, and pseudorandom sequences are transmitted by wire from the Institute of Marine Sciences Laboratory to the transducer, a distance of 12 miles. Bottom-mounted hydrophones are employed at various distances, with the principal sensors on the island of Bimini which lies 45 miles eastward across the Florida Straits. The reflector provides a mainlobe of 28° at the half-power points; the back lobes are down more than 25 dB and the directivity index is 15 dB. The effective mechanical Q of the transducer-reflector combination is 4. At the present depth, the transducer is cavitation limited to an acoustic output of 4 kW.