Acoustic metamaterial structures

Abstract

Disclosed is a modular kit for producing structures for controlling acoustic waves. The kit comprises modules providing respective arrangements of unit cells that can be connected together to define an overall form of the structure, and wherein the structure of a unit cell determines how an incident acoustic wave will locally interact with the module at the respective position of the unit cell, such that when multiple ones of the plurality of modules within the kit are connected together to define the structure, the distribution of local interactions due to the arrangement of the unit cells within the modules controls the propagation of acoustic waves interacting with the structure.

Description

[0001]163673 / 01 Acoustic Metamaterial Structures The technology described herein relates generally to structures for manipulating acoustic waves and techniques for manufacturing the same. In particular, the technology described herein utilises modules, which may be engineered at subwavelength level (often classified with the name “acoustic metamaterials”), to build larger structures that provide control over acoustic waves (e.g. for noise reduction). Cross-Reference to Related Application This application claims priority from and the benefit of United Kingdom patent application no.2301232.1 filed on 27 January 2023. The entire content of this application is incorporated herein by reference. Background The use of acoustic metamaterials to manipulate acoustic waves has been the subject of various developments in recent years. There are various applications where this might be desirable. For example, US-10,873,812 and WO 2020 / 208380 (both filed in the name of The University of Sussex) describe various acoustic metamaterial systems as well as applications of such systems. One area that is increasingly being explored is the use of acoustic metamaterials for noise reduction wherein the acoustic metamaterial system is designed to selectively reduce certain frequencies of sound, or correspondingly to accentuate certain other (desired) frequencies of sound. This may be particularly attractive since the use of acoustic metamaterials may help in realising lighter weight, and more flexible, structures compared to traditional noise-reducing techniques, such as those that rely on adding significant acoustic damping material, or rely on active noise cancellation. The Applicants however believe that there remains scope for improvements in this regard. Summary A first aspect of the present invention comprises a modular kit for producing structures for controlling acoustic waves, wherein the kit comprises: a plurality of modules that can be connected together to define an overall form of the structure, wherein respective modules of the plurality of modules comprise respective arrangements of unit cells, wherein the structure of a unit cell determines how an incident acoustic wave will locally interact with the module at the respective position of the unit cell, the arrangement of unit cells within a module thus defining a corresponding arrangement of local interactions, such that when multiples ones of the plurality of modules within the kit are connected together to define the structure, the distribution of local interactions for the overall structure controls the propagation of acoustic waves interacting with the structure. The present invention also extends to the structures that can be (and are) formed using such modular kit. Thus, a second aspect of the technology described herein comprises a modular structure for controlling acoustic waves, wherein the structure comprises a plurality of modules that are connected together to define an overall form of the structure, wherein respective modules of the plurality of modules comprise respective arrangements of unit cells, wherein the structure of a unit cell determines how an incident acoustic wave will locally interact with the module at the respective position of the unit cell, the arrangement of unit cells within a module thus defining a corresponding arrangement of local interactions, such that the distribution of local interactions for the overall structure controls the propagation of acoustic waves interacting with the structure. The present invention further extends to the form of the individual modules for use in forming such structures, as will be explained further below. The present invention relates to novel approaches for controlling acoustic waves (i.e. controlling sound) using a structure comprising an arrangement of unit cells. The structure of the individual unit cells determines how an incident acoustic wave will interact with the unit cell. Thus, the arrangement of unit cells for the structure defines a corresponding distribution of local interactions that will accordingly determine how an incident acoustic wave will interact with the overall structure. For example, and in preferred embodiments, as will be explained further below, the structures of the present invention use unit cells that are each capable of encoding a particular delay (or set of delays). However, it would also be possible to use unit cells that are designed to encode a particular resonance. Of course the structure may also comprise a combination of different types of unit cells, as desired. The structure may thus generally comprise of an “acoustic metamaterial” structure, in that the control of acoustic waves propagating within or through the structure is determined by the arrangement and sub-wavelength structures of the unit cells, as will be explained further below. Acoustic metamaterials present new avenues for controlling acoustic waves using more passive structures that can advantageously be constructed from various different materials, thus offering possibilities for lighter weight and / or smaller structures. For instance, considering noise cancellation as an example, acoustic metamaterial-based approaches can achieve significant noise reduction using relatively lighter weight and thinner structures, e.g., compared to solutions using active noise cancellation or that rely on sound absorption. Such acoustic metamaterial devices can potentially also be transparent to light and / or fluid (e.g. air) in a manner that could not typically be achieved with more traditional solutions. Despite these possibilities there are still difficulties in manufacturing acoustic metamaterial based structures to scale in a cost and time efficient manner. The present invention thus relates to improved techniques for constructing such structures or devices to provide a desired control of acoustic waves, such as noise reduction, that can preferably provide the benefits offered by metamaterial-based approaches, whilst facilitating improved (e.g. simpler and cheaper) manufacturing. This is achieved according to the present invention by providing a modular kit (i.e. or ‘set’ of modules), or a modular structure, in which individual modules of “unit cells” (which modules preferably comprises ‘strips’ of unit cells, as will be explained further below, although in general may also take other forms) can be interconnected to assemble larger structures (which may, e.g., be flat panels, polyhedral structures, or approximately curved or angled structures, as desired, depending on the arrangement of the modules) with the arrangement of unit cells controlling the interactions of acoustic waves with the structure, e.g., and preferably, by controlling the propagation of acoustic waves within or through the structure, to achieve a desired acoustic control (which may, e.g., be, and in a particularly embodiment is, noise reduction, but other arrangements are also contemplated and it is a potential benefit of the approaches described herein that there is flexibility in how the modules of unit cells may be connected to define different structures for performing different acoustic control operations). The individual modules may generally take any suitable and desired form. For example, the individual modules may, and preferably do, comprise ‘strips’ of Nx1 unit cells, that can then be assembled together as desired to create larger structures. However, in general, the individual modules may comprise other arrangements of unit cells, such as an ‘L’ or ‘T’ shaped arrangement of unit cells, that can be arranged together, as desired. In the case of a curved surface, for instance, the shape of the individual modules may be suggested by tessellation (e.g. they may be rhomboidal or trapezoidal) or bio-inspired (e.g. similar to the scutes of crocodile armours). Thus, in preferred embodiments, at least one module is provided in the form of an Nx1 strip of unit cells, where N is any suitable integer that is greater than or equal to 2, but in preferred embodiments may be greater than or equal to 3, 4, 5, 6, 7, 8, 9, 10, or higher. Preferably, there are a plurality of modules, and in some preferred embodiments all of the modules (in the kit (i.e. set) or structure), are provided in a similar (or the same) form, such that in some embodiments there are multiple modules, each taking the form of a respective strip of Nx1 unit cells. That is, in some embodiments, all of the individual modules that are provided within a kit or used within a given structure may comprise such Nx1 strips of unit cells. In some embodiments, all of the individual modules within a given kit or structure may comprise such Nx1 strips of unit cells. This can be particularly beneficial in simplifying the assembling of the modules together, as will be described further below, wherein individual modules can be connected end-to-end and / or stacked on top of each other to define a larger structure. This can also facilitate improved manufacture of the individual modules themselves, e.g., and preferably, by injection moulding, as will be described further below. That is, the use of modules in the form of such ‘strips’ has been found to particularly facilitate mass manufacturing of the modules (i.e. the strips), and hence reduce associated costs and time for assembling larger structures using such modules. In the case where the kit or structure comprises a plurality of individual modules that each comprise a respective Nx1 array of unit cells, each of such modules may have the same size (i.e. the value of N may be the same), or there may be different modules having different sizes (i.e. different values of N). Various arrangements are contemplated in that regard. It is also contemplated that such Nx1 strips of unit cells may be combined with modules having different shapes, as desired, to provide greater flexibility in how the structure is formed. For example, some other modules may comprise larger arrays of unit cells, or may comprise, e.g., an ‘L’ or ‘T’ shaped arrangement of unit cells or rhomboidal or trapezoidal-shaped arrangements of unit cells. Various arrangements are contemplated in this regard. That is, the modules that are used together to form a given structure may have the same size and shape (and in preferred embodiments do have the same size and shape), but it is also contemplated that different modules may be provided having different sizes and / or different shapes. In some preferred embodiments, for at least one module (which may, e.g., be an Nx1 strip, as discussed above, but could also take other suitable forms, as desired), and preferably a plurality of modules, preferably all of the modules, the respective unit cells of the module are open on a first, front face of the module to allow fluid to pass into the unit cells of the module through the first face, whereas the unit cells of the module are closed on a second, rear face of the module, such that an acoustic wave that is incident on the first face of the module will pass into a respective channel provided by a respective unit cell of the module, and then impinge on the closed second face of the module. A “front face” or “front surface” of a unit cell and corresponding “front face” or “front surface” of a module as referred to herein will generally be understood to be the same, front, face or surface of the “acoustic metamaterial” structure (or acoustic metasurface) thus formed from those unit cells / modules. Similarly, the “rear face” or “rear surface” of a unit cell and corresponding “rear face” or “rear surface” of a module as referred to herein will generally be understood to be the same, rear, face or surface of the “acoustic metamaterial” structure (or acoustic metasurface) thus formed from those unit cells / modules. It will be appreciated that “front” and “rear’ are used here primarily for semantic purposes and that whilst in some preferred embodiments the structures may be intended such that the “front” face is facing a source of noise, e.g., such that an incident acoustic wave to be controlled by the structure first impinges on the front face, and then subsequently impinges on the rear face having travelled from the front face and through the unit cells, the unit cells, modules, and structures, described herein, may also be used in the reverse sense, such that incident acoustic waves to be controlled are instead caused to first impinge on the “rear” surface of the structure (which may e.g. be a closed surface). Indeed, in general, the unit cells, modules, and structures described herein may be used to manipulate acoustic waves impinging upon it from any suitable and desired direction. In some embodiments, each module of the plurality of modules is identical, e.g. each module may be an identical Nx1 strip of unit cells (i.e. identical unit cells). However, there are provided at least two different types of modules, each having a different arrangement of unit cells. Preferably, each of the different types of individual modules are provided in a similar form, e.g., and in preferred embodiments, in the form of an Nx1 strip of unit cells, but with the arrangement of unit cells being different for the different types of module. This then allows improved flexibility in terms of being able to assemble different structures with desired acoustic control properties. Other arrangements would however be possible. For example different types of individual modules may be provided that differ in length (i.e. they are all Nx1 strips of unit cells, but the value of N is different for different types of unit cells) and / or shape. In a particularly preferred embodiment, the modular kit, or structure, comprises at least two different types of modules. For example, in preferred embodiments, the modular kit, or structure, may comprise at least first and second, different types of modules that can be connected together to define the overall form of the structure, wherein each of the first and second types of modules comprises an Nx1 strip of unit cells, wherein N may be the same or different for the first and second types of modules, and wherein the first and second types of modules can be stacked together to define a panel, the panel thereby comprising a two dimensional array of unit cells (e.g. an NxM array of unit cells, where M is the number of modules that are stacked together to form the panel), wherein the first and second types of modules comprise respective different arrangements of unit cells, and wherein the respective arrangement of unit cells for the first and second types of modules are configured such that the thus formed panel is configured to perform a desired acoustic control operation at least for acoustic waves that are incident substantially normally to the panel. That is, when a plurality of modules of the first and second types are stacked together in such alternating manner, with a module of the first type stacked on top of a module of the second type, and so on, the distribution of local interactions (e.g. delays and / or resonances) for the thus formed panel is configured to perform a desired acoustic control operation at least for acoustic waves that are incident substantially normally to the panel. Thus, in preferred embodiments, individual modules are provided that can be stacked on top of each other so as to define two-dimensional panels. A structure formed according to the present invention thus preferably comprises one or more panels, each panel preferably comprising a respective stack of modules of at least the first and second types of modules described above (although in principle a panel could also be formed by simply stacking a single type of module). Preferably, the individual modules are designed to facilitate such stacking. For example, and as will be explained further below, the features (and walls) that are present on an upper side of one module are preferably designed to closely fit with the features that are present on a lower surface of another module. For instance, the lower surface of a module may be provided with grooves that generally match the shape and position of the features that are present on the upper side of another module such that the two modules can be closely stacked together. Thus, in preferred embodiments, the individual modules (e.g. strips) can only be stacked on top of one another in one orientation (although they may also be connected end-to-end, as will be explained further below), and the modules (e.g. strips) are preferably designed to facilitate such stacking. That is, a module is preferably designed with a certain orientation. Again, this may help to simplify manufacture and assembly. For example, in preferred embodiments where there are at least first and second different types of modules, as described above, the first and second types of module are preferably designed so as to facilitate stacking a module of the first type of module on top of a module of the second type of module. Preferably, the modules of the second type are then also configured for stacking on top of modules of the first type, so that the modules can be stacked in an alternating fashion to define a corresponding panel of modules. Each module is thus preferably generally rectangular, i.e. defined by a pair of opposing end faces and with four longitudinal faces or planes therebetween. The four longitudinal faces or planes may be suitably referred to as the top face and bottom face (which are generally opposing each other) and front face and rear face (which are as defined above, are generally opposing each other, may be generally perpendicular to the top / bottom faces). It is also contemplated however that the modules need not be generally rectangular and may instead be substantially trapezoidal, or otherwise shaped, to facilitate stacking the modules into curved surfaces. That is, rather than stacking modules to form substantially two-dimensional panels, as described above, the modules may be configured such that when the modules are stacked together the resulting structure is a curved surface. In particularly preferred embodiments, each module has a substantially continuous, i.e. closed, bottom face which defines a bottom surface. In contrast, the top face of each module is preferably substantially open. This may facilitate stacking of one module on top of another module, wherein the bottom surface of the module stacked on top is positioned along the top face of the below module, such that the bottom surface of the module stacked on top acts as a top (i.e. closing) surface for unit cells of the below module. Each module may have a substantially continuous, i.e. closed, rear face which defines a rear surface, whereas the front face of each module is preferably substantially open. Each module may comprise a continuous rear surface connected to a continuous bottom surface along a longitudinal edge so as to define a longitudinal L-shape. Other arrangements would however be possible. Internal structures within the modules may thus define the unit cells therein. The internal structures may therefore be connected to the continuous rear and bottom surfaces, whereas at least some free edges of the internal structures may define the open top face and the open front face. As will be appreciated, modules may be stacked successively such that the top face of a module is facing the bottom surface of the next module stacked immediately on top. The bottom surface of a module may comprise features which correspond to the free edges of the internal structures which define the open top face of the module stacked immediately below. Thus, a, and preferably each, module comprises top and bottom faces that connect the respective front and rear sides of the unit cells, and wherein one of the top or bottom faces of the module is open, whereas the other one of the top or bottom faces of the module is closed, wherein the features on the closed face are designed to facilitate stacking of modules. There may in some embodiments be a gap between modules within a stack, which may allow some fluid (e.g. air) flow through the resulting panel. In some preferred embodiments, however, the stacking is performed such that the resulting panel is closed on one side, e.g. the rear face of the panel. For example, in preferred embodiments, the modules are designed such that the respective unit cells are open on one side (a front face) but are closed on the opposite side (the rear face). Thus, in embodiments, the structure comprises at least one module, and preferably a plurality of modules, for which the unit cells of the module open on a first, front face of the module to allow fluid to flow into the unit cells of the module through the first face, and for which the unit cells of the module are closed on a second, rear face of the module, such that an acoustic wave incident on the first face will pass into a respective channel provided by a respective unit cell of the module, and then impinge on the closed second face of the module. (As will be described further below, the respective channels for at least some of the unit cells are preferably structured, with the interactions of the acoustic wave with the structure of the channel for a unit cell determining the respective delay associated with that unit cell.) Preferably, each of the first and second types of modules are formed in this way. Thus, in embodiments, for each of the modules of the first and second type, the respective unit cells of the module are open on a first, front face of the module to allow fluid to pass into the unit cells of the module through the first face, whereas the unit cells of the module are closed on a second, rear face of the module, such that an acoustic wave that is incident on the first face of the module will pass into a respective channel provided by a respective unit cell of the module, and then impinge on the closed second face of the module. In that case, the modules of the first and second different types of modules are preferably configured to facilitate stacking of the modules such that when a module of the first type is stacked on top of a module of the second type as part of a panel, a substantially closed rear surface is provided to prevent direct flow of fluid through the rear surface of the panel. For example, and preferably, a module of the first type preferably comprises a groove or lip on its bottom surface that is arranged to sit over the corresponding upper edge of a module of the second type, and vice versa, such that when the modules are stacked together in the manner described above, they are stacked in such a way to prevent direct flow of fluid between the bottom surface of the first module and the upper edge of the second module. In a similar manner, the individual modules are preferably designed such that when two modules are stacked on top of each other in this way, there is no direct line of fluid (e.g. air) flow in between the two modules. For example, and in preferred embodiments, the lower surface of the module provides a lip along at least one of its edges that is designed to sit over the corresponding edge at the upper part of another module such that when the module is stacked on top of the other module, the lip that is provided on the lower surface of the top module extends slightly over the edge at the upper part of the bottom module to prevent fluid (e.g. air) from being able to pass between the two modules. This can then provide improved acoustic behaviour. In general, however, the modules may be stacked with or without fluid (e.g. air) flow, and both options may be desirable, depending on the desired application. That is, in some cases, it may be desirable to allow for a degree of fluid (e.g. air) flow, e.g. for ventilation purposes. In that case, the modules will typically not comprise any such lip on their lower surface, for example, such that a gap remains between adjacent modules in the stack. In various embodiments above the unit cells are closed on their second, rear face. It is also contemplated however that the unit cells may be open, or at least partially open, on that face. Thus, in some embodiments, one or more of the modules may comprise one or more unit cells which are open, or at least partially open, on the second, rear face of the module. For instance, the portion of the unit cell which would otherwise form part of the rear face of the module may be entirely open, or may comprise an aperture therethrough. As will be appreciated, this may facilitate fluid flow (e.g. air flow) therethrough, as desired. The individual modules are preferably also configured to be able to interconnect with each other in an end-to-end manner. In a particularly preferred embodiment, an individual module is configured to be able to connect end-to-end with another similarly or identically formed module. Thus, a (and preferably each) module is preferably provided with connecting parts on its two end faces that are designed to mate with corresponding connecting parts on other such modules. In this way, the modules can be connected end-on-end to form longer structures. As will be understood, connecting in an end-to-end manner comprises connecting the modules in a direction different to the stacking direction referred to above. For instance the modules may be connected end-to-end in a direction which is substantially perpendicular to the direction in which modules are stacked. Thus, in preferred embodiments, the modules are designed such that they can be stacked on top of each other, to define two dimensional panels, with the stacking thus being performed in a first direction, but can also be connected end-to-end in a second direction, e.g. to increase the length of the panels in the second direction. This can therefore provide great flexibility in assembly different structures. As will be described below, in preferred embodiments, there are at least two different types of modules that can be connected together in this way, as this gives greater flexibility in assembling larger structures for performing desired acoustic control operations. That is, there are preferably at least two different types of modules that are configured for mutual end-to-end interconnection with each other, and that can therefore be (and preferably are) connected together to define the overall structure. The two different types of modules thus preferably comprise similar (identical) connecting parts on the respective end faces, but may comprise different arrangements of unit cells, for example. Thus, at least some of the modules in the plurality of modules, and preferably each of the modules in the plurality of modules, are provided with a male connecting part on a first end face of the module and are further provided with a corresponding female connecting part on a second, opposite end face of the module such that the modules can be connected end-to-end. Accordingly, in addition to stacking to form two dimensional arrays (panels) having a size dictated by the length of a module and the number of modules which are thereby stacked, modules can also connect end-to-end to define longer modules (or thus longer panels). Preferably all modules can be connected end-to-end. For instance, first connecting parts on respective end faces of modules may be connectable together, second connecting parts on respective end faces of modules may be connectable together, or preferably the first connecting part on an end face of one module may be connectable with the second connecting part on an end of another module. The first connecting part may be a male connecting part and the second connecting part may be a female connecting part, such that modules may only be connected end-to-end in a male-female arrangement, however various other arrangements possible. As will be appreciated, by providing modules having a first connecting part on the same respective end face and a second connecting part on a second connecting end face, the panel thus formed from stacking these modules will have a stack on first connecting parts on one end and a stack of second connecting parts on another end. This consistency of connecting parts on either end may facilitate installation of the panel in an exterior frame, e.g. by providing a frame configured to cooperate with only first connecting parts on one side and with only second connecting parts on the other side. In some embodiments, the structure may be assembled only by connecting and / or stacking individual modules together. However, in preferred embodiments, additional supporting parts are provided that are able to connect between individual modules, for support purposes and for allowing modules to be connected in different ways to create larger and more complex structures. Thus, in embodiments, the modular structure further comprises one or more corresponding supporting parts that are configured to connect between modules of the plurality of modules to define the overall form of the structure. The corresponding supporting parts may generally take any suitable and desired form. For example, in preferred embodiments, the supporting parts are designed with similarly shaped connecting parts to those that are provided on the end faces of the individual modules such that can be connected in the same way. Preferably the connecting parts are identical to those provided on the end faces of the individual modules. However, in principle, the supporting parts may also be used to connect modules to other structures, such as a frame. In that case, only one of the connecting parts on the support part may need to match with a connecting part on the module, and another face of the supporting part may have a different form of connecting part for connecting to another structure, as desired. Various options are contemplated in this regard and it is a benefit of providing such supporting parts that these increase the range of structures that can be assembled. For example, in some preferred embodiments, the supporting parts may also facilitate connecting individual modules end-to-end at different angles. That is, as described above, two modules may be connected end-to-end to define a structure that is effectively a ‘longer’ module, but in that case the structure has the same direction as the individual modules (e.g. it will be straight if the individual modules are straight). The use of supporting parts may thus allow two modules to be connected at a different angle, e.g. to define an angled connection, e.g. a corner, or to approximate a curve, with the supporting part connected between the two individual modules. As will be understood, ‘angle’ is taken to mean an angle other than 180 degrees, A range of different supporting parts may be provided to this end. For instance, one type of supporting part may allow for straight connections, but other types of supporting part may allow for 45 degree, 60 degree or 90 degree connections. Various other examples would be possible. The supporting parts may also be used to provide more flexible connections between modules. The structure that is formed may thus comprise various arrangements of modules (or panels of modules) that develop in three dimensions, wherein individual modules can be (and preferably are) stacked, or connected end-to-end, within a two- dimensional plane to define a panel, and wherein the modules (or panels) can also be connected at different angles. For example, in this way, a box or enclosure could be formed, comprising various panels connected at right angles. As another example, the panels could be connected at shallower angles so as to approximate a curved surface. Various arrangements are contemplated in this regard. In general the supporting parts may thus be made from the same material as the individual modules or may be made from different materials to provide different material properties to the connections. The use of such separate supporting parts thus increases flexibility of assembly and allows a wider range of structures to be assembled. The supporting parts may be designed to match one or more dimensions, e.g. the width and height, of the individual modules, to provide a more seamless connection. In some preferred embodiments however the supporting parts may match the modules in height but have a relatively reduced width compared to the modules. This may be useful when the supporting parts are designed to fit into an external frame for supporting the overall structure, for example. Various other arrangements would be possible in this regard and in general the supporting parts may take any suitable and desired form. The benefit of this modular approach is that the individual modules can thus be designed for relatively easier (cheaper) manufacture but can then be assembled as desired into larger structures to provide the desired overall structure and acoustic properties depending on the desired application. This therefore offers a highly flexible approach that is relatively cheaper, e.g. compared to manufacturing bespoke structures. For example, in particularly preferred embodiments, the individual modules can be injection moulded (and are preferably thus further designed to facilitate this), thus enabling at scale manufacture with reduced part costs. As will be appreciated, more than one module may be injection moulded together to provide an injection moulded assembly of modules, which can be subsequently separated (e.g. by cutting) so as to provide the two individual modules. For instance, it is contemplated that a pair of modules may be injection moulded ‘back-to-back’ (or indeed three or four modules may be injection moulded together), or a plurality of modules may be injection moulded ‘end-to-end’, and the modules may be cut from this injection moulded assembly of modules to provide the individual modules as described herein. As mentioned above, a respective, individual module generally comprises an arrangement of unit cells. Thus, according to the present invention, a module (and preferably each module) comprises a respective arrangement of unit cells that defines a corresponding arrangement of local interactions (e.g. delays and / or resonances) for the module. When the modules are interconnected together to define the overall structure, the respective arrangements of local interactions for the individual modules are thus effectively combined to determine a larger delay distribution for the structure. The distribution of local interactions controls how acoustic waves interact with the structure. For example, a unit cell may introduce a certain time (or correspondingly, phase) delay to a portion of an acoustic wave interacting with that unit cell or may react to a specific bandwidth or may resonate or partially-resonate at a specific bandwidth. Of course, at least some unit cells within a module may be unstructured so that acoustic waves pass through that unit cell without slowing down and / or without increasing the effective path length travelled by the acoustic waves through that unit cell. Such unit cells may be referred to as “empty” unit cells. As mentioned above, a, and preferably each, module of the modular structure may comprise unit cells that are open on a front face in order to allow fluid to pass into the unit cell, but are closed on an opposite, rear face, such that fluid is unable to pass through the rear face and out of the unit cell. However, other arrangements are possible. At least some unit cells within a module preferably define a channel extending between the front face and rear face, wherein channel is structured to determine respective delay associated with that unit cell. The channel may further comprise various sub-wavelength structures or features that act to slow down the acoustic waves and / or increase the effective path length travelled by the acoustic waves through the channel thereby introducing a phase delay. For example, the channels may include a substantially labyrinthine, or meandered, structure (e.g. formed by one or more bars protruding into the interior volume of the unit cell), or a multi-slit, coil, helical, or Helmholtz resonator-type structure, or a leaky-resonator structure. The different delays introduced to the different portions of the acoustic wave interacting with different cells may thus result in interference that will thus determine how the acoustic wave is manipulated. For example, in the case where the delays effectively correspond to a phase delay of pi between two adjacent unit cells at a certain operating frequency, there will be destructive interference, and hence noise reduction, at least at that frequency. However, other arrangements of delays, and acoustic waves at other frequencies, may be controlled differently. Various aspects of this are described in more detail in US-10,873,812 and WO 2020 / 208380, both filed in the name of The University of Sussex, and the contents of which are incorporated herein in their entirety. In some preferred embodiments, the modular kit is for providing noise- reducing structures. For example, in preferred embodiments wherein there at least first and second different types of modules that can be stacked together to form panels, the respective arrangements of unit cells for the first and second types of modules are configured such that when the modules are stacked together to form a panel, the distribution of local interactions (e.g. delays and / or resonances) for the panel is configured to provide noise reduction, at least at one or more target operating frequencies. Thus, in embodiments, the modular structure comprises one or more such panels that are configured to provide noise reduction, at least at one or more target operating frequencies. In that case, the respective arrangement of unit cells for the first type of module preferably comprises a first pattern of unit cells, and wherein the respective arrangement of unit cells for the second type of module preferably comprises a second, complementary pattern of unit cells such that when the modules are stacked together to form a panel, the distribution of local interactions (e.g. delays and / or resonances) for the panel is configured to provide noise reduction, at least at one or more target operating frequencies. For example, in one preferred embodiment, a first module may comprise a first alternating pattern (‘ABAB’) of unit cells, and a second module may also be available comprising the reverse pattern (‘BABA’), such that when the two modules are stacked on top of each other, a ‘checkerboard’ pattern is realised. Such checkerboard patterns have been found to be particularly advantageous in noise control applications as the portions of acoustic waves passing through the different squares may destructively interfere with each other to provide a significant noise reduction effect. For instance, in an example the alternating pattern may be an alternating series of empty unit cells and unit cells that introduce a certain phase change, e.g., and preferably, that introduces a phase change of pi at least at a desired operating frequency (or correspondingly any other pair of suitable phase changes between adjacent cells that provide a similar effect). In general, the modules may have any suitable repeating or alternating pattern. Although the preferred embodiment envisages two types of unit cells in each module to thereby form a checkerboard, other, more complex arrangements are contemplated.. For instance, a first module may define a particular gradient of phase shifts, whereas the second module may define an opposing gradient. Further, a first plurality of modules could together define a first plurality of gradients, whereas a second plurality of modules could together define a second plurality of respectively opposing gradients. In the preferred embodiments described above, various structures are created by assembling at least first and second different types of modules. However, various other arrangements would be possible. For example, in some embodiments there may be only a single type of unit cell. In that case, by appropriate assembly of that unit cell, the distribution of local interactions (e.g. delays and / or resonances) defined by the unit cells can still be selected to perform different acoustic control operations, in a similar manner as described above. For example, by stacking identical unit cells with a slight offset, it may be possible to create a checkerboard arrangement. Or, all of the unit cells may provide the same gradient of phase shifts. Various other examples would of course be possible. In preferred embodiments at least one (and preferably at least some) unit cells within an individual module comprise a channel having an internal structure that defines a tortuous flow path within the unit cell. For example, the unit cell may comprise one or more bars extending into the channel. The bars may thus define a meandered structure to increase the effective path length for acoustic waves interacting with the unit cell. In this way the respective delay associated with the unit cell can be adjusted, e.g. by adjusting the number and / or length of the bars. Preferably the channel is closed on one end. For example, in preferred embodiments, as mentioned above, an individual module is open on at least one side (which is preferably the side on which acoustic waves are intended to impinge), but is closed on the opposite side. Fluid (e.g. air) or other ambient fluid can thus pass into the module, interacting with the unit cells at least through the at least one open side. Acoustic waves will primarily propagate through the fluid and so will follow the same path. The acoustic waves will thus preferably at least partly reflect from the closed side. Preferably, the unit cells (and the individual module as a whole) is closed on two sides and open on two sides. This also facilitates injection moulding. For example, in order to perform injection moulding of the modules, the module must of course be open on one side. Likewise, having the module closed at least on the opposite side defines a continuous element along the length of the module, which helps with the injection moulding. The form of the modules (and the unit cells thereof) may be further configured to facilitates their manufacture by injection moulding by providing tapered features and draft angles to facilitate removing the module from the mould. The individual unit cells within a module may therefore be slightly trapezoidal. The present inventors have recognised that this does not impact negatively on the acoustic properties. The present invention also extends to the individual modules themselves. Thus, a third aspect of the present invention relates to a module that comprises a respective arrangement of unit cells, the arrangement of unit cells within a module defining a corresponding arrangement of delays that the module will introduce to an incident acoustic wave. The modules are preferably configured for assembling into larger structures, as described above. Thus, the module preferably comprises, on its end faces, respective connecting parts that are configured to allow the module to connect end-to-end with another similar or identical module. Preferably the module is also configured to facilitate stacking similar modules on top of each other. As will be appreciated by those skilled in the art, the individual modules according to the third aspect of the present invention can, and preferably do, comprise any one or more or all of the preferred and optional features described above in relation to the first aspect, as appropriate. Thus, the modules preferably comprise strips, e.g. Nx1 arrangements, of unit cells, preferably designed as described above. The modules may be self-supporting to some degree such that a plurality of modules can be connected together into a standalone structure. However, as mentioned above, in order to make larger structures (e.g. panels), or to connect such modules or structures around a corner, for example, in embodiments there are also provided additional support elements that are designed with corresponding connecting parts as the individual modules so that the modules can also be connected with the support elements in the same way. The supporting elements may take any suitable and desired form. The supporting parts may be formed from plastic. Preferably, the supporting elements are also injection moulded. In some preferred embodiments the supporting elements comprise a through hole that is designed to receive a supporting rod (e.g. a metal rod), which may provide additional stability. Accordingly, when a plurality of modules are stacked together to form a two dimensional panel, with two or more supporting parts connected to the modules within the panel, a supporting rod may be inserted into the respective supporting parts to support the panel. This can further improve stability and strength of the modular structures formed in this manner. The overall structure may thus comprise one or more panels, each made up of a respective plurality of individual modules that are stacked on top of each other and / or connected end-to-end. The panels may then be interconnected with other panels, either end-to-end, or at various angles, preferably using such supporting elements. The structure may be self-supporting, e.g. in the form of a box, or enclosure. Alternatively, the structure may be connected into a supporting frame. It will be appreciated that various arrangements are possible for assembling the modules of the present invention into different structures and this is indeed another benefit of the present invention. According to the present invention, there may therefore be provided a kit of parts including a plurality of modules, preferably also a plurality of corresponding supporting elements, that can be (and are) assembled on demand into larger structures, as desired. Thus, the present invention provides a highly flexible and scalable approach for assembling devices for manipulating noise / sound, with individual modules that can be manufactured with relatively lower costs (e.g., and preferably, by injection moulding, as described above) and then assembled as desired into larger structures that can perform a desired acoustic control operation, whatever that might be. The present invention may therefore provide various benefits compared to other possible arrangements. Subject to the particular requirements of the present invention the structure may generally be assembled in any suitable and desired manner. For instance, subject to the particular requirements of the present invention, the structure (the modules), and the unit cells within the structure (modules), may be constructed and / or arranged according to any of the embodiments described in US- 10,873,812 and WO 2020 / 208380, both filed in the name of The University of Sussex, and the contents of which are incorporated herein in their entirety. The modules (and arrangement of unit cells within an individual module) are preferably fixed. In that case, the overall structure can be assembled and re- assembled as desired by connecting the modules in different arrangements. It is also contemplated however that at least some modules may be provided that allow a degree of re-configurability. It will be appreciated that such re-configurable modules may not be suitable for injection moulding, and this may complicate manufacture, but these may advantageously be used in combination with fixed modules, to provide a more dynamic structure. Various examples in this regard are described in US- 10,873,812 and WO 2020 / 208380. The structure may also be adjusted in other ways. For example, two or more structures formed according to the present invention may be used together, in which the structures can be selectively positioned, and re-positioned, in order to adjust the function of the device. In a similar fashion, the device may further comprise a masking element that is rotatable or otherwise movable relative to the arrangement of unit cells in order to adjust the function of the device. Various other arrangements would be possible in this regard. The modules, and generally the structure, may be formed from any suitable material as desired. In general, suitable materials may include, for example, metals, rubber, plastics, or wood. In general, any material may be used so long as it is suitable for the intended application of the device. Indeed, a benefit of the acoustic metamaterial approaches described herein is that the acoustic metamaterial cells can generally be fabricated from any suitable material since the noise reducing effect depends on the structure of the cells rather than the material properties per se. However, in preferred embodiments, where the modules are injection moulded, they are typically, and preferably, formed from plastics (thermoplastics), preferably fire- retardant and / or recyclable. In that respect, it will be appreciated that whilst preferred embodiments relate to injection moulding, and the modules are preferably designed to facilitate that, in general suitable modules may also be formed by additive manufacturing (e.g. three dimensional printing), or any other suitable technique, as desired. The (internal) surfaces of the acoustic metamaterial cells may be substantially smooth to minimise interactions between the flow and the surfaces of the acoustic metamaterial cells. However, in some cases, it may in fact be desirable to introduce a certain surface roughness at least to the surfaces of the acoustic metamaterial cells that define the tortuous flow path. In this respect, the inventors recognise that increasing the surface roughness will impact the flow (by changing the boundary conditions), and thus provides a further mechanism for controlling the interaction of the incident flow / sound with the acoustic metamaterial cells. For instance, by increasing the surface roughness, turbulent flow may be induced inside the acoustic metamaterial cells, which can further dissipate noise. The structure may in some embodiments comprise a standalone apparatus, e.g. a box or booth that is designed to provide a certain amount of acoustic (noise) control, but in embodiments the structure may also be integrated into a larger apparatus. Various arrangements are contemplated in this regard for using such structures. For example, such structures may advantageously find application in a wide range of situations in which improved acoustic control is desired, including, but not limited to, for loudspeaker design, household appliances, heating and ventilation systems, vehicles, etc.. Various other arrangements would be possible. Brief Description of the Drawings Various embodiments of the technology described herein will now be described by way of example only and with reference to the accompanying figures, in which: Figure 1A shows schematically a unit cell construction for introducing a phase delay to an incident acoustic wave, according to an embodiment of the present invention; Figure 1B illustrates how the unit cell construction shown in Figure 1A may be designed to introduce different phase delays to an incident acoustic wave; Figure 2A shows schematically a unit cell construction for introducing a phase delay to an incident acoustic wave and further including a closed end portion, according to an embodiment of the present invention; and Figure 2B shows schematically a unit cell construction for introducing no phase delay to an incident acoustic wave and further including a closed end portion, according to an embodiment of the present invention; Figure 3A shows a perspective view of a unit cell of the type shown in Figures 1A or 1A, according to an embodiment of the present invention; and Figure 3B shows a perspective view of a unit cell of the type shown in Figure 2B, or that of Figure 2B without a closed end portion, configured to introduce no phase delay, according to an embodiment of the present invention; Figure 4A shows a face-on view of a panel of an acoustic metasurface, according to an embodiment of the present invention; and Figure 4B shows a back view of the panel of Figure 4A; Figure 5A shows a face-on view of an acoustic metasurface formed from a pair of connected panels, according to an embodiment of the present invention; and Figure 5B shows a back view of the panel of Figure 5A; Figures 6A-6C show different perspective views of a module, according to an embodiment of the present invention; Figure 7A shows a perspective view of a female connecting part on an end face of a module, according to an embodiment of the present invention; Figure 7B shows a perspective view of a male connecting part on an end face of a module, according to an embodiment of the present invention; and Figure 7C shows a top- down view of two modules connected together end-to-end, according to an embodiment of the present invention; Figure 8A shows a perspective back view of a module stacked on top of another module, according to an embodiment of the present invention; Figure 8B shows a side on perspective view showing an end face of the stacked modules of Figure 8A; Figure 9A shows a perspective back view of a structure formed from the stacked modules of Figure 8A connected end-to-end with another pair of stacked modules, according to an embodiment of the present invention; Figure 9B shows a back view of the structure of Figure 9A; and Figure 9C shows a shows a face-on view of structure of Figure 9A; Figure 10A shows a cross-sectional view of the end-to-end connection between two modules taken along the line A-A in Figure 7C; Figure 10B shows a similar cross-sectional view taken along the line B-B in Figure 7C; Figure 10C shows a similar cross-sectional view taken along the line C-C in Figure 7C; Figure 10D shows a cross-sectional view of two stacked modules taken along the line D-D in Figure 9C; and Figure 10E shows a similar cross-sectional view of the end-to-end connection between two pairs of stacked modules taken along the line E-E in Figure 9C; Figure 11A shows a perspective view of a supporting part, according to an embodiment of the present invention; Figure 11B shows an opposing perspective view of the supporting part of Figure 11A; Figure 11C shows a perspective view of a supporting part connected on the end of a module, according to an embodiment of the present invention; and Figure 11D shows a perspective view of a supporting part connected on the end of a module, according to an embodiment of the present invention; Figure 12A shows a perspective back view of a pair of panels each formed from a pair of stacked modules connected end-to-end by a pair of stacked supporting parts, according to an embodiment of the present invention; and Figure 12B shows a perspective face-on view of the structure of Figure 12A; Figures 13A-13C shows different perspective views of a pair of stacked supporting parts on the end of a pair of stacked modules, having a supporting rod therethrough, according to an embodiment of the present invention; Figure 14A shows a top view of a an acoustic metasurface formed from three connected panels, according to an embodiment of the present invention; Figure 14B shows a back view of the structure of Figure 14A; Figure 14C shows a face-on view of the structure of Figure 14A; Figure 14D shows a bottom view of the structure of Figure 14A; Figure 14E shows a side view taken along the line E-E in Figure 14A; and Figure 14F shows a side view taken along the line F-F in Figure 14A; Figure 15A shows a perspective back view of a metasurface formed from three connected panels similar to that of Figures 14A-14F, according to an embodiment of the present invention; and Figure 15B shows a perspective front-view of the metasurface of Figure 15A; Figure 16A shows a supporting part which enables two modules to be connected at an angle to each other, according to an embodiment of the present invention; and Figure 16B shows a pair of modules connected at an angle to each other via the supporting part of Figure 16A, according to an embodiment of the present invention; Figure 17 shows a flow chart for a method of manufacture, according to an embodiment of the present invention. Like reference signs are used to denote like components in the figures. Detailed Description The concepts described herein generally relate to approaches for spatially manipulating sound using acoustic metamaterials. Thus, in embodiments a device for manipulating acoustic waves (hereinafter, a “sound modulation device”) may be provided. In particular, a plurality of unit cells each capable of encoding a particular time or phase delay, or plurality of time or phase delays, are arranged together in an array in order to construct an acoustic “metasurface” (or, metamaterial ”panel”). The time delay or phase distribution of the acoustic metasurface may thus be quantised in the spatial domain according to the positions and sizes of the unit cells. The spatial distribution of the time or phase delays across the acoustic metasurface generally determines how an acoustic wave incident on the metasurface will be transformed or manipulated as it passes through and interacts with the unit cells of the metasurface. The arrangement of unit cells within the metasurface may be configured for performing various different acoustic transformations or manipulations. In general, the concepts described herein may be used to perform various different types of acoustic control. However, various non-limiting examples and embodiments will now be described in the context of noise reduction to help illustrate these concepts. In particular, according to the present embodiments, the device comprises a plurality of unit cells that are each pre-configured to encode a particular, fixed time delay. The unit cells effectively therefore become, in isolation, the building blocks of the acoustic metamaterial panels or metasurfaces. The unit cells 22 may take various suitable forms so long as they act to introduce a certain time delay to an incident acoustic wave. Generally, the unit cells 22 may be designed to introduce a local phase shift at least within the range 0 to 2π for a selected operating frequency. In order to form a desired acoustic wave with the required accuracy, and in order to avoid spatial aliasing effects, the unit cells 22 desirably hold sub-wavelength resolution. The unit cells 22 may also be able to transmit sound effectively with minimal energy losses, particularly where it is desired to stack a plurality of metasurfaces (e.g. such that the unit cells of adjacent metasurfaces are lined up together). For instance, in embodiments, the unit cells 22 may define a central channel through which acoustic waves pass from one side of the unit cell to the other. The central channel may further comprise various sub-wavelength structures or features that act to slow down the acoustic waves and / or increase the effective path length travelled by the acoustic waves through the channel thereby introducing a phase delay. For example, the channels may include a substantially labyrinthine, or meandered, structure, or a multi-slit, coil, helical, or Helmholtz resonator-type structure. One suitable structure is illustrated by way of example in Figure 1A which shows in cross section an example of a labyrinth structure with meanders defined by four bars 31 extending into an open channel. The effective path length, Leff, for acoustic waves travelling through the unit cell is given by Leff = h + ΔL, where h is the height on the unit cell, and ΔL is the additional path length introduced by the structure of the unit cell. This additional path length introduces a phase delay φ = eik.Leff, where k is the wavenumber (k = 2π / λ) of the acoustic wave. The shape and / or dimensions of the unit cells may generally be selected to introduce a desired phase delay for acoustic waves of a particular wavelength. That is, the design of unit cells may be substantially optimised or configured for use with a particular operating wavelength, such that a desired phase delay is provided for incident acoustic waves at the operating wavelength, λ0. In embodiments, the device may be designed for use substantially only at a single operating wavelength, such that there is little or no response or transmission at other wavelengths. In other embodiments it is contemplated that the device may be designed for use with a range of wavelengths, such as a range of wavelengths around a central operating wavelength. It is also contemplated that the device may be configured to operate at a number of different operating wavelengths. Figure 1B illustrates how the exemplary unit cell construction shown in Figure 1A may be designed to encode a range of different phase delays. The unit cells shown in Figure 1B are generally in the form of a rectangular cuboid with a square base shape of side λ0 / 2 and height of λ0. Thus, the unit cells allow the acoustic metamaterial layers to be quantised with a resolution of λ0 / 2. This may be a good compromise between ease-of-manufacture and the need to realise diffraction-limited fields without spatial aliasing. Indeed, it has been found that it may be advantageous to keep the size of the unit cells (in the plane of the metamaterial layers) smaller than the wavelength corresponding to the Nyquist frequency. Thus, when designing a device that is optimised or configured for use at an operating wavelength, λ0, the unit cells may suitably have a dimension of λ0 / 2, or smaller. As shown in Figure 1B, and as mentioned above in relation to Figure 1A, the unit cells each comprise an open central channel having a structure that delays the incident wave, hence shifting the relative phase of the output. In particular, the open central channel is provided with a labyrinthine or meandered structure by a plurality of bars 31 extending into the channel. The length of, bl, and spacing between, bs, the meanders may then be varied in order to provide a range of effective path lengths as shown in Figure 1B. In Figure 1B, the thickness of the walls relative to the configured operating wavelength, λ0, is λ0 / 40 and the thickness of the meanders is λ0 / 20. However, these values may be selected as desired e.g. to achieve a desired strength or robustness, or based on manufacturing constraints. As shown in Figures 2A and 2B, and also explained in more detail below, some unit cells may not be open, but rather the channel may be closed at one end, i.e. by a closed end portion 23, which may cause at least a portion of the incident acoustic wave to be reflected, and wherein a portion of the incident acoustic wave may also be transmitted from the closed end 23 and / or a portion of the incident acoustic wave may be dispersed / dissipated through the material of the closed end 23. Figure 2A corresponds to the unit cell of Figure 1A but having a closed end portion 23. In contrast, Figure 2B shows a cross section of a unit cell which does not include a labyrinth structure with meanders defined by bars 31 (i.e., the unit cell of Figure 2B has a direct channel structure without any bars provided extending therein), and therefore provides substantially no phase delay given that the structure does not provide any additional path length. As shown, the unit cell Figure 2B also includes a closed end portion 23 (however it will be appreciated that other unit cells having a direct channel structure may be provided without the closed end portion). Figure 3A shows a perspective view of an example of a unit cell of the type shown in Figure 1A or 2A (i.e. with or without a closed end portion, respectively) that is pre-configured to encode a phase delay of 5π / 8 for an operating wavelength λ0. Figure 3B shows a perspective view of a pre-configured unit cell of the type shown in Figure 2B that is pre-configured to encode no phase delay. As will be appreciated, by varying the length and spacing of the bars 31 (or indeed the number of bars 31) in a unit cell allows the phase delay to be adjusted, and further by including a closed end portion 23 or not as appropriate present a further means for varying the acoustic operation of the unit cell. Accordingly, by selecting different unit cells as appropriate, practically any phase distribution may be introduced for manipulating an incoming wave incident on an array of the unit cells. As best shown in Figure 1B, the base portions of the bars 31 defining the meanders may have ‘shoulders’ such that they gradually taper into the channel to the desired end thickness (e.g. λ0 / 20). These ‘shoulders’ may help to increase robustness and stability during manufacture and / or may help contribute to impedance matching. In particular, the geometry of the unit cells may be selected so that the effective acoustic impedance of each unit cell is matched to that of the ambient medium within which the device is operating (e.g. air or water), thereby increasing the efficiency of transmission (and suppressing reflection). It is emphasised again that Figures 1 to 3 merely illustrate certain examples of suitable unit cells for introducing a time delay, and that the unit cells may generally take various forms including, but not limited to, other types of labyrinthine or meandered structures, multi-slit, helical or coiled structures, or Helmholtz resonator-type structures. The arrangement of the unit cells within an acoustic metasurface will determine how an acoustic wave incident on, and passing through, the acoustic metasurface will be manipulated. Thus, it is possible to design a vast range of acoustic metasurfaces that are arranged to perform various different acoustic manipulations. Thus, whilst various embodiments will now be described in the context of noise control (reduction), it will be appreciated that the techniques described herein may generally be used for any other desired acoustic control. Accordingly, an acoustic metasurface may be configured to provide a reduction in intensity for incident acoustic waves, i.e. to provide a noise cancelling (or reduction) operation. This could be realised, for example, as generally shown in Figures 4A and 4B, by providing an acoustic metasurface 21 having an alternating checkerboard pattern of unit cells 22a, 22b designed to introduce phase delays of 0 and ∆. For instance, by selecting a unit cell structure such that ∆=π, substantially total destructive interference may be achieved. An acoustic wave encountering these phase delays may then have its intensity reduced as a result of the interference between the components passing through the different unit cells. Moreover, as will be understood, the inventors have further discovered that for substantial noise reduction, ∆ need not necessarily be set to precisely π, depending, e.g., on various factors such as whether some or all of the unit cells 22a,22b are provided with a closed end portion 23. In particular, in Figures 4A and 4B, the unit cells 22a and 22b may correspond to the unit cells similar to those shown in Figures 3A and 3B respectively, but with one end 23 of the channel closed (i.e. both having closed end portions, as discussed above in relation of Figures 2A and 2B). That is, Figure 4A shows a face- on view of the open end of the channels, whereas Figure 4B shows a back view (i.e. such that the surface(s) of the closed ends 23 facing away from the unit cells is visible). It will be appreciated of course that the unit cell 22a need not have the channel which corresponds precisely to that shown in Figure 3A, but rather may be chosen to be truncated to be, e.g., λ0 / 2 or λ0 / 3 etc. in height as compared to λ0 in Figure 3A. This is shown for instance in the unit cells of Figures 6-14. In an earlier application it was contemplated that such individual unit cells may be interconnected together, or inserted into a frame, to form a two-dimensional array or metamaterial layer. The present embodiments relate to improved, e.g. cheaper, more efficient, techniques for manufacturing larger structures incorporating similar unit cells. In particular, the acoustic metasurface 21 of Figures 4A and 4B is itself composed of modules 24 of unit cells 22, wherein in Figures 4A and 4B each module 24 is an N-by-1 strip of unit cells. For instance, in Figure 4A, each module 24 is a 9- by-1 strip of unit cells 22. As shown, the modules 24 of Figures 4A and 4B are stackable on top of each other to define a two-dimensional panel 30 which constitutes the metasurface 21. As will be apparent, Figure 4A shows two distinct modules 24, labelled as ‘ABAB’ and ‘BABA’, wherein the relative positions of the different unit cells 22a and 22b are interchanged between the two distinct modules 24. As such, inter-changeably stacking on top of each other an ‘ABAB’ module 24 and a ‘BABA’ module 24 results in the checkboard metasurface 21 as shown. Figure 4A shows a front face of acoustic metasurface 21 wherein the unit cells 22 are open to allow fluid to flow into the unit cells 22, whereas Figure 4B shows the opposing rear face of acoustic metasurface 21 wherein the unit cells 22 are closed to prevent fluid flow through the rear face of the acoustic metasurface 21. As shown in Figures 5A and 5B, each individual module 24 is configured to be connected end-on-end with another module 24 via connecting parts 25 provided respectively on the two end faces. That is, the connecting parts 25 on one module 24 is designed to connect with the connecting parts 25 on another module 24. In this way, two panels 30 may be connected end-to-end to thereby constitute the larger metasurface 21. As will be appreciated, connecting parts 25 may be configured such that any connecting part 25 of a given module 24 may connect with the connecting part of any other (different) module 24, which enables versatility in the relative arrangement of modules 24 to form any panel 30 which is able to be connected to any other panel 30 (e.g., with a unique and different relative arrangement of modules 24 within that panel 30). Alternatively, a particular connecting part 25 may be configured to only connect or mate with a corresponding connecting part, such as male- and female-connecting parts 25. This may be advantageous when assembling a specific arrangement of modules 24. For instance, in Figure 4A, male connecting parts 25 may be provided at the end faces of the ‘ABAB’ module 24 and corresponding female connecting parts 25 may be provided at the end faces of the ‘BABA’ module 24, such that when assembling the modules 24 to form two connected panels 30 as in Figures 5A and 5B, a user is only able to connect them in the desired checkboard arrangement of unit cells 22. Figures 6A-6C show various perspectives of a particular exemplary ‘ABAB’ module 24. The module 24 is thus generally rectangular, having a male connecting part 25a at one end-face, a female connecting part 25b at the other opposite end- face (as shown in Figure 6C), and with four longitudinal faces or planes therebetween. A first longitudinal surface 26 (i.e. the ‘rear’ surface) comprises the closed-end portions 23 of the unit cells, as shown in Figures 6B and 6C. A second longitudinal surface 27 (i.e. the ‘bottom’ surface) is provided adjacent to the rear surface 26 such that they together provide a longitudinal L-shape. The bottom surface 27 comprises adjacent (and in-plane) bottom -walls 28 of the unit cells, as shown in Figures 6A and 6B. In contrast, the remainder of the module 24 is open. That is, the ends 29 of the internal structures such as the side-walls between adjacent unit cells 22a,22b and ends of the bottom-walls 28 (i.e. the edge of the longitudinal face 27) together define openings to the unit cells 22a,22b, and all lie in the same longitudinal plane (i.e. the ‘front’ face opposite to the rear surface 26), as shown in Figure 6A. Furthermore, the remaining side of the unit cells 22a,22b is left open, such that edges of the bars 31 and edges 32 of side-walls between adjacent unit cells 22a,22b lie in the same longitudinal plane (i.e. the ‘top’ face opposite to the bottom surface 27), as shown in Figure 6C. Thus, the longitudinal faces 27, 28 and their respective opposite longitudinal planes, and two-ends faces define the substantially rectangular shape. In this way, by stacking modules each having such a longitudinal L-shape, a structure like that in Figure 4A,4B may be constructed. A top- piece or plank may be provided so as to complete the unit cells of the upper-most module 24 (such that a single opening is provided into the unit cell, or a pair of openings on opposite sides of the unit cell is provided). In Figure 6B, longitudinal face 27 is provided with grooves 33 (i.e. on the surface of longitudinal face 27 facing away from the channel of the unit cells). These grooves 32 correspond to, and cooperate with, the edges of the bars 31 and edges 32 of side-walls between adjacent unit cells 22a,22b of another module 24 to be stacked on-top. The cooperation between the grooves 33 and edges of stacked modules 24 reduces fluid leakage therebetween (e.g. air flow), such that the acoustic properties of the overall panel 30 constructed from the modules 24 has minimal or substantially no difference to a corresponding panel which is integrally formed (e.g. by 3D printing the entire panel 30). Figure 7A shows the female connecting part 25b on an end face of the module 24 of Figures 6A-6C, and Figure 7B shows the male connecting part 25a on the opposite end face of that module 24. As shown, the male connecting part 25a is formed of a protrusion having a pair of tapered recesses 34 on opposing sides of the protrusion. The female connecting part 25b accordingly includes a pair of receiving arms 35 forming a receiving volume therebetween configured to cooperate or mate with the male connecting part 25a, wherein the receiving arms 35 correspond to the tapered recesses 34 and are configured to engage with the recesses 34 when the two connecting parts 25a,25b are connected. Figure 7C shows a top-down view of two modules 24 connected together end-to-end. The dashed arrow in Figure 7B shows the direction in which the material forming the internal structures of the module 24 narrows (e.g., the separating wall between adjacent unit cells). All of the internal structures narrow in this direction i.e. towards open top face. This facilitates stacking the module 24 in the corresponding groove 33 of an adjacent (stacked) module, and further simultaneously helps with removing the module 24 from a mould after fabrication in an injection moulding manufacture process. As will be understood, in an alternative embodiment, a pair of stacked Nx1 modules 24 may be provided as an integral piece (e.g. in a back-to-back manner with unit cells defined either side of the second longitudinal surface 27 which forms a central longitudinal surface of the integral pair of stacked Nx1 modules). In this embodiment, a separate longitudinal plank is provided for each integral pair of stacked Nx1 modules 24. Accordingly, a modular structured metasurface 21 of unit cells as described herein may thus be formed by stacking integral pairs of stacked Nx1 modules, separated by the longitudinal planks. Figure 8A shows a perspective back view of a ‘BABA’ module 24 stacked on top of an ‘ABAB’ module 24. Figure 8B shows a side on perspective view the stacked modules 24 of Figure 8A. As shown in Figures 7A, 7B and 8B, the connecting parts 25 are offset from the centre line of the respective end face. The inventors have recognised that by off-setting the connecting parts 25 in this manner may facilitate fitting the panels 30 into an external frame. In this embodiment, like connecting parts 25 are provided so as to be on the same side of the panel when the modules 24 are stacked, as best seen in Figure 8B wherein the male connecting parts are visible on the same side of the stacked modules 24. However, as will be appreciated, the type of connecting part 25 may alternate between male 25a and female 25b as modules are stacked on top of each other, as discussed above in relation to Figures 5A-5B. As best seen in Figure 8B, an edge of the first longitudinal face 26 includes a longitudinal lip 36. The present inventors have recognised that this lip 36 may act as an acoustic baffle. That is, in a similar manner to the grooves 33 discussed above in relation to Figure 6B, the lip 36 may reduce fluid leakage between the stacked modules 34 (e.g. air flow), thereby improving the acoustic behaviour of the panel 30. Returning again to Figures 7A and 7B, the male 25a and female 25b connecting parts respectively include overhangs 53a and 53b. The male overhang 53a cooperates with a respective recess in a male connecting part 25a of an adjacent stacked module 24 as shown in Figure 8B, as well as being configured to fit within and project upwards through the female connecting part 25b when connected- end-to-end with another module 24. The female overhang 53b cooperates with a respective recess in a female connecting part 25b of an adjacent stacked module 24 as shown in Figure 8A, as well as being configured to fit around project downwards of the male connecting part 25a when connected-end-to-end with another module 24. These overhangs 53a,53b accordingly help provide stability by resisting the longitudinal movement of a stacked module relative to the module immediately below, as well as ensuring there is minimal or no air-flow through the connecting parts in a similar manner as the grooves 33 and lip 36 as discussed above. Figure 9A shows a perspective back view of a structure formed from the ‘BABA’ module 24 stacked on top of the ‘ABAB’ module 24 of Figure 8A connected end-to-end with an ‘ABAB’ module 24 stacked on top of a ‘BABA’ module, so as to form the start of a metasurface 21 similar to that of Figure 5A-5B formed from two panels 30 connected together. Figure 9B shows a back view of the structure of Figure 9A, and Figure 9C shows a shows a face-on view of structure of Figure 9A such that the open ends of the channels of the unit cells 22a,22b are visible. Figure 10A is a cross-sectional view of the end-to-end connection between two modules 24 taken along the line A-A in Figure 7C. Figure 10B is a similar cross- sectional view taken along the line B-B in Figure 7C, whereas Figure 10C is a similar cross-sectional view taken along the line C-C in Figure 7C. Figure 10D shows a cross-sectional view of two stacked modules taken along the line D-D in Figure 9C, whereas Figure 10E shows a cross-sectional view of the end-to-end connection between two pairs of stacked modules 24 taken along the line E-E in Figure 9C. Figure 11A shows a perspective view of a supporting part 37 and Figure 11B shows an opposing perspective view of the supporting part 37. The supporting part has a male connecting part 25a on one side and a female connecting part 25b on the other, opposite side. Accordingly, the supporting part 37 can be connected to a corresponding connecting part at the end of a module 24, as shown in Figures 11C and 11D. As will be appreciated, a supporting part 37 may instead have only male connecting parts 25a or only female connecting parts 25b. The supporting part 37 has a hole 38 through it, wherein a rod 52 (e.g. a metal rod, such as an aluminium rod) can be passed through the hole 38 of the supporting part 37 so as to provide a rigid support to the metasurface 21 so formed. That is, as will be appreciated and described in more detail below, a number of supporting parts 37 may be used to provide support to a metasurface 21 built from panels 30. For instance, supporting parts 37 can be provided on the outer ends of panels 30 (e.g., on the outer most end face of each of the modules forming the panel 30) to thereby form a support frame. Optionally or alternatively, the supporting parts may be provided in-between panels 30 to connect the panels and provide a central support, as shown in Figures 12A-12B. A pair of stacked supporting parts 37 are also shown at the end of one of the panels in Figures 12A-12B. The rod 52 passed through the hole 38 is shown in particular in Figures 13A- 13C, which show various perspective views of the pair of stacked supporting parts at the end of one of the panels 30 of Figures 12A-12B. The rod 52 may advantageously provide a means for attaching or fixing the panel 30 or outer-most panel(s) of metasurface 21 formed of one or more panels 30 in place. For instance, the end portion of the rod 52 which projects beyond the panel 30 or the outer-most panel(s) of metasurface 21 may be inserted into a corresponding recess within an exterior base or frame (not shown). As shown, the end portion of the rod 52 in Figures 13A- 13C is threaded so as to be able to screw into a corresponding recess, which may further provide means for fixing the panel 30 or metasurface 21 in place. Indeed, as shown in Figures 13A-13C, substantially the entire length of rod 52 is threaded. By providing a kit of parts comprising modules 24 and supporting parts 37, a larger metasurface 21 may be constructed from a number of panels 30 which is structurally self-supporting. For instance, Figures 14A-14F and 15A-15B show an example of a metasurface 31 formed from three connected panels 30, which is supported via connecting parts 37 on the outermost side edges of the metasurface 21. Figure 14A shows a top view of metasurface 21, Figure 14B shows a back view, Figure 14C shows a face-on view, Figure 14D shows a bottom view, Figure 14E shows a side view taken along the line E-E in Figure 14A, and Figure 14F shows a side view taken along the line F-F in Figure 14A. Figure 15A shows a perspective back-view of a metasurface 21 formed from three connected panels similar to that of Figures 14A-14F. Figure 15B shows a perspective front-view of the metasurface 21 of Figure 15A. The present inventors have discovered that providing a metasurface 21 formed from such a modular structure enables structures to be easily and simply constructed which both cover a large surface area and are well supported. As will be appreciated, the metasurface 21 formed from one or more panels 30 as described above need not have a checkboard arrangement of unit cells. Rather, the above can be applied to construct any desired array of unit cells, e.g. any of those described in US-10,873,812 and WO 2020 / 208380 (both filed in the name of The University of Sussex). As will also be appreciated, the above assembly can be applied to create a metasurface 21 having a shape other than planar. For instance, Figure 16A shows a supporting part 37 which enables two modules 24 to be connected at 90 degrees to each other. That is, the supporting part has a male connecting part 25a orientated at an angle of 90 degrees to a female connecting part 25b, In this way, two panels 30 can be connected at 90 degrees to each other, as shown in Figure 16B, wherein a pair of modules 24 are connected via the angled supporting part 37 at 90 degrees to each other. Thus, a box shape with a module forming each side of the box may be constructed. As will be appreciated, angled supporting parts 37 may be provided with turns at any angle, such that a variety of different shapes can be constructed from panels 30 connected at corresponding angles. The above modular construction for metasurface 21 is particularly suitable for fabrication via mould-making techniques such as injection moulding or die casting. Accordingly, with reference to Figure 17, a method of manufacture includes the following steps: At step 40, an array of unit cells for a metasurface is selected or designed. At step 41, the array is truncated into modules for fabrication, such as N- by-1 strips of unit cells, or larger blocks such as M-by-N unit cells. As will be appreciated, a module may include e.g. two strips of N-by-1 which are to be separated via cutting after the module has been fabricated to provide separate N-by- 1 strips. At step 42, moulds are created for the modules (e.g. negatives). At step 43, the moulds are then used in a fabrication process such as injection moulding (particularly suitable for forming the modules out of plastic), cast moulding, or die casting (particularly suitable for forming the modules out of metals). At step 44, the modules are removed from the moulds, and at step 45, the metasurface is assembled by stacking and / or connecting the modules. As will be appreciated, the supporting parts described above may also be fabricated via injection moulding, cast moulding, or die cast the modules. The foregoing detailed description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the technology to the precise form disclosed. Many modifications and variations are possible in the light of the above teaching. The described embodiments were chosen in order to best explain the principles of the technology and its practical application, to thereby enable others skilled in the art to best utilise the technology in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope be defined by the claims appended hereto.

Claims

163673 / 01 Claims; 1. A modular kit for producing structures for controlling acoustic waves, wherein the kit comprises: a plurality of modules that can be connected together to define an overall form of the structure, wherein respective modules of the plurality of modules comprise respective arrangements of unit cells, wherein the structure of a unit cell determines how an incident acoustic wave will locally interact with the module at the respective position of the unit cell, the arrangement of unit cells within a module thus defining a corresponding arrangement of local interactions, such that when multiples ones of the plurality of modules within the kit are connected together to define the structure, the distribution of local interactions for the overall structure controls the propagation of acoustic waves interacting with the structure.

2. The modular kit of claim 1, wherein at least one module is provided in the form of an Nx1 strip of unit cells.

3. The modular kit of claim 1 or 2, comprising at least first and second, different types of modules that can be connected together to define the overall form of the structure, wherein each of the first and second types of modules are provided in the form of an Nx1 strip of unit cells, wherein N may be the same or different for the first and second types of modules, and wherein the first and second types of modules can be stacked together to define a panel, the panel thereby comprising a two dimensional array of unit cells, wherein the first and second types of modules comprise respective different arrangements of unit cells, and wherein the respective arrangement of unit cells for the first and second types of modules are configured such that the thus formed panel is configured to perform a desired acoustic control operation at least for acoustic waves that are incident substantially normally to the panel.

4. The modular kit of claim 3, wherein for each of the modules of the first and second type, the respective unit cells of the module are open on a first, front face of the module to allow fluid to pass into the unit cells of the module through the first face, whereas the unit cells of the module are closed on a second, rear face of themodule, such that an acoustic wave that is incident on the first face of the module will pass into a respective channel provided by a respective unit cell of the module, and then impinge on the closed second face of the module.

5. The modular kit of claim 4, wherein the modules of the first and second different types of modules are configured such that when a module of the first type is stacked on top of a module of the second type as part of a panel, a substantially closed rear surface is provided to prevent direct flow of fluid through the rear surface of the panel.

6. The modular kit of claim 1 or 2, wherein the plurality of modules includes a plurality of modules of the same type can be connected together to define the overall form of the structure, wherein the modules can be stacked together to define a panel, the panel thereby comprising a two dimensional array of unit cells, and wherein the respective arrangement of unit cells for the modules are configured such that the thus formed panel is configured to perform a desired acoustic control operation at least for acoustic waves that are incident substantially normally to the panel.

7. The modular kit of claims 3 or 6, wherein for each module, the respective unit cells of the module are open on a first, front face of the module to allow fluid to pass into the unit cells of the module through the first face, and wherein at least one of the unit cells comprise an opening on a second, rear face of the module.

8. The modular kit of any preceding claim, wherein at least some of the modules in the plurality of modules, and preferably each of the modules in the plurality of modules, are provided with a male connecting part on a first end face of the module and are further provided with a corresponding female connecting part on a second, opposite end face of the module such that the modules can be connected end-to- end.

9. The modular kit of any preceding claim, further comprising a corresponding one or supporting parts that are configured to: (i) connect between individual modules; and / or (ii) connect an individual module with a supporting structure.

10. The modular kit of claim 9, comprising at least some supporting parts that are configured to connect individual modules at an angle .

11. The modular kit of claim 9 or 10, wherein the supporting parts are provided with a through hole configured to receive a supporting rod, such that when a plurality of modules are stacked together to form a two dimensional panel, with two or more supporting parts connected to the modules within the panel, a supporting rod can be inserted into the respective supporting parts to support the panel.

12. A structure for controlling acoustic waves, wherein the structure comprises a plurality of modules that are connected together to define an overall form of the structure, wherein respective modules of the plurality of modules comprise respective arrangements of unit cells, wherein the structure of a unit cell determines how an incident acoustic wave will locally interact with the module at the respective position of the unit cell, the arrangement of unit cells within a module thus defining a corresponding arrangement of local interactions, such that the distribution of local interactions for the overall structure controls the propagation of acoustic waves interacting with the structure.

13. The structure of claim 12, wherein at least one module is provided in the form of an Nx1 strip of unit cells.

14. The structure of claim 12 or 13, comprising one or more panels, wherein each panel comprises at least first and second, different types of modules that are stacked on top of each other in an alternating manner to define the panel, wherein each of the first and second types of modules are provided in the form of an Nx1 strip of unit cells, wherein N may be the same or different for the first and second types of modules, the panel thereby comprising a two dimensional array of unit cells, wherein the first and second types of modules comprise respective different arrangements of unit cells, and wherein the respective arrangement of unit cells for the first and second types of modules are configured such that the thus formed panel is configured to perform a desired acoustic control operation at least for acoustic waves that are incident substantially normally to the panel.

15. The structure of claim 14, wherein the unit cells of each module are open on a first, front face of the module to allow fluid to pass into the unit cells of the respective module through the first face, whereas the unit cells of the each module are closed on a second, rear face of the module, such that an acoustic wave that is incident on the first face of the respective module will pass into a respective channel provided bya respective unit cell of the respective, and then impinge on the closed second face of the respective module.

16. The structure of claim 15, wherein said at least first and second, different types of modules are stacked on top of each other so as to provide a substantially closed rear surface of the respective panel, wherein the closed rear surface prevents direct flow of fluid through the rear surface of the panel.

17. The structure of any of claims 14-16, wherein the structure comprises a plurality of panels, the structure further comprising one or more supporting parts, the one or more supporting parts being connected between respective modules of the first and second panels in order to connect the first panel to the second panel, wherein the one or more supporting parts either connect the first panel to the second panel end-to-end or connect the first panel to the second panel at an angle.

18. The structure of claims 14-17, comprising two or more supporting parts each being provided with a through hole, wherein said two or more supporting parts are connected to the modules within a panel of the one or more panels, wherein the structure comprises a supporting rod inserted through the through hole of the two or more supporting parts so as to: (i) support the panel; and / or (ii) connect the panel into an external frame.

19. The modular kit of any of claims 3-5 or claims 8-11 when dependent on claim 3, or the structure of any of claims 14-19, wherein the respective arrangement of unit cells for the first type of module comprises a first pattern of unit cells, and wherein the respective arrangement of unit cells for the second type of module comprises a second, complementary pattern of unit cells such that when the modules are stacked together to form a panel, the distribution of delays for the panel is configured to provide noise reduction, at least at one or more target operating frequencies.

20. The modular kit or structure of any preceding claim, wherein at least one module within the structure, and preferably a plurality of, preferably each of, the modules within the structure, are injection moulded.

21. An individual module for use as part of a modular kit or structure as claimed in any preceding claim.

22. A module that defines a respective Nx1 strip of unit cells, wherein the arrangement of unit cells within the strip defines a corresponding arrangement of delays that the module will introduce to an incident acoustic wave impinging on the module, and wherein the module is designed to connect with other similar modules to define a larger structure.

23. The module of claim 22, wherein the unit cells are open on a front face of the module to allow fluid to flow into the unit cells of the module through the first face, and wherein the unit cells are closed on the opposite, rear face of the module, such that an acoustic wave incident on the first face will pass into a respective channel provided by a respective unit cell of the module, and then impinge on the closed second face of the module.

24. The module of claim 23, comprising a substantially continuous, closed bottom surface and a substantially open top face to facilitate stacking of the module on top of another similar module, such that, when stacked, the bottom surface of the module stacked on top is positioned along the open top face of the below module, such that the bottom surface of the module stacked on top acts as a closing top surface for unit cells of the below module.

25. The module of claim 24, comprising a lip along an edge of the closed rear face of the module, wherein, when the module is stacked on top of another similar module, the lip is configured to sit over an adjacent edge of the closed read face of the below module so as to form a closed rear surface.

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