ACOUSTIC BLACK HOLE

Abstract

 There is described an acoustic black hole 'ABH' comprising a tapered body comprising an upstream end and a downstream end, the tapered body tapering from a first characteristic towards the upstream end to a second characteristic towards the downstream end, the tapered body comprising at least one fold in between the upstream end and the downstream end, such that the downstream end at least partly points towards the upstream end of the tapered body.

Description

FIELD

[0001] The present invention relates to acoustic black holes and structures utilising the same.

BACKGROUND

[0002] In product design, it is often desirable to design a product that is both lightweight and low noise. However, this results in a conflict between reducing the weight and increasing the sound radiation from the structure. It is known to use a structure referred to as an acoustic black hole (ABH) to provide structural damping.

[0003] The acoustic black hole effect is typically achieved by introducing a power law taper into a beam or plate that changes the thickness over a set distance. This change in thickness profile causes the flexural waves propagating along the direction of the ABH to decrease in wave speed. In the theoretical limit, there is no reflection of the waves from the ABH. The ABH effect can also be achieved using other gradient functions, including a power-cosine curve, for example.

[0004] However, the dimensionality/shape of existing ABHs make them inappropriate for use in certain contexts. Hence an improved ABH which is more widely applicable is desirable.

SUMMARY

[0005] It is an aim of the present invention, amongst others, to provide a improved acoustic black hole and/or structure and/or address one or more of the problems discussed above, or discussed elsewhere, or to at least provide an alternative acoustic black hole or structure.

[0006] According to an aspect of the present invention, there is provided an acoustic black hole 'ABH', comprising a tapered body comprising an upstream end and a downstream end, the tapered body tapering from a first characteristic towards the upstream end to a second characteristic towards the downstream end, and comprising at least one fold in between the upstream end and the downstream end, such that the downstream end at least partly points towards the upstream end of the tapered body.

[0007] In an example, the first characteristic and second characteristics are thicknesses of a cross section of the tapered body.

[0008] In an example, the at least one fold is closer to the downstream end of the tapered body. Conversely, in an example, the at least one fold is closer to the upstream end of the tapered body.

[0009] In an example, a first region may be considered proximate to the upstream end and a second region proximate to the downstream end, the regions being separated by the at least one fold.

[0010] In an example, the first region comprises a first material and the second region comprises a second, different, material.

[0011] In an example, the first region comprises a first density and the second region comprises a second, different, density.

[0012] In an example, the first region comprises a first rigidity and the second region comprises a second, different, rigidity.

[0013] In an example, at least one of the first region and second region are actively controlled.

[0014] In an example, the tapered body may comprise at least two folds, and a be considered to comprise a third region in between the at least two folds and spacing apart the first region and the second region. Suitably, in an example, the first region and second region may be arranged substantially parallel.

[0015] In an example, the third region comprises a third material different to the first material and the second material.

[0016] In an example, the third region comprises a third density different to the first density and the second density.

[0017] In an example, the third region comprises a third rigidity different to the first rigidity and the second rigidity.

[0018] In an example, the third region is actively controlled.

[0019] In an example, a height of the third region in a y-axis does not exceed a height of the upstream end in the y-axis.

[0020] In a related aspect of the present invention, there is provided a compound ABH comprising a plurality of the aforementioned ABHs.

[0021] In an example the compound ABH comprises a first pair of ABHs abutting at their respective at least one fold.

[0022] In an example, the compound ABH comprises a second pair of ABHs abutting at their respective least one fold.

[0023] In an example, the first and second pair of ABHs may be arranged to form a cavity in between the two pairs of ABHs.

[0024] In a related aspect of the present invention, there is provided a structural damper comprising the aforementioned ABH.

[0025] In a related aspect of the present invention, there is provided a primary structure comprising the aforementioned structural damper.

BRIEF DESCRIPTION OF THE FIGURES

[0026] Embodiments of the invention will now be described by way of example only with reference to the figures, in which:

Figure 1 shows the principles of an acoustic black hole;

Figure 2 shows an example improved acoustic black hole - Fig. 2A shows a single fold acoustic black hole and Fig. 2B shows a double fold acoustic black hole;

Figure 3 shows a graph of displacement magnitude with distance for a range of acoustic black holes;

Figure 4 shows a graph of power dissipation for a range of acoustic black holes;

Figure 5 shows an example compound ABH - Fig. 5A shows a linear compound ABH while Fig. 2B shows a circular compound ABH;

Figure 6 shows another example compound ABH;

Figure 7 shows an example of a damper structure with controlled ABH; and

Figure 8 shows examples of a controlled folded ABH.

DETAILED DESCRIPTION

[0027] Figure 1 shows an example of an acoustic black hole (ABH) 1 on a beam 2. The ABH 1 may be provided with a layer of damping material 3. The flexural wave speed c_f (x), decreases as the taper height decreases as:

where E is the Young's modulus of the ABH material, h(x) is the height of the taper, ρ_s is the density of the ABH material and ω is the angular frequency.

[0028] From Equation 1 it can be seen that if the tip of the ABH reduces to zero thickness, i.e. h(x)=0, then the flexural wave speed at the tip will be c_f (x)=0. In this ideal, theoretical case, the incident wave will not be reflected from the end of the tapered beam and will therefore, be effectively attenuated.

[0029] In this respect, acoustic black holes are known in the art. For example, 'Higher-order WKB analysis of reflection from tapered elastic wedges' Journal of Sound and Vibration 449 (2019) 368-388 (Angelis Karlos, Stephen J. Elliot, Jordan Cheer), the contents of which are incorporated herein, provides examples of different types of 'one-dimensional' acoustic black holes. The thickness variations, of these acoustic black holes, are according to the expressions provided in Table 1 below:

Table 1

Thickness profile type	Thickness variation	Length of ideal wedge	Decay parameter
Power-law			-
Exponential	h = h0e-βx	∞
Power-cosine			-
Gaussian	h = h0e-γx2	∞
Compound power-law			-

where:

'x' is the distance, in the length direction, from the upstream end of the acoustic black hole (i.e. at the start of the taper);

'x1' is the length of the acoustic black hole;

'h' is the thickness of the acoustic black hole (at position (x));

'h0' is the thickness of the acoustic black hole at the upstream end of the acoustic black hole (i.e. at position (x = 0));

'h1' is the thickness of the acoustic black hole at the downstream end of the acoustic black hole (i.e. at position (x = x1));

'n' is power coefficient of the shape function (which must be greater or equal to 2).

[0030] In the description which follows, acoustic black holes, structural dampers, structurally damped structures, and methods, are described. The term "acoustic black hole" is used to refer to an element, member, or structure, which, in use, exhibits the acoustic black hole effect.

[0031] In the description herein, acoustic black holes comprise regions of taper. In the examples shown and described, the taper is a thickness taper. That is, the thickness of the acoustic black hole tapers (i.e., reduces or diminishes in thickness in a direction and along a line toward a point, line or region). Additionally, or alternatively, tapering may be in shape. A thickness or shape may be referred to generally as a "spatial property". Conventional ABHs incorporate tapers in thickness, from a first thickness to a second thickness. The first thickness is typically a non-zero thickness. The second thickness is, in the ideal case, a zero thickness. A thickness or shape taper may be advantageous in that it may be simpler to manufacture than, for example, a taper in material and/or material property.

[0032] However, in contrast to a thickness taper, the taper could also be a "functional taper" or a "functional grading". That is, the tapering could be a tapering function of the acoustic black hole, rather than a tapering thickness. For example, the tapering may be a tapering of material and/or material property. The material property may be, for example, density and/or rigidity. This may be achieved by use of additive layer manufacturing (e.g. 3D printing) to form an acoustic black hole having a tapering, graded, or varying, material property. A tapering in material and/or material property may be advantageous in that thin ABH regions need not be provided, which may improve the structural strength, and operational lifetime, of the ABH.

[0033] In this way, it is appropriate to refer to ABH tapers as tapering of a "characteristic". Tapering may be from a "first characteristic" to a "second characteristic". A similar or identical effect to a thickness tapering may be achieved by a tapering in material and/or material property. For example, a tapering from a region of high rigidity to a region of low rigidity may provide a reduction of the flexural wave speed to c_f (x)=0, as described above, thereby to provide the ABH effect. A variation from a first characteristic to a second characteristic includes a variation in a magnitude of a specific characteristic from a first to a second value, or a variation in a type of characteristic from a first to a second (different) type.

[0034] The term "structural damper" may be used to refer to an arrangement, assembly or kit of acoustic black holes and optionally active control components (e.g., comprising a sensor, an actuator, and a controller).

[0035] The term "primary structure" may be used to refer to a structure that a damper device, or structural damper, is arranged to provide structural damping to. The primary structure is a structure that, in use, has a vibration applied to it. The primary structure may be a structure that is vibrated, directly or indirectly, by a source of vibration (e.g., an engine, fluid flow, etc.).

[0036] An acoustic black hole may be formed in or on the primary structure. For example, the acoustic black hole may be embedded in the primary structure. Alternatively, or additionally, the acoustic black hole may be coupled to the primary structure. That is, the acoustic black hole may be manufactured separately and coupled, or connected, to the primary structure.

[0037] A structural damper may be formed in or on the primary structure. For example, the structural damper may be integral to the primary structure. Alternatively, or additionally, the structural damper may be coupled to the primary structure. That is, the structural damper may be manufactured separately and coupled, or connected, to the primary structure.

[0038] The term "structurally damped structure" may be used to refer to a structure, arrangement, assembly or kit comprising an acoustic black hole and a primary structure. The structurally damped structure may also comprise active control components (e.g., comprising a sensor, an actuator, and a controller). That is, the structurally damped structure may comprise a primary structure and a structural damper. As above, whilst in the exemplary embodiments described herein the structurally damped structure comprises a structural damper comprising a single compound ABH, the structurally damped structure may comprise a plurality of compound ABHs and associated control components (e.g., a plurality of actuators, sensors and/or controllers).

[0039] The term "damper structure" may be used to refer to a structure that is coupled to an ABH (e.g., mechanically coupled) such that it can transmit vibration and/or flexural waves through its structure to and from an ABH. The ABH may be formed in or on the damper structure. The damper structure may be coupled, or connected to, the primary structure, or the damper structure may be formed in or on the primary structure.

[0040] In examples herein, the acoustic black hole may be said to have a first axis and a second axis, and the primary structure may be said to have a first axis and a second axis. Throughout, the first axis of the primary structure is parallel to the first axis of the acoustic black hole, and the second axis of the primary structure is parallel to the second axis of the acoustic black hole. The first axis of the primary structure may be the same as the first axis of the acoustic black hole, and the second axis of the primary structure may be the same as the second axis of the acoustic black hole.

[0041] In some examples, the first axis and second axis are perpendicular. In this way, the acoustic black hole may comprise tapering (or variation) in directions parallel to, for example, the first axis, and no tapering (or variation), or tapering (or variation), in the second axis, the first and second axis being perpendicular. This may simplify construction, by avoiding construction or formation of tapering/varying features at relative angles. Nevertheless, in other embodiments, the first axis and second axis need not be perpendicular, which may lead to improvements in structural damping where vibrations emanate from multiple angles relative to the acoustic black hole.

[0042] In some examples a structurally damped structure may be said to comprise a primary structure having a "region of curvature" in at least one of the first axis and second axis, the region of curvature may be about the acoustic black hole, surrounding the acoustic black hole and/or in proximity of the acoustic black hole. It will be appreciated that the whole of the primary structure may comprise curvature in at least one of the first axis and second axis. Thus, it may be appropriate to refer to the primary structure comprising "at least a region" of curvature.

[0043] Figure 2 shows an example of an improved acoustic black hole 100. In particular, the acoustic black hole has been designed to save space compared to prior art ABHs, as well as allow for flexibility in length and height. Fig. 2A shows one example implementation. Fig. 2B shows another example implementation. Each of the example ABHs in Fig. 2 may be considered linear, or 1-D, ABHs.

[0044] Here the ABH comprises a body 102 having an upstream end 104 and a downstream end 106. The body 102 is tapered such that the body 102 tapers from a first characteristic at, or proximate to, the upstream end 104 to a second characteristic at, or proximate to, the downstream end 106.

[0045] In the example shown the tapering is a shape wise tapering, such that the first characteristic and second characteristics are thicknesses of a cross section of the body 102 in a x-z plane. That is, the first characteristic is a thickness greater than a thickness of the second characteristic.

[0046] It will however be appreciated from the discussion above that other approaches to tapering the body 102 consistent with ABH principles may be utilised.

[0047] The body 102 comprises at least one fold 112 in between the upstream end 104 and the downstream end 106 such that the downstream end 106 at least partly points back towards the upstream end 104.

[0048] In the examples shown, the downstream end 106 does not reach the upstream end 104 - i.e., the at least one fold 112 is closer to the downstream end 106 than the upstream end 104. In some alternative examples the downstream end 106 may instead extend past the upstream end 104 - i.e., the at least one fold 112 is closer to the upstream end 104 than the downstream end 106.

[0049] Put another way, given a Euclidian coordinates system in which the upstream end 104 starts at x=0, in examples where the at least one fold 112 is closer to the downstream end 106, a termination point 107 of the downstream end 106 may have an a positive x value, whereas in examples where the at least one fold 112 is closer to the upstream end 104, the termination point 107 of the downstream end 106 may have a negative x value.

[0050] Suitably, the at least one fold 112 may be considered to divide the body 102 into a plurality of regions comprising a first region 108 and a second region 110. The first region may be considered proximate to the upstream end 104 of the body 102 and the second region 110 proximate to the downstream end 106 of the body 102. Suitably the second region 110 may be considered at least partly oriented back towards the first region 108 as a result of the at least one fold 112. In this way an overall length of the AHB 100 may be reduced (in the direction of the first (x) axis).

[0051] More specifically, in the example of Fig. 2A, exactly one fold 112 is provided which separates the body 102 into the first and second regions 108, 110. Suitably, the downstream end 106 points back towards the upstream end 106 at an angle defined by the fold 112.

[0052] In the example of Fig. 2B, two folds 112 are provided which separates the body 102 into the aforementioned first and second regions 108, 110 and also a third region 114. Suitably, the third region 114 spaces apart the first and second regions 108, 110. In particular, in the present example the third region 114 allows for the first region 108 and second region 110 to be substantially parallel. That is, the third region 114 may be substantially orthogonal to the first region 108 and second region 11. Or put anther way, the first and second regions 108, 110 may be considered to have major (first) axes in an x direction, while the third region 110 may be considered to have major (first) axis in a z direction.

[0053] Suitably, a length of the third region 114 (i.e., the degree to which it separates the first and second regions 108, 110) controls a height (in a z direction) of the AHB 100. In the example shown, the size of the third region 114 is set such that a total height of the AHB 100 does not exceed the height of the upstream end 104; thus the example of Fig. 2B has advantages not only for overall length but also for height of the AHB 100.

[0054] Although the above has focused on tapering shape as the variable characteristic, it will however be appreciated that other characteristics can also be applied to the ABH 100 instead of or in addition to tapering shape. Also, it will be appreciated that other aspects of ABHs may be applied to the presently described ABH.

[0055] For example, the first region 108 may comprise a first material while the second region 110 may comprise a second material. The first region 108 may have a first density while the second region 110 has a second density. The first region 108 may have a first rigidity while the second region 110 has a second rigidity. The first region 108 may be actively controlled while the second region 110 is not actively controlled - or vice-versa - or indeed both the first region 108 and second region 110 may be both actively controlled.

[0056] Similarly, in examples where the body 102 also comprises the third region 114, the third region 114 may be the same material as one of the first or second regions 108, 110, or may be a different material to both of the first and second regions 108, 110. The third region 114 may have the same, or substantially similar, density to either the first or second regions 108, 110 or may have a different density to both the first and second regions 108, 110. The third region 114 may have the same or different rigidity to one or both of the first and second regions 108, 110. The third region 114 may be actively controlled in addition to both the first and second regions 108, 110 or instead of the second and third regions 108, 110.

[0057] Figure 3 shows an example of the space savings that can be achieved using an ABH designed as above. Here there is shown sound propagation through a standard bar 10, a standard ABH 20, an ABH 100 as above, and a coiled ABH 30. Versus the standard ABH 20 the improved ABH 100 has the same height (as in, how much vertical space it would take up in a structure) but much reduced length. Versus the coiled ABH 30, that device can be utilised in much shorter spaces, but similarly only in spaces with much greater height in order to account for the coil.

[0058] Figure 4 shows a graph demonstrating the attenuation of vibrations in the objects of Fig. 3. Line 11 shows dissipated power for a solid standard shaped bar 10 which has little to no attenuation of vibration. Line 21 shows dissipated power for a standard ABH 20. Line 101 shows dissipated power for an improved ABH 100. Line 31 shows dissipated power for a coiled ABH 30.

[0059] Figure 5 shows an example of a compound ABH 200 based on the principles above. Here, the compound ABH 200 comprises a plurality a folded ABHs 100. The compound ABH is particularly suited for damping vibrations from multiple angles.

[0060] As shown in Fig. 5A, two ABHs 100 are arranged back to back. That is, each ABH 100 is arranged with its upstream end 106 pointing (in the direction of taper) at the other ABH 100. Suitably, each ABH 100 abuts the other ABH 100 where the ABHs 100 are folded. Put another way, where the ABHs comprise the third region 114, the third region 114 of one ABH 100 may abut the third region 114 of the other ABH. Here the compound ABH 200 is assumed to have a substantially linear, or 1-D, geometry.

[0061] Fig. 5B shows an example of a compound ABH 200 as per Fig. 5A in a circular, or cylindrical, geometry as from a top down view. That is, in the context of Fig. 5B, Fig. 5A may be taken to represent a cross sectional slice through a cylindrical compound ABH 200. The cylindrical compound ABH may also be considered a 2-D ABH.

[0062] Put another way, it may be envisaged that the ABH 200 comprises a cylindrical symmetry about an axis defined by the third region 114. Suitably, in one example, the upstream end 204 of the compound ABH 200 may be taken to correspond to an outer circumference of the ABH 200, while the downstream end 206 has a radius less than the radius of the upstream end 204. In another example, the radius of the downstream end 206 may be greater than the radius of the upstream end 204.

[0063] The compound ABH 200 may form part of a structurally damped structure. For example, there may be provided a substantially two-dimensional primary structure in the form of a relatively thin flat plate. The plate may comprise a plurality of compound ABHs that are embedded in the plate; that is each compound ABH 200 may take the form of a generally circular indentation in a surface of the plate.

[0064] Figure 6 shows another example compound ABH 200 comprising a plurality of ABHs 100. Here the compound ABH 200 comprises two pairs of back-to-back ABHs analogous to Fig. 5A. The plurality of ABHs 100 are arranged to form a cavity 216 which separates the second regions 110a of the first pair of ABHs 100a and the second regions 110b of the second pair of ABHs 100b.

[0065] More specifically, a first pair of ABHs 100a are arranged abutting at the third regions 114a to form a lower housing 218, while a second pair of ABHs 100b are arranged abutting at the third regions 114b to form an upper housing. Suitably the two pairs of ABHs 100a,b are arranged to abut at the upstream ends 104a,b. It will also be appreciated that the present arrangement can be achieved in 3-D, or cylindrical, form analogous to Fig. 5B.

[0066] Figure 7 shows an example of an actively controlled acoustic black hole 300 as part of a structurally damped structure 30. The structurally damped structure 30 comprises a primary structure 32 in the form of a rectangular beam 32 and a structural damper 14, at the end of the beam 32, configured to provide structural damping of the beam 32.

[0067] In this respect, the beam 32 is attached, at one end, to a vibration source 36. The vibration source 36 produces a vibratory excitation force (V) at that end of the beam 32, which induces vibration and flexural waves in the beam 32. This also causes the beam 32 to emit acoustic radiation, i.e. noise.

[0068] In general, the structural damper 14 has a damper structure 38, that comprises an acoustic black hole 300, provided at the opposite end of the beam 32 to the vibration source 36. The damper structure 38 (and the acoustic black hole 300) may be considered embedded in the end of the beam 32. It will however be appreciated that in the example shown the damper structure 38 is the same as the ABH 300; or put another way, the ABH 100 is the damper structure. It will be appreciated that in other examples the damper structure may comprise other components in addition to the ABH 300.

[0069] The ABH 300 may be substantially the same as described with reference to the previous Figures for ABH 100 above. That is, the ABH 300 comprises a body 302 having an upstream end 304 and a downstream end 106 and a tapered body 302 tapering from a first characteristic at the upstream end 104 to a second characteristic at the downstream end 306. Suitably, the upstream end is proximate to the vibration source 36. The body 302 may comprise at least one fold 316 in between the upstream end 304 and the downstream end 306 (not shown in Fig. 7; shown in Fig. 8).

[0070] Here the ABH 300 comprises an actuator 308 configured to apply an actuating force to the ABH 300. The actuator 308 causes the ABH 300 to deform (i.e. through expansion or contraction) due to the application of the actuating force. Put another way, energy is input from the actuator 308 into the acoustic black hole 300.

[0071] In the present example, the actuating force is applied parallel to the plane of the flexural waves that travel along the beam 32 due to excitation by the vibration source 36. In other examples, the actuating force may be applied in a different direction with or without a component parallel to the flexural waves. The actuator 308 may be located remote from the ABH 300 such as on the beam 32, or a fold of the beam 218.

[0072] The ABH 300 also comprises a first sensor 310 configured to detect a deformation of the ABH 300. A signal from the sensor 310 is provided to a controller 312 which generates a control signal, based on the received sensor signal, for controlling the actuator 308. The control signal is suitably transmitted to the actuator 308 in order to attenuate the vibration of the primary structure / beam 32.

[0073] In the present example, the actuator 308 may comprise a piezo-electric transducer (PZT) patch 308 which is provided on at least one of the upper and lower surface of the ABH 300; shown on the upper surface in Fig. 7. The actuating PZT patch 108 may be provided on a partial area of a surface of the ABH or substantially covering an entire surface of the ABH 300.

[0074] Likewise, the sensor 310 may also comprise a PZT patch 310 provided on at least one of the upper and lower surface of the ABH 300; here shown on the lower surface in Fig. 7.

[0075] The upper and lower PZT patches 308, 310 are each of a material that has an intrinsic level of damping and so provide a passive damping of the ABH 300.

[0076] Suitably, the movement of the ABH 300 acts to deform the sensing PZT patch 310, which causes a voltage to be induced across the sensing PZT patch 310 that is representative of the deformation. The sensing PZT patch 310 is connected to the controller 312 such that the voltage induced across the sensing PZT patch 310 is passed to the controller 312 as a feedback signal. The output of the controller 312 is connected to the actuating PZT patch 308 such that the control signal, here in the form of a voltage, is applied across the actuating PZT patch 308 in dependence on the feedback signal.

[0077] It will of course be appreciated that any suitable type and arrangement of sensors and actuators may be used. It will also be appreciated that the upper and lower PZT patches could be reversed, i.e. the actuator provided on the lower surface and the sensor provided on the upper surface.

[0078] As shown, the structurally damped structure 30 may comprise a second sensor 314, mounted on the primary structure 32, which is configured to sense the vibration of the beam 32. As above, the second sensor 314 may be a PZT sensor, however any suitable type of sensor may be used. It will be appreciated that the second sensor 314 may be embodied as a set of sensors.

[0079] The second sensor 314 may be located upstream of the actuator 308. The second sensor 314 is connected to the controller 312 so as to provide a feed forward reference signal (r) to the controller 32 that corresponds to the vibrating movement of the beam 32 at the location of the sensor 34. In this way the controller 312 may better generate a signal to minimise the vibration of the beam 32. Where the second sensor 314 is embodied as a set of sensors, such sensors may be distributed in different locations across the primary structure 32 or indeed vibration source 36.

[0080] Figure 8 shows example arrangements of the actuator 308 and first sensor 310 specific to a folded ABH arrangement.

[0081] In the example of Fig. 8A, the actuator 308 is provided on an upper surface of the first region 318 on the side of the fold 316 proximate to the upstream end 304. Suitably, the first sensor 310 may be provided on the opposite surface of the first region 318 to the actuator 308. It will of course be appreciated that the sensor 310 and actuator 308 may be disposed the other way around.

[0082] In the example of Fig. 8B, the actuator 308 is disposed on an upper surface of the second region 320 on the side of the fold 316 proximate to the downstream end 306. Here "upper" is defined relative to the tapered shape, so that upper corresponds to the surface of the body 302 which is angled, or has curvature (compared to the opposite surface which may be flat). That is, here the "upper" surface of the second region 320 corresponds to the surface which faces the upper surface of the first region 318. Suitably, the sensor 310 may be provided on the opposite (e.g., flat) surface of the second region 320. It will also be appreciated that the arrangement of the actuator 308 and sensor 310 could be reversed.

[0083] Fig. 8C shows an example whereby the actuator 308 is provided on the upper surface of the third region 322. Here the upper surface of the third region may be the surface which faces the upper surfaces of the first and second regions 318, 320. Again the sensor may be on the opposite surface and optionally the arrangement of the actuator and sensor 308, 310 may be reversed.

[0084] Although the above examples have focussed on arranging the (PZT) actuators and sensors on specific regions of the body 302 with respect to the fold 316, it should be appreciated that the examples are non-limiting.

[0085] For example, a set of actuators 308 may be provided with each actuator in the set corresponding to a surface of one of the regions of the body 302; here some regions may also be left without an actuator 308. A corresponding set of sensors may be provided on the opposite surfaces, with control of each of the actuators 308 being independent based on a feedback from each corresponding sensor 310. For example, much like a basic example of a controlled ABH, an entirety of a surface of the folded ABH may be covered with PZT patches.

[0086] In some examples only one sensor 310 may be provided and the feedback signals suitably averaged before sending a common control signal to the actuators 308, or a plurality of sensors 310 may be used to generate a control signal for only one actuator 308 as either a sole actuator or an isolated member of a set of actuators.

[0087] In some examples, one or more actuators may be provided on one or more surfaces of the ABH 300, while the first sensor 310 is not provided on an opposite surface but elsewhere on the ABH 300. For example, an actuator may be provided on a surface of the first region 318 or the second region 320 (or both) and a sensor on the third region 322, as shown in Fig. 8D.

[0088] In summary, the above has described a new technique for acoustic black holes that achieves space savings compared to existing techniques.

[0089] Where, in the foregoing description, integers or elements are mentioned that have known, obvious, or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present disclosure, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the disclosure that are described as optional do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, while of possible benefit in some embodiments of the disclosure, may not be desirable, and can therefore be absent, in other embodiments.

Claims

1. An acoustic black hole 'ABH', comprising:

a tapered body comprising an upstream end and a downstream end, the tapered body tapering from a first characteristic towards the upstream end to a second characteristic towards the downstream end;

the tapered body comprising at least one fold in between the upstream end and the downstream end, such that the downstream end at least partly points towards the upstream end of the tapered body.

2. The ABH of claim 1, wherein the first characteristic and second characteristics are thicknesses of a cross section of the tapered body.

3. The ABH of claim 1 or 2, wherein the at least one fold is closer to the downstream end of the tapered body.

4. The ABH of claim 1 or 2, wherein the at least one fold is closer to the upstream end of the tapered body.

5. The ABH of any preceding claim, comprising a first region proximate to the upstream end and a second region proximate to the downstream end separated by the at least one fold.

6. The ABH of claim 5, wherein the first region comprises a first material and the second region comprises a second, different, material.

7. The ABH of claim 5 or 6, wherein the first region comprises a first density and the second region comprises a second, different, density.

8. The ABH of any of claims 5 to 7, wherein the first region comprises a first rigidity and the second region comprises a second, different, rigidity.

9. The ABH of any of claims 5 to 8, wherein at least one of the first region and second region are actively controlled.

10. The ABH of any of claims 5 to 9, comprising at least two folds, and a third region in between the at least two folds and spacing apart the first region and the second region.

11. The ABH of claim 10, wherein the first region and second region are arranged substantially parallel.

12. The ABH of claim 10 or 11, wherein a height of the third region in a y-axis does not exceed a height of the upstream end in the y-axis.

13. A compound ABH comprising a plurality of ABHs according to any preceding claim, comprising a first pair of ABHs abutting at their respective at least one fold.

14. The compound ABH of claim 13, comprising a second pair of ABHs abutting at their respective least one fold.

15. The compound ABH of claim 14, wherein the first and second pair of ABHs are arranged to form a cavity in between the two pairs of ABHs.

Drawing