Acoustic transmitter

Details Acoustic transmitter Acoustic transmitter

1996-11-01

1998-05-26

Eldred, J. Woodrow

Communications, electrical: acoustic wave systems and devices

Signal transducers

Underwater type

363174, 181110, 310337, H04R 1700

Patent

active

057577285

DESCRIPTION:

BRIEF SUMMARY
This invention relates in general to acoustic transmitter sources for emitting sound waves in water, in particular low-frequency sound waves e.g. for use in offshore seismic explorations. Such sources, beeing employed for generation of sound waves in water, are referred to under various terms, such as seismic transmitters, flextensional transmitters or acoustic pingers, cf. also Sonar.
The acoustic source according to the invention can with advantage find uses in connection in such sound or tone transmitters, whereby the soundwaves emitted into the water can be reflected from the sea bottom and underlying geological formations, with subsequent listening or detection by means of hydrophones or geophones of various types.
The invention is particularly directed to a special design of sound-emitting casing surfaces or shells in acoustic transmitters, and the design described here can be considered more or less related to certain forms of flextensional transmitters.
Flextensional transmitters are described in many publications, and one example is L. H. Royster: "The flextensional concept. A new approach to the design of underwater acoustic transducers" Appl.Acoust. 3 (1970) p. 117-226. Other examples are G. Bromfield: "Class IV flextensional transducers", Proceedings Second International Workshop on Power Transducers, Toulon, France, June 1990, and in G. Bringham and B. Glass: "Present status in flextensional transducer technology", J. Acoust. Soc. A., 68(1980), p. 1046-1052.
The known flextensional shell principle is based on volume changes in a vibrating ellipsoidic shell. When the long axis of an ellipsoid is put into vibration, the length of the short axes thereof will also vibrate, but with a much larger amplitude. This implies that the ellipsoidic shape leads to an efficient generator of acoustic energy at low frequencies. In general the fundamental resonance frequency of the ellipsoidic shell is so low that the length thereof is small, compared to the wavelength in water.
A limiting factor in this type of shell is the mechanical stresses to which the shell is subjected when large vibrational amplitudes are desired. Normally aluminium or glass fiber reinforced plastic materials are employed as the shell material. The use of such shells is also restricted as a consequence of the complicated shapes that these shells have, which leads to complicated manufacturing methods.
A particular example of a complex shape that in appearance and in function deviates rather strongly from the above discussed elliptical, round shells (egg shape), are hyperbolic, round shells. The advantage of this particular shape consists therein that the short axes will vibrate in-phase with the long axes. An embodiment based on this principle is described by D. F. Jones and C. G. Reithmeier: "Low frequency barrel-stave projectors" (Proceedings UDT 93, Nice, France, Microwave Exh. & Pub. Ltd. 1993, pp 251-253).
The latter embodiment in short comprises octagonal end plates joined to eight rod-shaped elements having a concave curvature and being placed in an essentially circular or polygonal arrangement about an axis of symmetry. A significant problem with this structure is the high number of gaps between the rod elements, associated with the question of sealing and possible contact between these elements.
This invention has for an object to provide an improved acoustic transmitter which by a simpler and more favourable structure than the previously known designs, gives a more reliable transmitter unit which at the same time has a high acoustic efficiency.
Thus in view of the known techniques discussed, the invention takes as a starting point an acoustic transmitter with sound-emitting surfaces having in the principle a concave configuration and being adapted to be excited into vibrational movement by means of a drive unit which activates the sound-emitting surfaces through the intermediary of at least one push element and two plate-like end pieces being connected to the sound-emitting surfaces.
What is novel and specific in the acoustic t

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