Application of thin, acoustic lenses in a 32-beam, dual-frequency, diver-held sonar

Often divers and small autonomous submersibles need the ability to detect objects and sense the terrain in turbid water, where optical systems fail. This paper discusses a prototype sonar built at the Applied Physics Laboratory that uses two thin acoustic lenses. The lenses simultaneously form 16 beams 0.5° in azimuth at 750 kHz and 16 beams 2° in azimuth at 200 kHz. The elevational beamwidth at both frequencies is 7°. The sonar processing forms a shadow-graph display similar to a sidescan sonar display. The sonar has a field of view of 40°, and the maximum range displayed varies from 3 to 60 m. The image refresh rate varies from 7 to 10 times per second. The sonar operates between 4 and 5 hours on a single charge of 7 A-h NiCad batteries. The lenses were designed with the help of desktop computer programs for optical lens design. Additional software was used to model the expected acoustic beampatterns. The polystyrene lenses form high-resolution beams acoustically and thus eliminate the need for a complex and power-intensive electronic beamformer. The lenses preform the beams before they are sensed on the array such that envelope detection takes place before sampling. This reduces the required sampling rate by over an order of magnitude compared with that of an electronic beamformer. The paper describes the lenses, transducer array, signal processing, results of in-water tests, and plans for a second prototype using 64 beams of which half are 0.5° wide and half are 0.75° wide