APPARATUS AND METHOD FOR LIMITING SOUND TRANSMISSION

Abstract

In summary, there is provided apparatus (1) for limiting transmission of sound from a sound emitter (2), the apparatus comprising a container (4) arranged to at least partially surround a sound emitter, the container retaining a liquid, wherein the liquid comprises at least 2% dissolved oxygen, and wherein the apparatus further comprises an agitator (6) configured to agitate the liquid, to thereby cause the formation of gas bubbles (8) in the liquid.

Claims

1. Apparatus for limiting transmission of sound from a sound emitter, the apparatus comprising a container arranged to at least partially surround a sound emitter, the container retaining a liquid, wherein the liquid comprises at least 2% dissolved oxygen by volume, and wherein the apparatus further comprises an agitator configured to agitate the liquid, to thereby cause the formation of gas bubbles in the liquid.

2. Apparatus according to claim 1, wherein the container has at least one opening defined therein, such that the bubbles can leave the container via the opening.

3. Apparatus according to claim 1, wherein the agitator comprises a bubble curtain generator, optionally arranged to generate a bubble curtain at least partially surrounding the sound emitter.

4. Apparatus according to claim 1, wherein the container retains the sound emitter, and wherein the sound emitter is submerged in the liquid.

5. Apparatus according to preceding claim 1, wherein the container is a first container comprising one or more first container walls, wherein the apparatus comprises a second container comprising one or more second container walls, the second container at least partially surrounding the sound emitter, and wherein the first container at least partially surrounds and is spaced apart from the second container, optionally wherein the liquid is retained between the first container walls and the second container walls, further optionally wherein the first container defines a volume of at least 0.5 m.sup.3.

6. (canceled)

7. Apparatus according to claim 1, wherein the liquid comprises water, optionally wherein the liquid comprises hydrogen peroxide, further optionally wherein the liquid comprises an enzyme for the decomposition of hydrogen peroxide, further optionally wherein the liquid comprises a surfactant.

8. (canceled)

9. (canceled)

10. (canceled)

11. Apparatus according to claim 1, wherein the liquid comprises at least 10% dissolved oxygen by volume.

12. Apparatus according to claim 1, wherein an agitation region surrounds the agitator, and wherein the agitator is configured to cause sufficient bubbles to form that the combined volume of bubbles is at least 1% of the total volume of the agitation region.

13. (canceled)

14. Apparatus according to claim 1, wherein the container comprises one or more barriers, the or each barrier extending across the interior of the container to thereby form a plurality of sub-containers, optionally wherein each of the plurality of sub-containers has a respective agitator provided therein.

15. Apparatus according to claim 1 comprising a pump configured to cause flow of liquid at the liquid surface, optionally in a downward direction.

16. Apparatus according to claim 1, wherein an agitation region surrounds the agitator, and wherein the agitator is configured to cause sufficient bubbles to form that, in the agitation region, the optical intensity of light having travelled through 10 centimetres of the liquid is reduced by at least 10% as compared to the intensity of that light before travelling through the liquid, optionally wherein the apparatus further comprises a light detector configured to output an indicator of the intensity of light having passed through a distance of the liquid.

17. (canceled)

18. Apparatus according to claim 16, comprising a controller configured to: receive the indicator of the intensity of light having passed through the distance of the liquid; and regulate the amount of agitation caused by the agitator, responsive to the received indicator to thereby maintain a target light intensity.

19. Apparatus according to claim 1, wherein the sound emitter comprises a marine piling apparatus.

20. An apparatus for limiting transmission of sound from a sound emitter, the apparatus comprising: a sound barrier, the sound barrier comprising a container retaining a liquid, wherein the liquid comprises at least 2% dissolved oxygen by volume; and, an agitator configured to agitate the liquid, to thereby cause the formation of gas bubbles in the liquid.

21. An apparatus according to claim 1, wherein the apparatus is configured to activate the agitator for an activation period, to thereby cause the generation of sufficient bubbles such that bubbles will then be suspended in the fluid for an attenuation period that is longer than the activation period.

22. A method of limiting transmission of sound from a sound emitter, the sound emitter being at least partially surrounded by a liquid, the liquid comprising at least 2% dissolved oxygen by volume, and the method comprising: causing agitation of the liquid in the container to thereby cause the formation of gas bubbles in the liquid, optionally wherein the liquid is retained by a container, the container at least partially surrounding the sound emitter.

23. A method according to claim 22, wherein the method comprises causing gas bubbles to form comprises causing oxygen to be released out of the liquid to thereby form bubbles.

24. A method according to claim 22, the method comprising: receiving an indicator of the intensity of light having passed through a distance of the liquid; and regulating the amount of agitation caused, responsive to the received indicator, to thereby maintain a target light intensity.

25. A method according to claim 22, the method further comprising: causing mixing of water, hydrogen peroxide, and a catalyst for the decomposition of hydrogen peroxide to thereby form the liquid comprising water and at least 2% dissolved oxygen by volume.

26. An apparatus according to claim 1, wherein the apparatus further comprises an agitator disposed in the liquid, the agitator configured to agitate the liquid proximal to the agitator, to thereby cause the formation of gas bubbles in the liquid.

27. (canceled)

Description

DESCRIPTION OF THE DRAWINGS

[0099] An example embodiment of the present invention will now be illustrated with reference to the following Figures in which:

[0100] FIG. 1 is a side elevation cross-sectional view diagram of a first example embodiment of an apparatus according to the invention;

[0101] FIG. 2 is a second side elevation cross-sectional view diagram of the first example embodiment of an apparatus according to the invention;

[0102] FIG. 3 is a side elevation cross-sectional view diagram of a second example embodiment of an apparatus according to the invention;

[0103] FIG. 4 is a side elevation cross-sectional view diagram of a third example embodiment of an apparatus according to the invention;

[0104] FIG. 5 is a plan elevation view diagram of a further example embodiment of an apparatus according to the invention;

[0105] FIG. 6 is a flow chart of steps in a method according to an example embodiment of the invention;

[0106] FIG. 7 is a schematic illustration of an apparatus according to an example embodiment of the invention; and

[0107] FIG. 8 is a plot of the speed of sound through a fluid.

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

[0108] It will be understood by those skilled in the art that any dimensions and relative orientations such as lower and higher, above and below, and directions such as vertical, horizontal, upper, lower, longitudinal, axial, radial, lateral, circumferential, etc. referred to in this description refer to, and are within expected structural tolerances and limits for, the technical field and the apparatus and methods described, and these should be interpreted with this in mind.

[0109] FIG. 1 is a side elevation view diagram of a first example embodiment of an apparatus 1 according to the invention. Here, the sound emitter 2 is a marine pile, part of a set of marine piling equipment 10. A container 4 has been arranged to surround the marine pile 2, the container 4 being filled with liquid, in this case sea water comprising 50% dissolved oxygen, by volume. In the lower portion of the container 4 there is provided an agitator in the form of a mixer 6 which, when in use, agitates the fluid to thereby cause oxygen to be released from the water and form bubbles 8. At the top of the container 4 there is provided a pump 22 configured to cause liquid near the top of the container and near the liquid surface 18 to flow in a downwards direction, as indicated by arrow 24. In FIG. 1 the mixer 6 has been operated for only a few seconds and therefore a relatively small number of bubbles have been generated.

[0110] The marine pile 2 is partially submerged in the liquid in the container, in that approximately 85% of the marine pile 2 is below the liquid surface 18. The container 4 is in turn partially surrounded by the sea, and thus by sea water. The container 4 extends from above the sea water surface 20 to the sea floor 26. The container 4 has a cylindrical outer wall 30, however it is open at its base and at its top. The cylindrical outer wall 30 is provided with weights (not shown) which weigh the wall down, bringing it sealedly into contact with the sea floor 26. The sea floor 26 may thus in this instance be thought of as providing the base of the container 4. The open top provides an opening in the container 4, via which bubbles 8 may leave the container, e.g. as they rise through the liquid under buoyancy. The outer cylindrical wall 30 of the container 4 extends both above the liquid surface 18 and above the sea water surface 20. The liquid surface 18 is slightly above the sea water surface 20 in this instance, because the fluid is slightly less dense than the sea water, and therefore hydrostatic forces act on the container 4 (which is slightly flexible), thereby compressing it slightly and causing a hydrostatic head in the container 4.

[0111] FIG. 2 is a second side elevation view diagram of the first example embodiment of the apparatus 1. Here, the mixer 6 has been in operation for a longer period (.sup.60 minutes) and as a result a larger number of bubbles 8 have formed. The bubbles 8 are continuously formed as a result of the mixer 6 agitating the fluid. The bubbles 8 tend to float upwards through the fluid due to buoyancy. As the bubbles 8 approach the fluid surface 18 the flow caused by the pump 22 tends to cause a portion of the bubbles 8 to move back downwards through the fluid, rather than allowing all bubbles 8 to leave the fluid as they reach the fluid surface 18. As a result, an annular bubble curtain 32 has formed, which partially surrounds the marine pile (i.e. the portion of the pile which is beneath the liquid surface 18 is radially surrounded by the bubble curtain 32).

[0112] FIG. 3 is a side elevation cross-sectional view of a second example embodiment of an apparatus 101 according to the invention. As with the embodiment of FIGS. 1 and 2, the apparatus 101 includes a container arranged to surround the sound emitter (again in the form of marine piling equipment 110) and the container is filled with liquid. In this instance, the container has two horizontal barriers 114a, 114b, each in the form of a flat disc with a central hole 116a, 116b. The horizontal barriers 114a, 114b each extend partway across the interior of the container, from the interior side of the cylindrical wall towards the central part of the container, with the hole 116a, 116b surrounding the marine pile, such that there is a space between each barrier 114a, 114b and the pile. The container may thus be understood to be made up of three sub-containers 104a, 104b, 104c. Each sub-container 104a, 104b, 104c is provided with two mixers 106a, 106b, 106c, 106d, 106e, 106f which, when in use, agitate the fluid in the respective sub-container 104a, 104b, 104c to thereby cause oxygen to be released from the water and form bubbles 108.

[0113] Barrier 114b forms the top of the lowermost container 104c and, as the barrier 114b has the hole 116b surrounding the pile, arranged so as to provide a space between the pile and the barrier 114b, bubbles 108 reaching the top of the sub-container 104c can leave the sub-container 104c and move into the middle sub-container 104b via the hole 116b. Similarly, barrier 114a forms the top of the middle sub-container 104b and, as the barrier 114a as the hole 116a surrounding the pile, arranged so as to provide a space between the pile and the barrier 114a, bubbles 108 reaching the top of sub-container 104b can leave the sub-container 104b and enter the uppermost sub-container 104a via the hole 116a. The uppermost sub-container 104a is open at its top surface and thus bubbles 108 reaching the top of the uppermost sub-container 104a can leave at the liquid surface 118. At the top of each sub-container 104a, 104b, 104c there is provided a pump (not shown) configured to cause liquid near the top of each respective sub-container 104a, 104b, 104c and near the liquid surface 118 to flow in a downwards direction. In FIG. 3 each mixer 106a, 106b, 106c, 106d, 106e, 106f has been operated for only a few seconds and therefore a relatively small number of bubbles have been generated by each mixer 106a, 106b, 106c, 106d, 106e, 106f. However, as each sub-container 104a, 104b, 104c is smaller than the container 4 of FIGS. 1 and 2, and as each sub-container is provided with two mixers 106a, 106b, 106c, 106d, 106e, 106f, a significantly larger quantity of bubbles can be generated per litre of liquid, and in a relatively short time.

[0114] As with FIGS. 1 and 2, the marine pile is partially submerged in the liquid in the container, in that approximately 85% of the marine pile is below the liquid surface 18. The container is in turn partially surrounded by the sea, with sub-containers 104b and 104c being surrounded by the sea and sub-container 104a being partially surrounded by the sea and partially extending above the sea surface 120. The container extends from above the sea water surface 120 to the sea floor. The container has a weighted cylindrical outer wall, however it is open at its base and at its top. The sea floor may thus in this instance be thought of as providing the base of the lowermost sub-container 104c. The open top provides an opening in the uppermost sub-container 104a, via which bubbles 8 may leave the container, e.g. as they rise through the liquid under buoyancy. The outer cylindrical wall of the container extends both above the liquid surface 118 and above the sea water surface 120.

[0115] In use, the mixer 6, 106a, 106b, 106c, 106d, 106e, 106f, is switched on and begins agitating the fluid in the container 4 or sub-container 104a, 104b, 104c. As the fluid is agitated, the oxygen dissolved in the fluid is released from the fluid, thereby forming bubbles 8, 108. The bubbles 8, 108 travel upwards in the fluid and in the container 4 or sub-container 104a, 104b, 104c towards the liquid surface 18, or the top of the sub-container 104b, 104c. At the liquid surface 18, or the top of the sub-container 104b, 104c, the flow caused by the pump 22 (not shown in FIG. 3), causes the liquid and a portion of the bubbles 8, 108, therein to travel in a downwards direction, as indicated by arrow 24 (not shown in FIG. 3). The bubbles 8, 108 thereby circulate within the container 4 or sub-container 104a, 104b, 104c and a bubble curtain 32 is formed (or may be formed in each respective sub-container), surrounding the sound emitter 2.

[0116] When the bubble curtain 32 has been formed, the sound emitter (here the marine pile) 2 may be activated. Sound travels outwards from the sound emitter 2, and most especially from the point where the marine pile 2 meets the sea floor 26. Sound waves travel through the liquid until they meet the bubble curtain 32. When the sound waves reach the bubble curtain 32, they repeatedly pass through and are scattered by multiple liquid-gas boundaries. Each bubble 8, 108 encountered by a sound wave represents two such boundaries (a first boundary as the sound wave travels from the liquid into the gas of the bubble 8, 108, and a second as the sound wave travels from the gas of the bubble 8, 108 back into the liquid surrounding the bubble 8, 108). Without wishing to be bound by theory, the inventor believes that sound transmission is limited as a result of the scattering and absorption of the sound waves that takes place at each such interface, with the result that sound energy is absorbed and the sound is attenuated after a high number of interactions with bubbles 8, 108 in the bubble curtain 32. As a result, very little sound is able to travel all the way through the bubble curtain 32 and escape beyond the container 4 (or sub-container 104a, 104b, 104c) and into the sea.

[0117] Marine piling can produce pressures of 180 decibels and above at 750 metres from the marine pile. This is harmful to marine life. The bubbles 8, 108 generated by operation of the apparatus 1, 101, 201 limit the transmission of sound, such that the sound pressure on the other side of the bubbles 8, 108 to the sound emitter 2 is reduced by 40 decibels compared to the sound pressure on the same side of the bubbles 8, 108 as the sound emitter 2. Accordingly, application of the apparatus 1, 101, 201 is beneficial to marine life and leads to a reduced environmental acoustic footprint.

[0118] When the marine piling operation has been completed (or is paused) the mixer 6, 106a, 106b, 106c, 106d, 106e, 106f and pump 22 are stopped. Bubbles 8, 108 steadily leave the fluid and the container 4 as they rise to the fluid surface 18, 118 under buoyancy. If no further marine piling is required, the apparatus 1, 101 can be removed, and optionally moved to the next marine pile 2.

[0119] FIG. 4 is a side elevation cross-sectional view of a third example embodiment of an apparatus 201 according to the invention. As with the embodiment of FIGS. 1, 2, and 3 the apparatus 201 includes a container 204 arranged to surround the sound emitter 202 (here a generator), the container 204 being filled with liquid. In this instance, the container 204 is an annular outer container 204 with an interior space 212 which is filled with air. The interior space 212 may be considered as an inner cylindrical container 212 which retains the sound emitter 202. The outer container 204 is provided with an agitator in the form of a mixer 206 which, when in use, agitates the fluid in the outer container 204 to thereby cause oxygen to be released from the water and form bubbles 208.

[0120] At the top of the outer container 204 there is provided a pump 222 configured to cause liquid near the liquid surface 218 to flow in a downwards direction as indicated by arrow 224. In FIG. 4 the mixer 206 has been operated for only a few seconds and therefore a relatively small number of bubbles have been generated.

[0121] In this example embodiment, the sound source is within the inner container 212, which is in turn partially surrounded by the outer container 204 and thus by the liquid in the outer container 204. The outer container 204 has an outer cylindrical wall 230, an inner cylindrical wall 236 and a base 238, however it is open at its top. The inner cylindrical wall 236 of the outer container 204 forms the cylindrical wall of the inner container 212. The inner container 212 also has a base 240 and an upper wall 242. The open top of the outer container 204 provides an opening in the outer container 204, via which bubbles 208 may leave the outer container 204, e.g. as they rise through the liquid under buoyancy. The outer cylindrical wall 230 of the outer container 204 extends above the liquid surface 218.

[0122] The apparatus 201 also has a light detector 215 (here a photodetector) and a light source 217 (here in the form of an LED) positioned within the outer container 204. The light detector 215 is configured to detect light emitted by the LED 217 and output an indicator of the intensity of light detected. The distance between the light detector 215 and the LED 217 is 20 centimetres. The apparatus 201 also has a controller (not shown) configured to receive the indicator of the intensity of light detected and regulate the amount of agitation caused by the mixer 206, responsive to the received indicator to thereby maintain a target light intensity.

[0123] The bubbles 208 are continuously formed as a result of the mixer 206 agitating the fluid. The bubbles 208 tend to float upwards through the fluid due to buoyancy. As the bubbles 208 approach the liquid surface 218 the flow caused by the pump 222 tends to cause a portion of the bubbles 208 to move back downwards through the fluid, rather than allowing all bubbles 208 to leave the fluid as they reach the fluid surface 218. As a result, an annular bubble curtain (not shown in FIG. 4) forms in the outer container 204, and thus the sound emitter 202 is partially surrounded by the bubble curtain.

[0124] Sound waves travel from the sound emitter 202, through the air in the inner container 212 until they reach the inner cylindrical wall 236. The sound waves travel through the inner cylindrical wall 236 and enter the fluid within the outer container 204 and continue travelling through this fluid until they reach the bubble curtain. Here, the sound waves are absorbed and scattered by the bubbles 208, similarly to the case as described in relation to FIGS. 1, 2, and 3. The mixer 206 and pump 224 may be operated continuously or may be operated only when the sound emitter 202 is emitting sound.

[0125] Light travels from the LED 217, through the liquid in the outer container 204 and is scattered by the bubbles. Accordingly, when the mixer 206 is not in operation (and thus relatively fewer bubbles are present) less scattering of the light occurs and more light will be detected by the light detector 215. The light detector 215 will therefore output an indication of a relatively higher intensity of light being detected. Conversely, when the mixer 206 is in operation (and thus relatively more bubbles are present) more scattering of the light occurs and the light detector 215 will therefore output an indication of a relatively lower intensity of light being detected.

[0126] If bubbles are not present to scatter light, they are also not present to scatter sound. Accordingly, when the controller receives an indication that the intensity of light received by the light detector 215 is below a threshold, the controller causes the degree of agitation caused by the mixer 206 to be increased (for example, by increasing the speed of the mixer 206). When the controller receives an indication that the intensity of light received by the light detector 215 is above a threshold, the controller causes the degree of agitation caused by the mixer 206 to be decreased (for example by decreasing the speed of the mixer 206) or stopped. In this way, it is possible to operate the apparatus 201 in a more efficient way, as it is not necessary to continually operate the mixer 206. In an example, it is has been found that it is possible to pause the mixing of the mixer 206 for 20 seconds before an increased intensity of light is detected, at which point the mixer can be caused to mix the fluid until the intensity of light is found to have decreased again.

[0127] FIG. 5 is a plan elevation view diagram of a further example embodiment of an apparatus 301 according to the invention. As with the embodiment of FIGS. 1 to 4 the apparatus 301 includes a container 304 arranged to surround the sound emitter 302, the container 304 being filled with liquid. In this instance the container is an open-topped cuboid with four vertical side walls 330a, 330b, 330c, 330d and a base (not shown). The container 304 is provided with an agitator in the form of a mixer 306 which, when in use, agitates the fluid in the container 304 to thereby cause oxygen to be released from the water and form bubbles (not shown in FIG. 5). At the top of the container 304 there is provided a pump (not shown in FIG. 5) configured to cause liquid near the liquid surface to flow in a downwards direction.

[0128] In this instance, the sound emitter 302 is a control valve 302 of a gas distribution pipe 311. The container has apertures to allow for the ingoing and outgoing pipe 311. The container 304 is sealed at the apertures, around the pipe 311, so that no liquid can leave the container therethrough.

[0129] Bubbles are continuously formed as a result of the mixer 306 agitating the fluid. The bubbles tend to float upwards through the fluid due to buoyancy. As the bubbles approach the top of the container 306 the flow caused by the pump tends to cause a portion of the bubbles to move back downwards through the fluid, rather than allowing all bubbles to leave the fluid as they reach the fluid surface. As a result, a bubble curtain (not shown in FIG. 5) forms in the container 304, and thus the sound emitter 302 is partially surrounded by the bubble curtain. Sound waves travel from the sound emitter 302, through the fluid within the container 304 and continue travelling through this fluid until they reach the bubble curtain. Here, the sound waves are absorbed and scattered by the bubbles, similarly to the case as described in relation to FIGS. 1, 2, and 4. This is particularly helpful in limiting the sound transmission from the control valve 302 into the air that would otherwise surround the control valve. Without the apparatus 301 the control valve 302 would be default be surrounded by air, which can lead to Helmholtz resonance.

[0130] The average (mean) diameter of the bubbles 8, 108, 208 is 0.05 cm, when considered in terms of the maximum size a bubble 8, 108, 208 will have from the instant it is generated to 3 seconds after it has been generated. Subsequently to this, the bubbles 8, 108, 208 will typically vary in size, particularly as they move upwards through the liquid. The bubble curtain 32 contains at least 1,000 bubbles in each 1 Litre volume where the bubble curtain 32 is present. However the skilled person will appreciate that relatively more bubbles 8, 108, 208 will typically be present closer to the mixer(s) 6, 106a, 106b, 106c, 106d, 106e, 106f, 206, 306 and that, over time, bubbles 8, 108, 208 will disperse somewhat so that fewer bubbles 8, 108, 208 will be present further from the mixer(s) 6, 106a, 106b, 106c, 106d, 106e, 106f, 206, 306.

[0131] The walls of the containers 4, 104, 204 are formed of tarpaulin with a reinforced webbing layer. The walls of container 304 is formed of high density polyethylene (HDPE). The or each mixer 6, 106a, 106b, 106c, 106d, 106e, 106f, 206, 306 is a paddle stirrer having two steel paddles which are rotatable about an axis via an electric motor at 100 rpm. The pump 22, 222 is a positive displacement pump configured to displace 1000 L per minute (although the skilled person will appreciate that the choice of pump will depend on the size of the container). The barriers 114a, 114b (where present) are formed of HDPE discs.

[0132] The liquid in the container 4, 104, 204, 304 is water comprising 50% dissolved oxygen by volume. The water also contains 20 mg/L of hydrogen peroxide and 5 kU of catalase per 1 litre of water. The catalase causes the decomposition of hydrogen peroxide into water and oxygen. Some of the oxygen immediately forms additional bubbles, whilst some dissolves into the water and may form additional bubbles as the liquid is agitated by the mixer 6, 106a, 106b, 106c, 106d, 106e, 106f, 206, 306. The catalase increases the rate at which the hydrogen peroxide is decomposed. The mixer 6, 106a, 106b, 106c, 106d, 106e, 106f, 206, 306 prevents the catalase from settling at the base of the container, and keeps it in suspension, which improves the efficiency with which the catalase acts on the hydrogen peroxide.

[0133] Advantageously, the formation of oxygen bubbles 8, 108, 208 in the liquid limits the transmission of the sound through the liquid. Agitation of liquid comprising at least 50% dissolved oxygen by volume has been found to be a particularly effective way to cause the formation of bubbles 8, 108, 208, as some of the oxygen dissolved in the liquid is released from the liquid to thereby form a large quantity of oxygen bubbles 8, 108, 208 as the liquid is agitated. Thus bubbles 8, 108, 208 can be formed without the use of compressors or similar devices to force gas bubbles through the liquid, and the apparatus, 1, 101, 201, 301 is therefore also more efficient than would be the case if a compressor were used.

[0134] FIG. 6 is a flow chart of steps in a method according to an example embodiment of the invention. Here the steps include providing 50 a sound emitter in a container and 52 causing agitation of the liquid in the container.

[0135] FIG. 7 is a schematic illustration of an apparatus 1 according to an example embodiment of the invention. The apparatus 1 has at least one mixer 6 and a controller 40. The controller 40 is configured to send signals 42 to the mixer 6. The controller 40 is also typically configured to transmit data elsewhere, for example to further components of the apparatus 1, and/or to devices external to the apparatus 1, via a wireless data connection. The signals 42 include signals generated by the controller 40 in dependence on data received by the controller 40, for example from user inputs and/or from the light detector 215. The controller 40 in this example is realised by one or more processors 44 and a computer-readable memory 46. The memory 46 stores instructions which, when executed by the one or more processors 44, cause the apparatus 1 to operate as described herein. Although the controller 40 is shown as being part of the apparatus 1, it will be understood that one or more components of the controller 40, or even the whole controller 40, can be provided separate from the apparatus 1. For example, the controller may be remote from the apparatus 1 and may exchange signals with the mixer 6 by wireless communication.

[0136] FIG. 8 is a plot of the speed of sound through a fluid, as a function of the proportion of water vs air in that fluid. As can be seen from this figure, the speed of sound is relatively high in 100% water and also relatively high in 100% air, however, the speed of sound is relatively low where the fluid is a combination of water and air. The reduction in the speed of sound has the result that sound of a given frequency has a correspondingly lower wavelength in a fluid containing a mixture of water and air, than it would in a fluid made up only of water, or a fluid made up only of air. This in turn changes the way in which the sound can be attenuated, either by individual bubbles, or by a bubble curtain (e.g. a bubble curtain generated by mixing of a liquid containing a high proportion of dissolved oxygen to thereby form bubbles, as herein described).

[0137] Although in the examples described hereinabove the agitator 6, 106, 206, 306 is provided in the form of a mixer, this is not required, and other means may be used to agitate the liquid in the alternative. For example, in some embodiments, the agitator may be a bubble curtain generator in the form of a compressor connected to a perforated pipe to thereby force bubbles through the liquid. The compressor may be a single compressor or may be provided by a group of (e.g. 7 bar) compressors, together outputting 10,000 L/min. The compressor may be arranged to create a bubble curtain at the lower outer perimeter of the container. The use of a bubble curtain generator to agitate the liquid allows for an even greater number of bubbles to be generated in the liquid (i.e. some arising as a result of the agitation causing dissolved oxygen to be released from the liquid and to thereby form bubbles, and some from the bubble curtain generator itself). A higher number of bubbles provides more effective sound attenuation and more effectively limits sound transmission.

Summary of Experimental Testing

[0138] The inventor carried out a test of an example of the apparatus and methods as described herein. The following summary of this test provides a nonlimiting example of how the apparatus and methods may be used.

[0139] A 3 m.sup.3 container was formed with tarpaulin container walls and this container was filled an aqueous solution, in this case seawater. 30 L of hydrogen peroxide was added to the container along with 100 ml of catalase (which encourages decomposition of hydrogen peroxide). A mixer (in this case an air mixer) was operated for 2 minutes, leading to the generation of a large number of buoyant bubbles, with an approximate average (mean) diameter of 5 mm. The resulting fluid was transparent.

[0140] A sound emitter in the form of a frequency generator was activated and caused to apply 10 ms sound pulses at approximately 190 dB frequency pulses. The pulses started at approximately 2 kHz and were increased in .sup.500 Hz steps up to approximately 13 kHz, with 100 ms between each pulse.

[0141] Hydrophones were used to record sound at 0.5 m, 1 m, and 2 m distances from the sound emitter, through the aqueous solution at 1 m and 2 m depths. Following the 2-minute activation time of the mixer, the sound level was not measurable with the hydrophones for a period in excess of 2 hours. It was however found that sound way discernible by replaying with amplification. By this method, it was possible to confirm that attenuation increased as a function of increasing distance (i.e. the distance the soundwaves travelled through the fluid).

[0142] Over the 2-hour period, it was also found that the concentration of hydrogen peroxide decreased by approximately 27%. In this test, it was also noted that attenuation effect was greatest at shallower depths, with the sound being detected by the deeper hydrophones sooner than by the shallower hydrophones. It should be noted that speed of sound is a function of pressure (and that pressure is greater at greater depths), and that bubble size is also a function of pressure (with higher pressures leading to smaller bubbles). It has further been observed that, at least in some cases, the attenuation effect is observed at shallower depths before it is observed at greater depths (corresponding to the effect also ending sooner at greater depths). The skilled person will thus appreciate that depth and buoyancy are just two of several parameters to be considered when seeking to put the invention into effect. It should also be understood that the rate of dissolved gas production is believed to be dependent on (e.g. at least) the hydrogen peroxide concentration, the quantities of enzymes (e.g. catalase) present, and the presence or absence (and quantities) of any biomass (e.g. fish).

[0143] It was noted that wave action on the tarpaulin provided a gentle movement of the fluid and that this provided further mixing during the test, likely contributing the relatively long period during which sound attenuation was effective.

[0144] Further tests with frequent mixing gave similar attenuation but initial results suggest that some mixing regimes may lead to less stable fluid conditions (e.g. with more variation in the numbers of bubbles, etc). That said, it also appears that varying the concentrations of hydrogen peroxide and catalase, will also effect stability of the fluid in this sense (i.e. in addition to the mixing).

[0145] The attenuation effect appears to be most significant with smaller gas bubbles rather than larger gas bubbles. The attenuation achieved was found to be greater than previous noise mitigation strategies (e.g. simple bubble curtains). It was surprisingly found that excessive mixing may in some circumstances reduce the effectiveness of the sound attenuation. An optimal rate of mixing to encourage hydrogen peroxide decomposition (and thus bubbles and dissolved oxygen), without reducing the effect, will depend on the conditions within the liquid.

[0146] Throughout the description and claims of this specification, the words comprise and contain and variations of them mean including but not limited to, and they are not intended to and do not exclude other components, integers, or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0147] Features, integers, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0148] In summary, there is provided apparatus (1) for limiting transmission of sound from a sound emitter (2), the apparatus comprising a container (4) arranged to at least partially surround a sound emitter, the container retaining a liquid, wherein the liquid comprises at least 2% dissolved oxygen, and wherein the apparatus further comprises an agitator (6) configured to agitate the liquid, to thereby cause the formation of gas bubbles (8) in the liquid.