WO2003032012A1

WO2003032012A1 - Underwater sound source

Info

Publication number: WO2003032012A1
Application number: PCT/GB2002/004473
Authority: WO
Inventors: Peter Dobbins
Original assignee: Bae Systems (Land And Sea Systems) Limited
Priority date: 2001-10-05
Filing date: 2002-10-02
Publication date: 2003-04-17
Also published as: GB0123987D0

Abstract

Described herein is an underwater sound source which utilises an rgan pipe assembly (2) powered by a gas/fluid mixture (8). The pipe assembly (2) comprises a pipe (2a) which comprises first and second portions (2b, 2c) separated by a wall portion (2d). The gas/liquid mixture (8) is forced into the pipe (2) through inlet (6), and due to the pressure, is routed through a secondary inlet (12) formed in wall portion (2d) and into the second portion (2c) of the pipe assembly (2). By varying the ratio of gas to liquid in the gas/liquid mixture and/or the diameter of the gas bubbles in the gas/liquid mixture, the frequency of sound generated can be varied for a pipe having predetermined fixed dimensions.

Description

UNDERWATER SOUND SOURCES

The present invention relates to underwater sound sources and is more particularly, although not exclusively, concerned with the generation of sound waves in a liquid, for example, seawater. In underwater applications where a continuous sound source is required, for example, in applications such as sonar and signalling, a number of different techniques have been utilised. Large transducers and other electrically powered sound sources have been used in systems using such techniques, and the power requirements for ship- and submarine-borne systems are not insignificant. Methods for producing continuous sound underwater have been an area of significant research.

It is known to use water in pipes for musical instruments. Such an arrangement is described in JP2001306060. Here, a musical instrument similar to an organ is described which comprises a plurality of pipes of differing size each of which is partly filled with liquid. The musical instrument operates by releasing single drops into the pipes as required, thus exciting resonance in the air-filled portion of the pipe to produce different musical sounds.

DE20008371 discloses a mouth organ arrangement which can be plunged into water. The arrangement includes a musical vibrating air column pipe which has an extension piece enabling a brief tone to sound when the end of the air column pipe is plunged into water. In operation, the arrangement produces the brief tone when the pipe is plunged into water so that the water drives air through the pipe to produce the tone.

As a modification to a musical instrument, US3924560 discloses a birdcall whistle which has water filled chambers.

It is also known to use water in pipes as warning devices. Such a device is described in RU2023788. Here, an arrangement of apertures in the form of expansion chambers which are located in an underwater part of a wall of a quay. The chambers are connected by curved water pipes which narrow to seawater sprayers and organ pipes of different tones. Impellers are located in the water pipes and have generators which are connected to different coloured bulbs on the organ pipes. The arrangement operates to provide warning that the sea is rough. When the water is wavy, water passes through the water pipes with increasing speed and onto the seawater sprayers and the organ pipes. The organ pipes sound and the coloured bulbs are illuminated to provide the warning. In this device, each organ pipe has a different fixed tone, and hence frequency, and there is no change in the frequency of the sound generated by each pipe.

Examples of underwater organ pipes are known (see Shigeru

Yoshikawa, "Air (or water) jet as a vibrating diaphragm in aerial (or underwater) organ pipes", J. Acoust. Soc. Am., 91 (4), 2411 (1992)) but such disclosures utilise conventional pipes blown by the same fluid as the surrounding medium

(for example, seawater).

In the underwater communications field, it is known to mix air, in the form of bubbles, with water to enlarge the amplitude of a signal sound wave. JP7203579 discloses such an arrangement. An air bubble-mixing device is connected to both a gas supply and a water supply to form and discharge a water/bubbles mixture in the vicinity of the signal sound wave which is generated by a separate transmitter.

GB2322506 discloses an underwater acoustic arrangement in which a low frequency sound wave is generated. The arrangement comprises a low frequency underwater sound source and a bubble generator. The bubble generator forms pressurised gas bubbles in a plume from an outlet of the generator. The sound source applies a varying pressure to the plume of bubbles to modulate the plume and to generate the low frequency sound wave. Whilst JP7203579 discloses the use of bubbles to modulate a sound source and GB2322506 discloses the use of bubbles as a resonator, neither document discloses using a bubbly liquid as the driving medium of a sound generator.

It is therefore an object of the present invention to provide sound generating apparatus which utilises a gas/liquid mixture to modify the sounds speed or resonant frequency of a pipe assembly. In accordance with one aspect of the present invention, there is provided sound generating apparatus comprising a pipe assembly located in a liquid and having an inlet aperture adapted for the input of a gas/liquid mixture to determine the frequency of sound generated. The pipe assembly may have fixed dimensions. Alternatively, the pipe assembly may have variable dimensions.

Advantageously, the liquid in which pipe assembly is located is the same liquid as that used in the gas/liquid mixture.

In one embodiment of the present invention, the pipe assembly is constructed of a material which has a density that is very much greater than that of the liquid.

In another embodiment of the present invention, the pipe assembly is constructed of a material which has a density that is very much less than that of the liquid. Preferably, the ratio of gas to liquid in the gas/liquid mixture is less than

1 :100 and is more preferably less than 1 :1000. The bubbles within the gas/liquid mixture may have diameters which are less than 2mm.

Additionally, the apparatus further includes means for supplying the gas/liquid mixture. The means for supplying the gas/liquid mixture may comprise a container in which the gas/liquid mixture is stored under pressure. Alternatively, the means for supplying the gas/liquid mixture may comprise a mixing device which is connected to a source of the gas and the liquid. As a further alternative, the means for supplying the gas/liquid mixture may comprise a Venturi device which is connected to a source of the gas and which entrains liquid and mixes it with the gas. The gas may be stored under pressure.

It is preferable that the gas/liquid mixture is applied under pressure. The pressure may be less than 10kPa in excess of atmospheric pressure.

It is also preferable that the volume flow rate of the gas/liquid mixture is less than 10m³s^"1. In one embodiment, the gas may be air and the liquid may be water. In a sea environment, the liquid may be seawater.

In accordance with a second aspect of the present invention, there is provided a method of producing a frequency variable acoustic signal in a liquid, the method comprising the steps of:- locating a pipe assembly in a liquid; supplying the pipe assembly with a gas/liquid mixture under pressure to generate the acoustic signal at one frequency; and varying the gas/liquid mixture to change the frequency of the acoustic signal generated.

Step c) may comprise varying the ratio of gas to liquid in the gas/liquid mixture. Alternatively, step c) may comprise varying the diameter of the bubbles of the gas in the gas/liquid mixture. It is also possible that step c) comprises varying both the ratio of gas to liquid in the gas/liquid mixture and the diameter of the bubbles of the gas in the gas/liquid mixture.

The method may include the further step of varying the dimensions of the pipe assembly.

For a better understanding of the present invention, reference will now be made to the accompanying drawing, the single Figure of which shows a diagrammatic representation of organ pipe assembly adapted for producing acoustic signals underwater in accordance with the present invention.

A water filled pipe is a classic example of the organ pipe resonance in which the resonant frequency of the pipe is known to be inversely proportional to the length of the pipe and directly proportional to the speed of sound in the pipe. The water in the pipe provides a virtually solid medium which serves only to vary the effective length of the pipe and hence the resonant frequency.

It is known that the speed of sound in a fluid medium is inversely proportional to the square root of the product of the fluid density and its compressibility. Therefore, by adding a very small volume of air (or other gas) to a liquid in the form of very small, dispersed bubbles, a new fluid having different physical properties is produced.

If the volume of gas is small compared to the volume of liquid, the density change is also comparatively small. However, the change in compressibility is quite significant resulting in a change in the speed of sound within the liquid. Whereas known devices have changed the resonant length of the pipe whilst the sound speed remains constant, the present invention depends upon changing the sound speed whilst the length is kept constant.

For organ pipes (whether wood, metal or any other solid material) operating in air, there is a natural acoustic impedance mismatch at the air pipe surface which ensures the energy in the organ pipe wave is contained to form the standing acoustic wave. In the underwater case, the density mismatch is substantially reduced. This necessitates that the density of the material from which the pipe is made must be substantially greater or substantially less than that of the fluid. It is to be noted that this mismatch may be achieved by utilising a double walled pipe having a gas or foam infill.

The present invention will now be described with reference to Figure 1.

In Figure 1 , a pipe assembly 2 is shown immersed in a liquid 4. In a typical application, the liquid 4 comprises seawater. In accordance with the present invention, the pipe assembly 2 can be considered to be identical to an organ pipe assembly whose operation is generally well known.

The pipe assembly 2 comprises a pipe 2a which comprises a first portion 2b and a second portion 2c separated by a wall portion 2d. As shown, the first portion 2b is smaller in size than the second portion 2c and can be considered to be a pressure reduction chamber. The second portion 2c is open at the end 2e remote from the wall portion 2d and can be considered to be the main body of the pipe 2a. The second portion 2c is a resonating device which generates a sound in accordance with the frequency of a standing wave generated inside it. The standing wave is formed by the reflection of an acoustic wave at the end 2e due to an interface between the inside of the second portion 2c and the surrounding water 4. An inlet 6 is formed in the first portion 2b and an aperture 10 is formed in the second portion 2c. In a typical organ pipe assembly, the relationship between the size of the aperture 10 and the diameter of the main body 2c of the pipe 2a determines the frequency of the sound wave generated. This is the same in the present invention, but in this case, it is not air or water which is passed through the pipe 2 to set up the standing wave, but a gas/liquid mixture 8 as will be described in more detail below.

In operation of the pipe assembly 2, the gas/liquid mixture 8 is forced into the pipe 2a through the inlet 6 under pressure and into first portion 2b as indicated by arrow 'A'. Due to the pressure, the gas/liquid mixture 8 is routed through a secondary inlet 12 formed in wall portion 2d and into the second portion 2c of the pipe assembly 2. A standing acoustic wave is developed within the second portion (or main body) 2c of the pipe assembly 2 as described above. In accordance with the present invention, the combination of an organ pipe assembly and a bubbly gas/liquid mixture provides an organ type sound generator which, whilst having fixed physical parameters, such as length and diameter, can generate sound having a range of frequencies. The bubbly gas/liquid mixture modifies the speed of sound in the pipe assembly. In particular, the frequency can be lowered, relative to the size of the pipe, so that a compact sound source can be made which is smaller than that which can be achieved using existing techniques. Whilst normally such a device is only capable of producing a single tone, the amplitude and frequency of that tone can be continuously variable by modifying the gas/water mixture proportions and the driving pressure. It is thus capable of producing many signal types that are useful in sonar and communications systems such as chirps.

In order to contain a standing wave within the main body 2c of the pipe

2a, the material from which the pipe 2a is made must have a density which is either very much greater than or very much less than water as discussed above. Moreover, it is preferred that the material has a relatively low elastic constant to minimise structural vibrations. Such materials may include, but are not limited to, heavier metals such as lead, iron, copper, gold, natural rocks/minerals, concrete (denser than water), and syntactic foams, double-walled structures containing air/gas (like double glazing) which are lighter than water. In accordance with the present invention, it is preferred that air is the gas and seawater is the liquid in the gas/liquid mixture when used in an underwater environment. However, it will be appreciated that any other suitable combination of gas and liquid could be used if the apparatus of the present invention is intended to operate in a different liquid environment. In the seawater environment, preferably the diameter of the bubbles used is less than 2mm and the ratio of air to seawater is less than 1 :100 and is more preferably less than 1 :1000. However, it will be appreciated that other suitable bubble diameters and ratios of air to seawater may be used if dictated by a particular application. The pressure and volume flow requirements for the gas/seawater mixture used by the invention are modest - the excess pressure being less than 10kPa (pressure above atmospheric pressure), and the volume flow rate less than 10mV¹.

Using different combinations of pipe materials, gases, and liquids, bubble diameters and gas/liquid ratios, it is possible to produce a range of frequencies substantially within the range of 10Hz to 10kHz, and more preferably, within the range of 10Hz to 1kHz.

There are different ways of supplying the gas/liquid mixture to the pipe assembly, and these may include:- • Pumping a stored gas/liquid mixture (as in carbonated drinks, for example) through the pipe assembly;

• Pumping the gas and liquid separately, and mixing them before they pass through the pipe assembly;

• Pumping the gas and using a Venturi device (similar to a carburettor) to entrain liquid from the surrounding medium and to mix it with the gas; and • Using gas stored under pressure (such as a SCUBA air bottle, for example) along with a Venturi device to entrain liquid from the surrounding medium and to mix it with the gas.

The pipe assembly in accordance with the present invention may be suspended from a platform on the seabed, floating in the sea or floating on the surface of the sea. The pipe assembly may either have a bubble generator connected directly to it or it may be connected to a remote bubble generator. The bubble generator may be attached to the pipe assembly by any suitable means. In addition to the pipe assembly having fixed dimensions as discussed above, further scope for altering the frequency of the sound generated could be provided by having a pipe assembly which has variable dimensions, that is, dimensions that could be varied in use.

Furthermore, the pipe assembly in accordance with the present invention requires no electrical or other power supplies, and the gas could be supplied from a pressurised container as described above to provide an entirely independent acoustic source which could be used, for example, as a distress beacon.

Claims

1. Sound generating apparatus comprising a pipe assembly located in a liquid and having an inlet aperture adapted for the input of a gas/liquid mixture.

2. Apparatus according to claim 1, wherein the pipe assembly has fixed dimensions.

3. Apparatus according to claim 1 , wherein the pipe assembly has variable dimensions.

4. Apparatus according to any one of the preceding claims, wherein the liquid in which pipe assembly is located is the same liquid as that used in the gas/liquid mixture.

5. Apparatus according to claim 4, wherein the pipe assembly is constructed of a material which has a density that is very much greater than that of the liquid.

6. Apparatus according to claim 4, wherein the pipe assembly is constructed of a material which has a density that is very much less than that of the liquid.

7. Apparatus according to any one of the preceding claims, wherein the ratio of gas to liquid in the gas/liquid mixture is less than 1 :100.

8. Apparatus according to claim 7, wherein the ratio of gas to liquid in the gas/liquid mixture is less than 1:1000.

9. Apparatus according to any one of the preceding claims, wherein the bubbles within the gas/liquid mixture have diameters which are less than 2mm.

10. Apparatus according to any one of the preceding claims, further including means for supplying the gas/liquid mixture.

11. Apparatus according to claim 10, wherein the means for supplying the gas/liquid mixture comprises a container in which the gas/liquid mixture is stored under pressure.

12. Apparatus according to claim 10, wherein the means for supplying the gas/liquid mixture comprises a mixing device which is connected to a source of the gas and the liquid.

13. Apparatus according to claim 10, wherein the means for supplying the gas/liquid mixture comprises a Venturi device which is connected to a source of the gas and which entrains liquid and mixes it with the gas.

14. Apparatus according to claim 13, wherein the gas is stored under pressure.

15. Apparatus according to any one of the preceding claims, wherein the gas/liquid mixture is applied under pressure.

16. Apparatus according to claim 15, wherein the pressure is less than 10kPa in excess of atmospheric pressure.

17. Apparatus according to any one of the preceding claims, wherein the volume flow rate of the gas/liquid mixture is less than 10m³s^"1.

18. Apparatus according to any one of the preceding claims, wherein the gas is air and the liquid is water.

19. Apparatus according to claim 18, wherein the liquid is seawater.

20. A method of producing a frequency variable acoustic signal in a liquid, the method comprising the steps of:-

a) locating a pipe assembly in a liquid; b) supplying the pipe assembly with a gas/liquid mixture under pressure to generate the acoustic signal at one frequency; and c) varying the gas/liquid mixture to change the frequency of the acoustic signal generated.

21. A method according to claim 20, wherein step c) comprises varying the ratio of gas to liquid in the gas/liquid mixture.

22. A method according to claim 20, wherein step c) comprises varying the diameter of the bubbles of the gas in the gas/liquid mixture.

23. A method according to claim 20, wherein step c) comprises varying both the ratio of gas to liquid in the gas/liquid mixture and the diameter of the bubbles of the gas in the gas/liquid mixture.

24. A method according to any one of claims 20 to 23, further including the step of varying the dimensions of the pipe assembly.