Low frequency underwater sound source

Details Low frequency underwater sound source Low frequency underwater sound source

1998-09-08

1999-12-07

Pihulic, Daniel T.

Communications, electrical: acoustic wave systems and devices

Signal transducers

Underwater type

367 1, G01V 102, G10K 1118

Patent

active

059994916

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to devices for the production of sound and in particular to the production of sound of low frequencies in underwater applications.
2. Discussion of the Prior Art
Considerable effort has been expended in reducing the operating frequency of transducers that are used in underwater communication, sonar and other instrumentation. Whereas it is relatively straightforward to measure and record low frequency sound, such sound generation is more difficult. The current generation of low frequency transducers is generally based on piezoelectric or magnetostrictive materials operating within a flextensional capsule which increases the displacement of water produced by the device and hence renders its operation more efficient. However these transducers suffer from the disadvantage that to retain a given degree of efficiency in operation, their size must increase as the inverse of the operating frequency. Size, weight and cost considerations therefore limit the lower usable frequency of existing transducers to 200 Hz or so. Also the device becomes increasingly resonant as frequency decreases thereby limiting the frequency range over which it can be used.
U.S. Pat. No. 3,437,170 discloses acoustic generators which release bubbles underwater from a number of chambers having different volumes for producing waves having predetermined energy spectra. Also U.S. Pat. No. 3,493,072 discloses an acoustic generator for marine seismic exploration in which pressurised gas is released from an underwater chamber impulsively to create air bubbles having varying fundamental frequencies.


SUMMARY OF THE INVENTION

The present invention is directed towards an improved low frequency acoustic generating system which is capable of overcoming certain disadvantages of prior art systems.
Thus according to the present invention an acoustic generating system comprises bubble generation means for forming pressurised gas bubbles, means for directing the bubbles in a plume from an outlet, and means for applying a varying pressure to the base of the plume of bubbles for modulating the plume of bubbles, the means for applying a varying pressure comprising a low frequency sound source whereby a source of low frequency sound waves is produced and preferably being located adjacent to the outlet. In use, the outlet of the acoustic generating system is preferably immersed in a liquid medium such as sea water.
According to a further aspect of the invention, there is provided a method emitting sound of a known low frequency by use of an acoustic generating system as hereinbefore described.
It has been found that plumes of bubbles in water possess an unusual acoustic property, namely that of directing sound. Small proportions of air in water (as bubbles), typically tenths or hundredths of a percent, make substantial changes to the sound velocity. The presence of air bubbles in larger proportions further decreases the sound speed, to a minimum of about 35 ms.sup.-1 for a few percent of air. In a bubble plume, the highest concentration of air is found along the flow axis and hence sound is trapped within the plume by diffraction towards the sound speed minimum. In addition, the acoustic impedance of the plume varies continuously along its length from a very low value at its point of formation to a high value at the point where the bubbles are dispersed or absorbed into the water. This leads to the situation in which small pressure changes caused by changes in air volume at the point of plume formation can produce large pressure changes (albeit associated with small changes in volume) at the end of the plume and ultimately in the surrounding fluid. The effect which is achieved by pulsing the air supply to the plume is very similar to that of an acoustic horn or loud-hailer in air. Thus the possibility exists of making use of a small source which is required to generate bubbles only and utilising volume changes in the neck of the plume to generate high levels of low frequency s

REFERENCES:
patent: 3177466 (1965-04-01), Arnoldi
patent: 3437170 (1969-04-01), Brock et al.
patent: 3493072 (1970-02-01), Johnston
Patent Abstracts of Japan vol. 95, No. 008 & JP 07 203579 A (Mitsubishi Heavy Ind Ltd), Aug. 4, 1995, see abstract.

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