Sound-absorption device

Abstract

In general terms, the present invention provides a sound-absorption device configured to absorb sound within a desired frequency range, in which a plurality of tuned resonator elements project from a panel absorber, such as a constrained plate. In a first aspect, a sound-absorption device is configured to absorb sound within a desired frequency range. The device comprises: a plate configured to be mounted adjacent a sound reflective surface to absorb sound reflected from the sound reflective surface; and a plurality of resonator elements. Each resonator element comprises a base extending from the plate and a free end, whereby each resonator element extends from the plate at an angle with respect to the plate.

Description

[0001] SOUND-ABSORPTION DEVICE

[0002] TECHNICAL FIELD

[0003] This invention relates to a sound-absorption device, an assembly of sound-absorption devices, and a method of absorbing sound.

[0004] BACKGROUND

[0005] Noise is a prevalent and ever-increasing threat to human health. Road traffic noise in England alone is estimated to cost in the region of £9billion per annum. There is a need for sound absorbing devices in many applications, including in the built environment (including domestic and industrial buildings), consumer appliances, and the automotive, aerospace and defence sectors, for example.

[0006] Sound absorbers are already used extensively to reduce noise, but established solutions are too bulky to be practical in many applications.

[0007] SUMMARY OF THE INVENTION

[0008] In general terms, the present invention provides a sound-absorption device configured to absorb sound within a desired frequency range, in which a plurality of tuned resonator elements project from a panel absorber, such as a constrained plate.

[0009] In a first aspect, the invention provides a sound-absorption device configured to absorb sound within a desired frequency range, the device comprising: a plate configured to be mounted adjacent to a sound reflective surface to absorb sound reflected from the sound reflective surface; and a plurality of resonator elements, each resonator element comprising a base extending from the plate and a free end, wherein each resonator element extends from the plate at an angle with respect to the plate.

[0010] This arrangement provides for sound absorption across a wider frequency range than known resonating devices. Moreover, this can be achieved with a relatively thin plate, which is important in applications where device size and / or mass are important design considerations.

[0011] By arranging the resonator elements so that they extend from the plate at an angle to the plate such that they have a free end, the resonator elements are free to vibrate at one or more tuned frequencies to thereby attenuate sound at those frequencies. The angle is a non-zero angle. It may be an acute or an oblique angle. That is, the angle is preferably not a right angle.

[0012] In some prior art arrangements features acting as resonators are co-planar with the plate, rather than at an angle as per the present invention. The present inventors have established that the claimed arrangement overcomes disadvantages with a co-planar arrangement while providing unexpected advantages. In particular, the claimed arrangement in which the resonator elements are provided at an angle to the plate provides a good compromise between performance and ease of manufacture.

[0013] The plate is preferably a thin plate excitable to one or more vibration modes at one or more resonant frequencies within the desired frequency range. The plate is preferably constrained at its peripheral edges. The plate is preferably a non-perforated plate.

[0014] The device comprises a backing cavity between the plate and the sound reflective surface in use. Thus, the plate and cavity together form a panel absorber, in which the backing cavity serves as a spring and the plate as a distributed mass. In preferred embodiments the backing cavity comprises an enclosed cavity, and the device comprises means to enclose the cavity in use. The sound reflective surface may be a wall of a building, the structure of a vehicle such as an automotive or aerospace vehicle, or may be any other surface in an application where sound absorption is desired. Similarly, the plate may comprise a structural element of a building or vehicle, such as a body panel of an automotive vehicle or an aerodynamic panel of an airframe. The resonator elements may be located on an internal face of such a panel, so that they are not visible during normal use.

[0015] In some embodiments the device may include a sound reflective surface arranged adjacent to the plate, such that the backing cavity is defined between the plate and the sound reflective surface. For example, the sound reflective surface and the plate may be mounted to a frame such that the backing cavity is defined therebetween.

[0016] The desired frequency range of the device may comprise up to three octaves. For example, between one and three octaves. This may allow for particular utility of the device in applications such as those in the built environment, where a wide range of frequencies may be encountered.

[0017] The plate and resonator elements together may provide a metamaterial plate. That is, together these features provide an engineered material that exhibits emergent properties not usually found in naturally occurring materials. Metamaterials derive their properties not from the properties of the base elements from which they are engineered, but from their shape and / or configuration.

[0018] The plate preferably has (i.e. is tuned to) one or more plate resonant frequencies, the backing cavity has (i.e. is tuned to) a cavity-plate resonant frequency, and each resonator element has (i.e. is tuned to) one or more element resonant frequencies different to the one or more plate resonant frequencies and the cavity-plate resonant frequency. The one or more plate resonant frequencies, the cavity-plate resonant frequency and the one or more element resonant frequencies are preferably within the desired frequency range. In this way, the plate resonant frequencies, cavity-plate resonant frequency and element resonant frequencies can be distributed across the desired frequency range to maximise the sound absorption across that range.

[0019] The one or more plate resonant frequencies preferably include one or more resonant frequencies of the plate. For example, the plate may have one or more natural frequencies at which it is excited into one or more modes of vibration. The cavity-plate resonant frequency may be a frequency at which the backing cavity acts as a spring to cause the plate to vibrate. Together these resonant frequencies combine to provide an absorption spectrum with multiple peaks corresponding to the resonant frequencies.

[0020] One or more of the plurality of resonator elements may have one or more first resonant frequencies and a further one or more of the plurality of resonator elements may have one or more second resonant frequencies different to the first resonant frequencies. That is, each resonator element may have a different set of resonant frequencies to other of the resonator elements. In this way, together the resonator elements can provide sound absorption across a broad band of frequencies where absorption is required, for example between the multiple peaks of the plate resonant frequencies.

[0021] The one or more plate resonant frequencies, the cavity-plate resonant frequency and the one or more element resonant frequencies are preferably distributed throughout the desired frequency range. This arrangement enables good levels of sound absorption to be achieved throughout the desired frequency range.

[0022] In preferred embodiments, the one or more element resonant frequencies are distributed over approximately one-third octave above and / or below one or each of the one or more plate resonant frequencies and / or the cavity-plate resonant frequency. This has been found to provide particularly good results.

[0023] The resonator elements are preferably generally plate-like. That is, each element is generally planar, with a thickness that is small in comparison to its other dimensions. Each resonator element may have a substantially uniform thickness between the base and the free end. Such elements provide an excellent performance, while also being relatively straightforward to manufacture. In other embodiments each resonator element may have a non-uniform thickness. For example, the thickness of each resonator element may be varied to tune the resonant frequency of the resonator element.

[0024] In preferred embodiments the plurality of resonator elements each have a shape that widens from the base towards the free end. The shape may be tapered from a narrower base to a wider region. Such shapes mimic the function of the scales found on a moth wing in a simplified form, and have been found to perform particularly well. The inventors have noted that a relatively narrow base may be beneficial because it allows the wider region to be excited in any direction. The stalk, or narrow base, is where the bending occurs so its dimensions control the stiffness, while the wider part acts like a mass. Having a narrow base allows the resonator element to vibrate normal to the direction of the element direction (a wide base would not allow this at a relevant frequency). A narrow base also means the location point on the plate can be where the plate is rotating (rather than translating out of the plate plane).

[0025] At least a portion of the resonator elements may extend from a first surface of the plate such that they extend towards the sound reflective surface in use. That is, the resonator elements may extend into a space between the plate and the sound-reflective surface, so that the elements are obscured in normal use. The front face of the device can therefore be smooth to provide an aesthetically pleasing finish that can be decorated as the end user desires. In some embodiments a further portion of the resonator elements may extend from a second surface of the plate opposed to the first surface.

[0026] The resonator elements may be arranged on the plate in one of an orthogonal grid, a radial grid, or an irregular pattern. The particular arrangement pattern is selected based on the vibration mode of the plate. That is, the resonator elements are preferably each arranged on the plate at an active region of a vibration mode of the plate. An active region comprises a region of the plate at which the resonator element will be excited at its resonator resonant frequency in response to excitation of the plate at its one or more plate resonant frequencies. The location of the active region(s) thus depends on the mode of the resonator element. For example, if the resonator element is tuned to resonate in its bending mode (out-of-plane to the element direction) then the active region of the plate is where the plate is translating out of its plane (at an antinode). Similarly, if the resonator element is vibrating normal to its direction (a hand waving motion) then the active area of the plate is where it is rotating (at a nodal line). If the two modes are at the same frequency then most of the plate is the active region.

[0027] The resonator elements may each extend from the plate at an angle of 10 degrees or more, and / or 80 degrees or less.

[0028] The plate may be configured to be mounted adjacent the sound reflective surface such that the device has a depth at or below 1 / 35, most preferably 1 / 40, of the longest wavelength in the desired frequency range. This provides for a device with a smaller depth (or thickness) than other known devices, especially for the longer wavelengths (and lower frequencies) encountered in the built environment, which are not well catered for by existing devices. For example, the desired frequency range may have an upper threshold of 250 Hz, or optionally 200 Hz.

[0029] The device may comprise a frame constraining the plate on one or more peripheral edges of the plate, for example around an entire periphery of the plate, or at one or more discrete peripheral positions. The frame may inhibit movement of the peripheral edge. The frame may comprise a peripheral wall that extends towards the sound reflective wall in use to define an enclosed backing cavity between the plate and the sound reflective surface. In this way, the frame can have multiple roles: constraint of the plate, attachment of the device to the sound-reflective surface (or other structure), and defining an enclosed backing cavity.

[0030] In some embodiments the plate comprises a plurality of layers, including first and second external layers sandwiched together by an elastomeric layer. This arrangement enables straightforward manufacture of the plate. The elastomeric layer can serve to bond the first and second external layers. The elastomeric layer can mechanically couple the first and second external layers. The elastomeric layer, e.g. an elastomeric adhesive, enhances and allows for tuning of the structural damping of the plate. The loss factor can be tuned according to the thickness of the elastomeric layer to optimise the amplitude of the absorption peaks.

[0031] Each resonator element may comprise a cut-out portion integrally formed with the plate, each cut-out portion corresponding to a complementarily shaped recess or opening in the plate. This arrangement provides for a particularly efficient manufacturing process. The cutout portion can be die cut or laser cut, for example, from the material of the plate (or from the material of an exterior layer of the plate) and then bent relative to the plate to provide the resonator element.

[0032] A second aspect of the invention provides an assembly of a plurality of devices according to the first aspect. In some embodiments each device has a different desired frequency range. In this way, sound can be absorbed over an even broader frequency range.

[0033] For example, at least one device of the plurality of devices may have a plate configured to be mounted at a different distance and / or angle from the sound reflective surface than the plate of at least one other device of the plurality of devices, to thereby absorb sound at a different desired frequency range.

[0034] Alternatively, or in addition, at least one device of the plurality of devices may have a plate with a different thickness, shape and / or material properties than the plate of at least one other device of the plurality of devices, to thereby absorb sound at a different desired frequency range.

[0035] Alternatively, or in addition, at least one device of the plurality of devices may have resonator elements with a different thickness, shape and / or material properties than the resonator elements of at least one other device of the plurality of devices, to thereby absorb sound at a different desired frequency range. A third aspect of the invention provides a method of absorbing sound within a desired frequency range, the method comprising mounting a device according to the first aspect or an assembly according to the second aspect adjacent a sound reflective surface to thereby define the backing cavity between the plate and the sound reflective surface.

[0036] The step of mounting the device may comprise defining an enclosed backing cavity between the plate and the sound reflective surface. An enclosed backing cavity may be substantially air-tight or otherwise configured to prevent escape of gases within the cavity. The backing cavity may be defined by the frame of the first aspect.

[0037] The step of mounting the device may comprise facing the resonator elements towards the sound reflective surface. That is, the resonator elements may extend into a space between the plate and the sound-reflective surface, so that the elements are obscured in normal use. The front face of the device can therefore be smooth, for example to provide an aesthetically pleasing finish that can be decorated as the end user desires. In some embodiments a further portion of the resonator elements may extend from a second surface of the plate opposed to the first surface.

[0038] The step of mounting the device may comprise arranging the plate adjacent to the sound reflective surface such that the device has a depth of or less than 1 / 40 of the longest wavelength in the desired frequency range. This provides for method of sound absorption that requires a device with a smaller depth (or thickness) than other known devices, especially for the longer wavelengths encountered in the built environment, which are not well catered for by existing devices.

[0039] The step of mounting the device may comprise comprising mounting a plurality of devices according to the first aspect adjacent a sound reflective surface, wherein the plate of at least one device of the plurality of devices is mounted at a different distance and / or angle from the sound reflective surface than the plate of at least one other device of the plurality of devices. In this way, sound can be absorbed over an even broader frequency range.

[0040] A fourth aspect of the invention provides a method of manufacturing a device according to the first aspect, including the steps of: providing a first layer to form the plate; forming a plurality of cut-out portions in the first layer; bending each cut-out portion at a bend line to form the plurality of resonator elements.

[0041] The method may further include laminating a second layer with the first layer. In preferred embodiments an elastomeric layer is laminated between the first layer and second layer.

[0042] An alternative aspect of the invention provides a method of manufacturing a device according to the first aspect, including the steps of forming the plate and the resonator elements using an additive manufacturing technique. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, integers or steps. Moreover 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.

[0043] Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible.

[0044] BRIEF DESCRIPTION OF THE DRAWINGS

[0045] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0046] Figure 1 is an orthographic view showing a front face of a sound-absorbing device according to an embodiment of the invention;

[0047] Figure 2 is an orthographic view showing a front face of a panel comprising an array of sound-absorbing devices according to an embodiment of the invention;

[0048] Figure 3 is an orthographic view showing a rear face of the panel of Figure 2;

[0049] Figure 4 is a sketch illustrating a cross-sectional view of a sound-absorbing device according to an embodiment of the invention mounted to a sound-reflective surface;

[0050] Figure 5 is an orthographic view showing a rear face of a sound-absorbing device according to an embodiment of the invention;

[0051] Figure 6 is an exploded view of the device of Figure 5;

[0052] Figure 7 illustrates the absorption spectra of a sound-absorbing device according to an embodiment of the invention as compared to the absorption spectra of a rigid plate (panel absorber) and a non-rigid plate, with horizontal lines to indicate the improvement in mean sound absorption achieved by the invention;

[0053] Figure 8 illustrates the absorption spectra of a sound-absorbing device according to two embodiments of the invention as compared to a similar device with no resonator elements; Figure 9 shows a detail view of a resonator element and plate manufactured by a cut-out and laminating process;

[0054] Figure 10 shows the resonator element and plate of Figure 9 prior to bending of the resonator element at an angle to the plate;

[0055] Figure 11 provides an exploded view of the layers of a device according to an embodiment of the invention manufactured by a laminating process;

[0056] Figure 12 illustrates a possible regular grid arrangement of resonator elements on the plate;

[0057] Figure 13 illustrates a number of possible shapes for the resonator elements;

[0058] Figure 14 is a detail view and illustrates a possible arrangement in which the resonator elements are provided on two faces of the plate;

[0059] Figure 15 shows a panel according to an embodiment of the invention in which each unit cell, or sound-absorbing device, is configured to be mounted at a different distance from a sound reflective surface;

[0060] Figure 16 illustrates absorption spectra as a function of thickness ratio (t / A) for unit cells according to embodiments of the invention;

[0061] Figure 17 illustrates a unit cell according to an embodiment of the invention with a radial grid arrangement of resonator elements;

[0062] Figure 18 illustrates a panel according to an embodiment of the invention arranged in the corner of a room or other volume where sound attenuation is required; and

[0063] Figure 19 illustrates a panel according to another embodiment of the invention arranged in the corner of a room or other volume where sound attenuation is required.

[0064] DETAILED DESCRIPTION

[0065] Sound-Absorption Devices

[0066] The illustrated embodiments show a sound absorbing panel 100 for use in the architectural field, in particular for lining one or more walls of a room to provide a sound-absorbing wall covering. The skilled reader will understand that the features and concepts illustrated herein can be applied to other applications, such as providing sound-absorbing devices within automotive or aeronautical structures.

[0067] Each panel 100 comprises a plurality of (nine in the illustrated embodiments) soundabsorbing unit cells 10 arranged in an array. Each unit cell 10 comprises a plate 20 mounted, in use, such that it is offset from a sound-reflecting surface 200 (such as a wall) by a predetermined distance and at a predetermined angle. This arrangement provides a backing cavity 300 between the sound-reflecting surface 200 and the plate 20, the backing cavity 300 and plate 20 together forming a panel absorber in which the backing cavity 300 serves as a spring and the plate as a distributed mass. The plate 20 can be positioned at a distance parallel or oblique to the sound reflective surface 200 so that the backing cavity 300 has a predetermined depth and / or volume. The cavity 300 will typically contain air, but may contain one or more other compressible gases through which sound waves can be transmitted. In some embodiments, the cavity 300 may also contain other material such as thermal insulation material, or other components such as systems or structural components.

[0068] The structural properties of the plate 20 are tuned so that the plate resonant frequencies (vibrational modes or natural frequencies) of the plate 20 are distributed generally adjacent to the sound absorption peak at the cavity-plate resonant frequency of the cavity 300 and plate 20 (combined to form a panel absorber). In this way, the overall frequency range over which the unit cell 10 can absorb sound is defined. Preferably, the frequency range is between one octave and three octaves. That is, the frequency range may be distributed up to 1.5 octaves below the cavity-plate resonant frequency, and up to 1.5 octaves above the cavity-plate resonant frequency. The vibrational modes of the plate 20 are determined by the size, shape and constraints of the plate 20, and can be tuned by changing the density and / or elasticity of the plate material, the constraints and / or boundary conditions of the plate, and / or the size and / or shape of the bounded plate 20. Thin plates or membranes, for example, may have several vibrational modes over a narrower frequency bandwidth when compared to thick plates. Conversely, thinner plates and less dense materials tend to increase the frequency at which the cavity-plate resonant frequency occurs due to its lower distributed mass.

[0069] A frame 30 extends around the full periphery of the plate 20 and constrains displacement of the plate 20 at its peripheral boundaries. In other embodiments the plate 20 may be constrained at only a portion of its periphery. The unit cell 10 is mounted adjacent to the sound-reflecting surface 200 via the frame 30. In the illustrated embodiments the frame 30 has a depth that is equal to the offset distance between the plate 20 and the soundreflecting surface 200. That is, the frame 30 provides four walls which surround and enclose the backing cavity 300. In other embodiments the frame 30 may act simply to constrain the peripheral edges of the plate 20, and the backing cavity 300 may or may not be enclosed by one or more other features. In yet further embodiments the sound-reflective surface 200 may be mounted to the frame 30, and the unit cell 10 (or panel 100) may be mounted offset from a wall or other structural feature.

[0070] An array of resonator elements 22 extends from a first face of the plate 20. Each resonator element 22 comprises a generally planar plate-like member with a base 24 attached to the plate 20 and a free end 26. The resonator elements 22 each extend from the plate 20 at an angle, typically an acute angle within the range 10° to 80°. An appropriate angle is selected in order to ensure unobstructed movement or vibration of the resonator elements 22. In some embodiments a further array of resonator elements extend from a second face of the plate 20 opposite to the first face, as illustrated in Fig. 14.

[0071] The resonator elements 22 are each tuned to one or more element resonant frequencies. Typically, the one or more element resonant frequencies are within a range bounded by the one or more plate resonant frequencies and / or the cavity-plate resonant frequency. In this way, the range of frequencies over which the unit cell 10 can absorb incident sound is maximised.

[0072] In preferred embodiments one or more of the elements 22 are tuned to a first set of one or more frequencies, one or more of the elements 22 are tuned to a second set of one or more different frequencies, and one or more of the elements 22 are tuned to a third set of one or more further different frequencies, and so on. In this way, the resonator elements 22 can together be tuned to frequencies spanning the desired frequency range over which sound is to be absorbed.

[0073] The shape and / or dimensions of the resonator elements 22 can be controlled to control the element resonant frequencies. Numerical modelling may be used to determine the appropriate shape / and or dimensions of the resonator elements 22 to achieve particular resonant frequencies. A particularly favoured shape for the resonator elements 22 is shown in Figs. 9, 10 and 13(d). The base 24 is relatively narrow, and the shape progressively widens to a relatively broad portion at the free end 26. The base 24 and free end 26 have parallel edges, while the mid part tapers from base 24 to free end 26. The taper angle is governed by the length and width of the base 24 and free end 26 portions. The respective width, length and shape of the resonator elements 22 are selected according to the resonant frequencies targeted. Alternative shapes are illustrated in Fig. 13(a) to (i). While the illustrated shapes are symmetrical, it is noted that asymmetrical shapes may be suitable in some applications. The inventors have determined that shapes which mimic the shape of the scales on the wings of moths are particularly suitable. Such shapes are typically narrower at the base 24 and widen towards the free end 26. More generally, the width of the base 24 portion ranges from at least the thickness of the plate 20 up to two times the width of the free end 26 portion. The resonator elements 22 have a uniform thickness along their length in the illustrated embodiments. In preferred embodiments the thickness of each resonator element 22 is at least the same as the thickness of the plate 20, or alternatively at least the same as a metamaterial layer 20-1, 20-2 of the plate 20.

[0074] The positions of the resonator elements 22 on the panel 20 are chosen according to the vibrational modes of the panel. The resonator elements 22 are distributed on a regular or irregular grid leaving enough space between neighbouring resonator elements 22 to allow their unobstructed movement or vibration. For example, the resonator elements 22 may be positioned on a regular orthogonal grid, a radial grid, or an irregular grid. Examples of regular grid arrangements are shown in Figs. 5, 6 and 12. Figure 17 shows a unit cell 10 with a circular plate 20 and a radial grid arrangement of resonator elements 22. The selection of grid type is based on the vibrational modes of the panel 20. The position of each resonator element 22 is then selected so that each element is located on the most active area of a plate mode, where larger strain energy is stored. This boosts the sound absorption that can be achieved, and provides additional frequency peaks where otherwise absorption would drop.

[0075] The number of resonator elements 22 provided on each plate 20 of each unit cell 10 depends on the size of the plate 20, the predetermined dimensions of the resonator elements 22 and the predetermined separation distances between resonator elements 22.

[0076] The shape of each unit cell 10 is determined by the shape of the metamaterial plate 20. The plates 20 are square in the illustrated embodiments, but can be any shape suitable for the application. For example, the plates 20 may be regular or irregular polygons. Specific examples include rectangular, circular, hexagonal or ellipsoidal shaped plates.

[0077] The panel 100 is assembled from multiple unit cells 10 having either the same plate shape or alternatively combining varied predetermined plate shapes generating a grid or periodic arrangement. Adjacent unit cells 10 can be tightly coupled or separated by a predetermined distance.

[0078] In some embodiments each unit cell 10 may be configured so that it is tuned to frequencies spanning a different frequency range to that of one or more other unit cells 10 of the panel 100. For example, each unit cell 10 of the panel 100 can be designed to absorb sound over a predetermined frequency range spanning over at least one octave band. By mixing several unit cells 10 comprised of different plate 20 configurations (e.g. different plate thicknesses) and / or different resonator element 22 configurations (e.g. different element shapes), and / or different cavity 300 configurations (e.g. different cavity 300 depth or volume), the panel 100 can absorb frequencies over an even wider frequency range.

[0079] Fig. 15 illustrates an example panel 100 configuration in which each unit cell 10 is located in a different position relative to a sound reflective surface so that the depth and volume of the backing cavity 300 of each unit cell 10 is different, and thus the cavity-panel resonant frequencies are different. The plates 20 and resonator elements 22 of each unit cell 10 may be tuned to frequencies in a range adjacent to or spanning the respective cavity-plate resonant frequency, as previously discussed.

[0080] Figures 18 and 19 illustrate possible configurations for mounting panels 100 according to the invention within a room or other volume within which sound attenuation is required.

[0081] In Figure 18 the panel 100 is mounted at a three-sided corner of a room subtended by three orthogonal edges 700, 702, 704 defining three orthogonal planar walls. The plate 20 of the panel 100 is triangular in shape, such that each of its three edges abuts a planar wall defined by two of the orthogonal edges 700, 702, 704. Each of the three walls acts as the sound reflective surface, and the volume defined between the walls and the panel 100 provides an enclosed backing cavity 300.

[0082] In Figure 19 the panel 100 is mounted between two orthogonal walls 710, 712. The plate 20 of the panel 100 is rectangular in shape, such that its side edges each abut a respective one of the walls 710, 712. Each of the two walls 710, 712 acts as the sound reflective surface, and the volume defined between the walls and the panel 100 provides a backing cavity 300. The backing cavity 300 is not enclosed, since it is unconstrained at its upper and lower portions, such that air (or other compressible gas) can move freely into or out of the cavity 300.

[0083] Panels 100 according to embodiments of the invention thus absorb sound that otherwise is reflected from a surface 200 and can cover a wide range of frequencies thanks to the contribution of combined resonances from the metamaterial plate 20, each individual resonator element 22, and the backing cavity 300. The panel 100 dissipates energy from impinging sound waves through the interactive resonances of the plate 20, backing cavity 300, and resonator elements 22, with the result that the panel 100 absorbs sound reflected from the sound-reflective surface 200.

[0084] Fig. 7 illustrates how the present invention achieves sound absorption over a wide frequency bandwidth. The solid line 410 illustrates the sound absorption spectrum of a unit cell 10 comprising a sound-absorbing device according to an embodiment of the present invention.

[0085] The dotted line 420 illustrates a single sound absorption peak at the cavity-plate resonant frequency for a plate in combination with the backing cavity 300 (together forming a panel absorber), but without the effects of any distortion of the plate itself due to the resonant frequencies (i.e. vibration modes) of the plate (i.e. assuming that the plate is a rigid plate). This cavity-plate resonant frequency provides sound absorption across a relatively narrow frequency band.

[0086] The solid grey line 430 illustrates the additional absorption peaks of the plate 20 of the unit cell 10, but without the effects of the resonator elements 22 and the cavity 300. The plate 20 is constrained at its peripheral boundaries, in the manner that the plate 20 of the illustrated embodiments is constrained, and is able to distort due to its resonant frequencies (i.e. vibration modes). That is, a finite plate with clamped boundary conditions possesses natural resonant frequencies defined by the mechanical properties of the plate, such as its mass and bending stiffness. When such plate resonant frequencies are adjacent to the cavity-plate resonant frequency this can create additional absorption peaks, as shown. The width and amplitude of the sound absorption peaks is determined by the damping characteristics of the plate (i.e. viscoelastic behaviour). Thus, line 430 shows an example of the sound absorption peaks caused by the plate's vibrational modes, and how such peaks can broaden the frequency bandwidth.

[0087] However, while the overall frequency range is broadened by combining the cavity-plate resonant frequency (attributed to the backing cavity) and the plate resonant frequencies (attributed to the vibrational modes of the plate), there are nevertheless some frequencies with a relatively low absorption coefficient. It is these frequencies that are targeted by the resonator elements 22 of the present invention.

[0088] The shaded region 440 illustrates the frequency band within which the resonator elements 22 have resonant frequencies (i.e. vibrational modes). That is, the resonator elements 22 are configured to vibrate and dissipate energy over a targeted frequency range 440 between a first cavity-plate resonant frequency peak 432 and a second cavity-plate resonant frequency peak 434. This is achieved by tuning each of the resonator elements so that they each have one or more different element resonant frequencies, and together the element resonant frequencies span the targeted frequency range as highlighted by the unit cell sound absorption spectrum 410. In this way, the mean absorption coefficient is increased, and it is possible to reduce or eliminate frequencies where a relatively low absorption coefficient is achieved within the targeted frequency range 440. Fig. 8 provides a further illustration of how the present invention achieves sound absorption over a wide frequency bandwidth.

[0089] The solid line 510 illustrates the sound absorption spectra of a unit cell 10 comprising a sound-absorbing device according to an embodiment of the invention, in which the resonator elements 22 are each tuned to one or more different element resonant frequencies, such that together they span the targeted frequency band 540.

[0090] In contrast, the grey line 520 illustrates the sound absorption spectra of the same unit cell 10 but in which the resonator elements 22 are each tuned to 100 Hz. It can be seen that a better absorption performance across the targeted frequency band 540 can be achieved when the resonator elements 22 are tuned to a distributed range of frequencies, rather than when the elements 22 are each tuned to the same frequency.

[0091] The dotted line 530 illustrates the performance of the unit cell 10 without any resonator elements 22, and is useful for comparison.

[0092] Experimental Results

[0093] The performance of three prototypes was assessed by measuring their absorption coefficient spectra, which defines the fraction of acoustic energy that is absorbed from an incident sound field as a function of frequency. Typically, low frequencies (long wavelengths) are absorbed by thick and heavy acoustic panels and known products differ in their thickness. Therefore, a thickness-to-wavelength ratio (t / A) was used to compare how efficiently lower frequencies are absorbed. Thickness (t) is the depth of the unit cell 10 from the soundreflecting surface 300 to the outer face of the plate 20.

[0094] Fig. 16 shows absorption spectra of four prototypes, identified as Panels 1 to 4, with different unit cell 10 thicknesses (20mm, 50mm, 57mm and 95mm). Line 610 indicates the spectrum for Panel 1, line 620 the spectrum for Panel 2, line 630 the spectrum for Panel 3 and line 640 the spectrum for Panel 4. The table below provides details about the characteristics of each of the prototypes.

[0095] Note that Panels 3 and 4 were identical, other than the depth of the backing cavity 300.

[0096] That is, the thickness (t) of Panels 3 and 4 is different, but other characteristics are identical. Panels 1 and 2 were measured experimentally using the two-microphone transfer function method (ASTM E1050). Laser-Doppler vibrometry was used to validate the frequency responses of the embedded resonators. The spectra for Panel 3 and Panel 4 were computationally modelled. Experimental work has demonstrated that the prototyping method employed provides a very good agreement between experimental and computational results.

[0097] The most recent prototype iterations (Panels 3 and 4) demonstrate that the invention can provide a thickness-to-wavelength ratio of between 1 / 40 and 1 / 100 at greater than 40% absorption, which approaches the established maximum performance of a moth wing.

[0098] Methods of Manufacture The unit cells 10 of the panel 100 can be manufactured by a number of suitable methods. One appropriate method uses additive layer manufacturing techniques to produce the plate 20 and resonator elements 22 as one unitary part. Suitable materials include thermoset resins, thermoplastic materials or metal.

[0099] In another method the plate 20 and resonator elements 22 are formed by a cut-out and laminating process. They are formed from at least three layers, as illustrated in Figs. 9, 10 and 11. First 20-1 and second 20-2 external layers are laminated to either face of an internal elastomeric adhesive layer 20-3 that bonds the external layers together. The first external layer 20-1 is die cut or laser cut along the cut-out contour line 20-4 illustrated in Fig. 10 and then bent along bend line 20-5 to form each resonator element 22. In the illustrated embodiments the resulting cut-out portions 20-6 are closed (i.e. blocked off or otherwise obscured) by the elastomeric adhesive layer 20-3 and / or the second external layer 20-2 so that a recess is formed.

[0100] The first external layer 20-1 thus forms a metamaterial layer. In some embodiments the second external layer 20-2 is also provided with resonator elements 22 formed by the same cut-out and bend method, so that this layer 20-1 is also a metamaterial layer. The metamaterial layer or layers 20-1, 20-2 can be plastic, metal, paper, or cardboard. The secondary external layer 20-2 that is not a metamaterial layer can be plastic, elastomer, metal, paper, cardboard, or plasticised fabric.

[0101] In related embodiments, it is envisaged that the cut-out portions 20-6 may instead remain open, or alternatively be closed (i.e. blocked off or otherwise obscured) by other means, for example in embodiments in which the adhesive layer 20-3 and / or the second external layer 20-2 are dispensed with.

Claims

CLAIMS1. A sound-absorption device configured to absorb sound within a desired frequency range, the device comprising: a plate configured to be mounted adjacent a sound reflective surface to absorb sound reflected from the sound reflective surface, a space between the plate and the sound reflective surface defining a backing cavity in use; and a plurality of resonator elements, each resonator element comprising a base attached to the plate and a free end, wherein each resonator element extends from the plate at an angle with respect to the plate.

2. A device according to claim 1, comprising means to enclose the backing cavity between the plate and the sound reflective surface in use.

3. A device according to claim 1 or claim 2, wherein the plate has one or more plate resonant frequencies, the backing cavity defined between the plate and the sound reflective surface in use has a cavity-plate resonant frequency, and each resonator element has one or more element resonant frequencies different to the one or more plate resonant frequencies and the cavity-plate resonant frequency, wherein the one or more plate resonant frequencies, the cavity-plate resonant frequency and the one or more element resonant frequencies are within the desired frequency range.

4. A device according to claim 3, wherein the cavity-plate resonant frequency is substantially at a mid-point of the desired frequency range, optionally wherein a first plate resonant frequency is below the cavity-plate resonant frequency and a second plate resonant frequency is above the cavity-plate resonant frequency, and further optionally wherein a first element resonant frequency of the one or more element resonant frequencies is below the cavity-plate resonant frequency and a second element resonant frequency of the one or more element resonant frequencies is above the cavity-plate resonant frequency.

5. A device according to claim 3 or claim 4, wherein one or more of the plurality of resonator elements has one or more first resonant frequencies and a further one or more of the plurality of resonator elements has one or more second resonant frequencies different to the first resonant frequencies.

6. A device according to any of claims 3 to 5, wherein the one or more plate resonant frequencies, the cavity-plate resonant frequency and the one or more element resonant frequencies are distributed throughout the desired frequency range.

7. A device according to any of claims 3 to 6, wherein the one or more element resonant frequencies are distributed over approximately one-third octave above and / or below one or each of the one or more plate resonant frequencies and / or the cavity-plate resonant frequency.

8. A device according to any preceding claim, wherein the resonator elements are generally plate-like.

9. A device according to any preceding claim, wherein each resonator element has a substantially uniform thickness between the base and the free end.

10. A device according to any preceding claim, wherein the plurality of resonator elements each have a shape that widens from the base towards the free end.

11. A device according to claim 10, wherein the shape is tapered from a narrower base to a wider region.

12. A device according to any preceding claim, wherein at least a portion of the resonator elements extend from a first surface of the plate such that they extend towards the sound reflective surface in use.

13. A device according to claim 12, wherein a further portion of the resonator elements extend from a second surface of the plate opposed to the first surface.

14. A device according to any preceding claim, wherein the resonator elements are arranged on the plate in one of an orthogonal grid, a radial grid, or an irregular pattern.

15. A device according to any preceding claim, wherein the resonator elements are each arranged on the plate at an active region of a vibration mode of the plate.

16. A device according to any preceding claim, wherein the resonator elements each extend from the plate at an angle of 10 degrees or more, and / or 80 degrees or less.

17. A device according to any preceding claim, wherein the plate is configured to be mounted adjacent the sound reflective surface such that the device has a depth at or below of 1 / 40 of the longest wavelength in the desired frequency range.

18. A device according to any preceding claim, wherein the desired frequency range comprises up to three octaves.

19. A device according to any preceding claim, comprising a frame constraining the plate at one or more peripheral edges of the plate.

20. A device according to claim 17, wherein the frame comprises a peripheral wall that extends towards the sound reflective wall in use to define an enclosed backing cavity between the plate and the sound reflective surface.

21. A device according to any preceding claim, wherein the plate comprises a plurality of layers, including first and second external layers sandwiched together by an elastomeric layer.

22. A device according to any preceding claim, wherein each resonator element comprises a cut-out portion integrally formed with the plate, each cut-out portion corresponding to a complementarily shaped recess or opening in the plate.

23. A device according to any preceding claim, comprising a sound reflective surface arranged adjacent to the plate such that the backing cavity is defined between the plate and the sound reflective surface.

24. An assembly of a plurality of devices according to any preceding claim.

25. An assembly according to claim 24, wherein each device has a different desired frequency range.

26. An assembly according to claim 24 or claim 25, wherein at least one device of the plurality of devices has a plate configured to be mounted at a different distance and / or angle from the sound reflective surface than the plate of at least one other device of the plurality of devices, to thereby absorb sound at a different desired frequency range.

27. An assembly according to any of claims 24 to 26, wherein at least one device of the plurality of devices has a plate with a different thickness, shape and / or material properties than the plate of at least one other device of the plurality of devices, to thereby absorb sound at a different desired frequency range.

28. An assembly according to any of claims 24 to 27, wherein at least one device of the plurality of devices has resonator elements with a different thickness, shape and / or material properties than the resonator elements of at least one other device of the plurality of devices, to thereby absorb sound at a different desired frequency range.

29. A method of absorbing sound within a desired frequency range, the method comprising mounting a device according to any of claims 1 to 22 adjacent a sound reflective surface to thereby define the backing cavity between the plate and the sound reflective surface.

30. A method according to claim 29, wherein mounting the device comprises defining an enclosed backing cavity between the plate and the sound reflective surface.

31. A method according to claim 29 or claim 30, wherein mounting the device comprises facing the resonator elements towards the sound reflective surface.

32. A method according to any of claims 29 to 31, wherein mounting the device comprises arranging the plate adjacent the sound reflective surface such that the device has a depth at or less than 1 / 40 of the longest wavelength in the desired frequency range.

33. A method according to any of claims 29 to 32, comprising mounting a plurality of devices according to any of claims 1 to 23 adjacent a sound reflective surface, wherein the plate of at least one device of the plurality of devices is mounted at a different distance and / or angle from the sound reflective surface than the plate of at least one other device of the plurality of devices.

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