Acoustic device for forming a wall of sound underwater

Abstract

An acoustic device for forming a wall of sound underwater. Walls of sound are useful in water for guiding fish away from areas. Hitherto, systems have used bubbles of water to facilitate the speed of travel of the sound. Those systems have suffered from the ability to maintain a consistent wall of bubbles and have often been inefficient. The present invention seeks to provide an acoustic device for forming a wall of sound underwater and comprises a transducer connected to an acoustic waveguide. The waveguide comprises one or more projections, whereby sound generated by the transducer travels along the projection or projections to form the wall of sound. The waveguide may typically comprise compressible projections formed from neoprene. These projections are arranged in a fan or frond shape. In addition or alternatively, the projections may be laminar. The projections could be of different lengths.

Claims

1. An acoustic device for forming a wall of sound underwater, said device comprising a transducer connected to an acoustic waveguide, the waveguide comprising one or more projections, whereby sound generated by the transducer travels along the projection or projections to form the wall of sound, wherein the improvement lies in that the or each projection is formed from a gas containing foam material.

2. An acoustic device as claimed in claim 1, wherein there are two or more projections.

3. An acoustic device as claim in claim 1, in which there is a greater volume of the waveguide where connected to the transducer.

4. An acoustic device as claimed in claim 3, in which the projections have different lengths.

5. An acoustic device as claimed in claim 4, in which there are three or more projections and are disposed such the length of the middle projection is shorter than the length of the outer projections.

6. An acoustic device as claimed in claim 2, wherein the projections are arranged in a fan/frond.

7. An acoustic device as claimed in claim 4, in which there are three or more projections and are disposed such the length of the middle projection is longer than the length of the outer projections.

8. An acoustic device as claimed in claim 2, wherein the projections are laminar.

9. An acoustic device as claimed in claim 1, in which each projection is tapered at the end not connected to the acoustic waveguide.

10. An acoustic device as claimed in claim 1, wherein there are two or more transducers and that the transducers are disposed so as to provide a contiguous wall of sound.

11. An acoustic device as claimed in claim 10, in which each end of the or each projection is connected to a transducer.

12. An acoustic device for forming a wall of sound underwater, said device comprising a transducer connected to an acoustic waveguide, the waveguide comprising one or more projections, whereby sound generated by the transducer travels along the projection or projections to form the wall of sound wherein the improvement lies in that the or each projection is formed from a tube of thin rubber containing a gas.

13. An acoustic device as claimed in claim 12, in which said tubes are pressurised prior to deployment.

14. An acoustic device as claimed in claim 12, wherein there are two or more projections.

15. An acoustic device as claim in claim 12, in which there is a greater volume of the waveguide where connected to the transducer.

16. An acoustic device as claimed in claim 15, in which the projections have different lengths.

17. An acoustic device as claimed in claim 16, in which there are three or more projections and are disposed such the length of the middle projection is shorter than the length of the outer projections.

18. An acoustic device as claimed in claim 14, wherein the projections are arranged in a fan/frond.

19. An acoustic device as claimed in claim 16, in which there are three or more projections and are disposed such the length of the middle projection is longer than the length of the outer projections.

20. An acoustic device as claimed in claim 14, wherein the projections are laminar.

21. An acoustic device as claimed in claim 12, in which each projection is tapered at the end not connected to the acoustic waveguide.

22. An acoustic device as claimed in claim 12, wherein there are two or more transducers and that the transducers are disposed so as to provide a contiguous wall of sound.

23. An acoustic device as claimed in claim 22, in which each end of the or each projection is connected to a transducer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will be described by way of further example only and with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of the present invention;

(3) FIG. 2 is a schematic diagram of an alternative embodiment;

(4) FIG. 3 is a schematic diagram of a further alternative embodiment; and

(5) FIG. 4 is a schematic diagram of a further alternative embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(6) FIG. 1 illustrates a first embodiment of the present invention. In this instance, the present invention is described in the sense of using the system to generate a sound field. The embodiment comprises two elements, means for generating a sound and fronds of material of a compressible nature to alter the propagation of the sound.

(7) A set of transducers or loudspeakers or projectors 2 are provided at intervals at a convenient position underwater. The transducers 2 are located on the seabed 4 or platform (not shown) but below the water surface 6. The transducers may for instance be mounted at the mid-water depth where the water is shallow.

(8) The transducers 2 are provided with a diaphragm 8, coil 10 and magnet 12 housed in a flexible surround 14 in accordance with known underwater loudspeakers.

(9) A waveguide 16 is connected to each transducer 2. Each waveguide 16 is formed from a set of one or more compressible projections 18. These compressible projections 18 serve to carry the sound from the underwater projectors 2. These projections are buoyant and so intrinsically float away from the respective transducer.

(10) Depending on the nature and quantity of the compressible projections 18 which are put in the water, the waveguide 16 can both match the impedance of the surrounding water to the transducer 2. Accordingly, sound is transduced using small volume displacements of the radiated sound field at high acoustic pressure instead of large volume displacements of the projectors at low acoustic pressure. The waveguide 16 also conducts the sound through the area around the projectors through the surrounding water by locally reducing the impedance of the water. Finally, and if required, by gradually reducing the quantity of the compressible projections 18 at the periphery of the waveguide 16, the sound travelling in the waveguide can be efficiently conducted into the surrounding water.

(11) FIGS. 2, 3 and 4 illustrate alternative waveguides 16 with different arrangements of the compressible projections 18.

(12) Typically, the waveguide 16 may be comprised of strings of a compressible material, typically oriented in the direction in which the sound is to travel. The strings need to be relatively compressible when compared with the surrounding water. For instance, the strings may be a compressible gas-containing foam such as foam neoprene. Since a small proportion of compressible gas is sufficient to significantly locally vary the acoustic impedance of the mixture of water, the volume of the strings may not have to be particularly large to form an effective acoustic waveguide. A system might, for instance, comprise strings of perhaps a few mm square, spaced at a distance of a few tens of mm. The strings could be fixed by tying at each end, or could be secured at the bottom and allowed to float under their own buoyancy in the water.

(13) While the strings might be of constant cross-section along their length, it would be possible to ensure that any particular volume of compressible gas that is necessary to provide an efficient waveguide is provided at any depth, by simply allowing for the effects of depth in compressing the material. For instance, if it were desired to achieve a constant proportion of compressible gas with depth, the volume of the compressible gas containing material when cut at the surface would be roughly twice as great for the material that would finish up at 10 meters depth as for that at the surface, and three times as great for that at 20 meters. When the material was deployed in the water, the material at 20 m would be compressed by a factor of about three, and that at 10 m by a factor of two, ensuring a constant volume of the deployed material along its entire length.

(14) It may be beneficial to provide material whereby the volume of material is not constant with depth, or not constant along the wall. For instance, a significant problem with underwater projectors is the fact that they must provide a very high force if they are to radiate sound into the dense medium of water. However, if a much larger volume of material is provided near to the projector, it will greatly reduce the acoustic load on it. This means that a transducer that provides large volume displacements, but at a much lower force, can radiate the same sound power. Such transducers are much easier to manufacture, and have a greater life than traditional underwater transducers. If the material tapers away from the transducer, it can act as an acoustic transformer, allowing the low forces and pressures at the transducer to be converted to high pressures elsewhere in the curtain. This is of benefit in providing a maximum sound from the system.

(15) There are many alternative ways by which the waveguide 16 may be provided. For instance as shown in FIG. 4, the strings may be formed of a compliant material such as a rubber, formed into a balloon with gas material inside. Such balloons could be fixed along a cord or rope, being spaced sufficiently closely that they behave in a similar way to the continuous compressible material of above. Alternatively, the compressible material could be provided by a tube of thin rubber or a similar material containing gas. The tube could be pressurised prior to deployment.

(16) Where boats or shipping may use the water in which the system is located, the waveguide 16 may be terminated at a suitable depth below the surface. In this case, sound will still leak out from the tip of the strings and hence the system may still have effectiveness for fish in the water above the system.

(17) The device therefore has several important advantages. The principal advantage of the system is that since no compressor is required to form bubbles, the system is much more economical to run in applications such as fish guidance systems, where it may have to be run for long periods of time. In addition, it is possible to form an accurate wall of sound. This is because the projections 18 are tethered to the transducer and any movement of the projections is more restricted and more in synchronisation with neighbouring projections. Hence the wall of sound is more constant and less chaotic as is the case with bubble curtains which is more susceptible to disturbance. Finally, as any arbitrary amount of compressibility at any point may be readily provided, it is possible to optimise the performance of the system.

(18) The afore going description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention.