Underwater Helmholtz-Resonator Transducers: General Design Principles
NAVAL UNDERWATER SYSTEMS CENTER NEW LONDON CT
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Underwater Helmholtz resonators excited by piezoelectric drivers show promise as compact sound sources for very low frequencies. Since 1969, Groves and Henriquez of the Naval Research Laboratory have been using small spherical shells of piezoelectric ceramic as Helmholtz-resonator transducers. The object of the present study was to investigate more general configurations for Helmholtz resonators and to evolve optimum designs. Primary interest was in developing a new approach to generating sound at frequencies below 100 Hz, where existing capabilities were very limited. Some of the principal avenues of development were 1 application of flexural disks to drive the resonators and comparative evaluation of disk and ring drivers, 2 introduction of compliant tubes into the compliance chamber to decrease its size, and 3 application of acoustoelectrical feedback to broaden the response at resonance. The study was conducted using a lumped-parameter approach and electroacoustical circuit analysis. Experiments with small-size models confirmed the general validity of this approach. A design procedure for disk-driven Helmholtz resonators is outlined. A numerical example illustrating the procedure indicates that a flexural disk 1-meter in diameter driving a Helmholtz resonator resonant at 40 Hz should be capable of producing a narrowband source level of 196 dBi micrometers Pa.m. It would also be useful as a broadband source, producing 185 dB source level over a band 37-100 Hz. The transducer would have a maximum depth capability of 450 m and a required minimum depth of 11 m to suppress internal cavitation.
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