Field of invention

The present invention relates to a consumer device such as an ear covering, which uses and/or comprises acoustic metamaterials.

Background

Noise control is a ubiquitous aspect of modern society. Noise control, also called sound attenuation, may be sought for safety reasons, e.g. to protect a person’s hearing, or for comfort reasons, for example to reduce unwanted sounds to a user, or for environmental or legal compliance reasons. Acoustic noise is an unresolved engineering challenge, that has a negative effect on the population health and wellbeing. Portable devices such as headphones or ear mufflers tackle this problem mainly by using active noise cancellation and/or passive noise control such as foams.

Active noise control, also known as active noise reduction or active noise cancellation, relies on the emission of a sound wave designed to interfere with and thereby cancel a particular incoming sound wave. Thus, active noise control requires the use of a power source.

Passive noise control reduces noise through the use of noise-isolating materials such as insulation, sound-absorbing tiles, foam, or the like. According to the mass-law, noise attenuation is proportional to wall thickness, frequency and density.

Active noise cancellation is most effective for low frequencies - generally up to 500 Hz - which typically relates to steady noise such as traffic noise on roads and airplanes, and is based on the waves interference principle. For higher frequencies, the spacing requirements for free space and zone of silence techniques become prohibitive. In acoustic cavity and duct-based systems, the number of nodes grows rapidly with increasing frequency, which quickly makes active noise control techniques unmanageable.

Passive treatments become more effective at higher frequencies and often provide an adequate solution without the need for active control. Passive noise control, such as foams, works by absorbing the incoming external sound waves and, at the same time, they avoid the reflection of waves (for example, in headphones, between a speaker and the eardrum). The absorption property of foams does not provide complete acoustic insulation, since - according to the mass-law - the transmission of sound is inversely proportional to the thickness, density and frequency of the wave. Acoustic metamaterials have emerged over the past decade, as a technology capable of attenuating and controlling sound waves in new ways. A metamaterial is an assembly of multiple individual elements - called “unit cells” or “meta-atoms” - that in the bulk exhibit properties not found in nature, such as negative bulk modulus or negative density. When these properties are negative, a frequency band called “bandgap” is formed, where sound is deeply attenuated. The meta-atoms are usually periodic, and their attenuation properties are a by-product of their acoustic resonance. Acoustic metamaterials typically consist of several arrays of unit cells. Unit cells in the same array must be identical, to leverage periodicity, while unit cells in different arrays can vary in type, geometry or material.

A particular problem is the attenuation of noise in small-scale electroacoustic or consumer device technology. Typically, active noise cancellation is capable of attenuating noise by detecting the external sound field with a microphone and by generating an antiphase signal. In terms of passive noise control, various types of foams are typically used to attenuate noise in electroacoustic devices by leveraging their density according to the mass law.

It is an object of at least one embodiment of at least one aspect of the present invention to alleviate and/or mitigate one or more problems or disadvantages associated with the prior art.

Summary

According to a first aspect there is provided an acoustic apparatus comprising: a housing comprising at least one opening in a wall thereof; and a metamaterial structure provided within the housing.

Advantageously, the acoustic apparatus may be configured to provide sound attenuation in the housing in a frequency range of about 300 to about 3000 Hz, e.g. about 500 to about 3000 Hz, e.g. about 1000 to about 3000 Hz. The acoustic apparatus may be configured to provide sound attenuation in the housing in a frequency range corresponding to human speech.

The housing may be configured to be fitted over or around a user’s ear. The acoustic apparatus may be or may comprise an ear covering. Typically, the housing may have a generally cup-like shape arranged to fit over or around a user’s ear.

The housing may comprise one or more walls defining a chamber configured to encapsulate and/or surround a user’s ear. The metamaterial structure may be provided within the chamber of the housing. The housing may define an open end or open side configured, in use, to face a user’s ear.

The acoustic apparatus may comprise a contacting element configured to contact and/or engage with a user’s body portion such as a skull and/or face, typically around a user’s ear.

The contacting element may define the open end or open side.

The contacting element may form part of or may be associated with or may be attached to the housing, e.g. at an open end or open side thereof.

The contacting element may be unitary with the housing, e.g. may be for part of the housing and/or may define the open end or open side thereof.

The contacting element may be attached to the housing at an open end or open side thereof. The contacting element may comprise padding configured to minimise noise permeation at the interface between the contacting element and the user.

The acoustic apparatus may be or may comprise an ear muffler, e.g. earmuffs or ear defenders.

The acoustic apparatus may be or may comprise a wearable electroacoustic device, e.g. headphones. In such instance, the apparatus may further comprise one or more electroacoustic transducers, e.g. loudspeakers, in one or more housings.

The acoustic apparatus may be provided as a pair, i.e., there may be provided a pair of the acoustic apparatus, e.g. a pair of housings, for example one for each of a user’s ears.

The acoustic apparatus may comprise an attachment portion configured to secure the acoustic apparatus, e.g. housings thereof, in place, typically configured to secure the apparatus on or around a user’s head. The attachment portion may comprise a band, strap, wire(s), or the like. Typically, the attachment portion may be adjustable to fit a geometry, e.g. head geometry, of the user.

The pair of acoustic apparatus, e.g. housings thereof, may be connected to each other. Typically, the pair of acoustic apparatus, e.g. housings thereof, may be connected by the attachment portion.

The housing may have a cross-section of any suitable shape, such as generally elliptical, circular, square, or rectangular.

Conveniently, the housing may have a generally elliptical shape in crosssection. The housing may have a height of about 60-120 mm, e.g. about 70-110 mm, e.g. about 80-100 mm, e.g. approximately 90 mm.

The housing may have a width of about 30-70 mm, e.g. about 40-60 mm, e.g. about 45-55 mm, e.g. approximately 49 mm.

The housing may have a depth of about 40-100 mm, e.g. about 50-90mm, e.g. about 60-80 mm, e.g. approximately 73 mm.

The housing may have a height of about 80-100 mm, a width of about 40- 60mm, and a depth of about 60-80mm.

The metamaterial structure may comprise an acoustic metamaterial structure. The metamaterial structure may comprise one or more types selected from the group consisting of Helmholtz resonators, pipes, and membranes or thin plates. Typically, the metamaterial structure may comprise or may be made of one or more arrays of unit cells. Typically, unit cells in a given array may be identical, in order to leverage periodicity, while unit cells in different arrays may either be identical or may vary in type, geometry and/or material. The metamaterial structure may be fabricated through one or more CAD-driven manufacturing technologies such as 3D printing, machining, laser-cutting or the like.

When the metamaterial structure is based on Helmholtz resonators, the geometry of the Helmholtz resonators may be varied to attenuate sound in a desired frequency range, for example by selecting specific neck and cavity sizes and shapes. The shape of the unit cells and/or of the neck portions thereof, may be cylindrical, parallelepipedic, prism-based e.g. with a hexagonal base, cubic, etc.

The metamaterial structure, e.g. one or more arrays thereof, may be made from metal, polymers, e.g. plastics, or any other suitable material. For example, the metamaterial structure, e.g., membranes or thin plates, may be made from Poly ethylene glycol diacrylate (PEGDA), Poly methyl methacrylate (PMMA), optionally with additives including SUDAN I, BaTiO3, Irgacure® 819, etc.

A/the housing, e.g. a wall thereof, may comprise one or more openings or apertures. The metamaterial structure may be provided or disposed against or adjacent an inner surface of the housing wall. The metamaterial structure may be provided or disposed adjacent or near one or more openings or apertures. Typically, the one or more openings or apertures ad/or the housing wall comprising the one or more openings or apertures, may be provided opposite the pen end of the housing.

Without wishing to be bound by theory, it is believed that sound attenuation by the acoustic metamaterials may be optimised if the sound waves carrying the sound to be attenuated are dissipated by the metamaterial structure. Therefore, it may be advantageous to allow the sound to be attenuated to penetrate or enter the housing, e.g. a chamber defined by the housing, for example trough the one or more openings or apertures of the housing wall, so as to allow the associated sound wave to interact with the metamaterial structure.

The metamaterial structure may be disposed or provided between the housing and a user’s ear. The metamaterial structure may be disposed or provided between one or more openings or apertures and an open end of the housing. By locating the metamaterial structure adjacent or near one or more openings, unwanted noise can be attenuated at the earliest point of entry into the acoustic apparatus, e.g. housing thereof.

When the acoustic apparatus is a wearable electroacoustic apparatus (such as headphones), the metamaterial structure may be provided between the housing, e.g. an opening or aperture thereof, and an electroacoustic transducer, e.g. loudspeaker, of the apparatus.

By providing or locating the metamaterial structure against or adjacent an inner surface of the housing wall, e.g. adjacent or near one or more openings or apertures, sound attenuation may be permitted regardless of the type of acoustic apparatus, e.g. whether for sound mufflers, electroacoustic apparatus, or the like. In other words, sound attenuation may be permitted regardless of the specific type of acoustic apparatus being used.

The openings or apertures may be created as part of the manufacturing design during manufacture. For example, the housing may be manufactured, e.g. may be moulded, 3D-printed, laser-cut, or the like, with openings and/or apertures therein.

Alternatively, the openings or apertures may be provided, e.g. may be made by drilling, sawing, laser cutting, or any other machining method, on a manufactured housing.

Advantageously, the provision of one or more openings or apertures in the housing may reduce the weight of the acoustic apparatus, thus also improving comfort to a user. The provision of one or more openings or apertures in the housing may also reduce the amount of material used during manufacture of the acoustic apparatus, e.g. housing, thus reducing manufacturing costs and/or environmental impact. The provision of one or more openings or apertures in the housing may also improve breathability and/or may prevent or reduce any thermal build up within the housing, thus further improving comfort to a user.

The frequency, frequencies or range of frequencies attenuated by the metamaterial structure may depend on a number of parameters including, for example, the type of metamaterials (e.g., Helmholtz resonators, pipes, or membranes or thin plates), and/or the dimensions of the metamaterial structure, e.g. dimension of the unit cells of one or more arrays thereof, such as height, depth, and/or width (e.g. radius) of a cavity of the unit cells and/or the dimensions of the neck portion thereof e.g. height, depth, and/or width thereof.

The metamaterial structure, e.g. type and/or dimensions thereof, for example when using Helmholtz resonators, may be selected to provide sound attenuation, e.g. within the housing, in a frequency range corresponding to human speech and/or in a frequency range from about 300 to about 3000 Hz, e.g., from about 500 to about 3000 Hz, e.g. from about 1000 to about 3000 Hz. For example, when based on Helmholtz resonators, the height, depth, and/or width (e.g. radius) of the cavities of the unit cells, and/or the height, depth, and/or width (e.g. radius) of the neck portion thereof, may be selected to provide sound attenuation, e.g. within the housing, in a frequency range corresponding to human speech and/or in a frequency range from about 300 to about 3000 Hz, e.g., from about 500 to about 3000 Hz, e.g. from about 1000 to about 3000 Hz

The height of the cavity of a/each unit cell may be in the range of 20-50 mm, e.g. about 25mm.

A side length (e.g. width and/or depth) of the cavity of a/each unit cell may be in the range of about 5-25 mm. For example, when the cavity is substantially square in cross section, a side length of the cavity may be in the range of 11.7-25.4 mm. In another example, the cavity may have a width of about 21 mm and a depth of about 6.5 mm.

The height of the neck of a/each unit cell may be in the range of 0.5-2.5 mm, e.g. about 1 mm.

A side length (e.g. width and/or depth) of the neck of a/each unit cell may be in the range of about 2-8 mm. For example, when the neck is substantially square in cross section, a side length of the neck may be in the range of 2-8 mm, e.g. about 4 mm.

The physical properties of the material of each unit cell, e.g. when using thin plates or membranes, may vary. For example, the Young’s Modulus may be in the range of about 1 x 10 ⁶ - 1 x 10 ⁹ Pa. The density may be in the range of about 1000 - 3000 Kg/m ³ . The Poisson’s Ratio may be in the range of about 0.3 - 0.5. The thickness may be in the range of about 20 x 10 ^-6 - 0.5 x 10 ^-3 m. The radius may be in the range of about 3 x 10 ^-3 - 10 x 10 ^-3 m.

The amount, e.g. weight or volume, of the metamaterial structure in the housing may be selected to provide an adequate level of sound attenuation. It will be appreciated that, whilst increasing the amount of metamaterials will generally increase the level of sound attenuation, practical constraints such as space, weight, and cost, will limit the amount of metamaterials that may be included within the housing.

The acoustic apparatus may comprise a control mechanism and/or an actuation mechanism for controlling and/or actuating the sound attenuation by the metamaterial structure.

The control mechanism and/or an actuation mechanism may comprise an actuator, e.g. a switch. The actuator may be associated with one or more openings or apertures. The actuator or switch may selectively and/or controllably allow partial and/or full opening and/or closing of one or more openings or apertures, e.g. of the one or more openings or apertures. A single actuator or switch may operate the opening or closing of the openings or apertures. Alternatively, there may be provided a plurality of actuators or switches, each allowing opening or closing of one or more openings or apertures. By such provision, a user may selectively control activation of the sound attenuation by the acoustic apparatus, and/or may control the degree or level of sound attenuation provided by the acoustic apparatus, e.g. by the metamaterial structure.

The control mechanism and/or an actuation mechanism, e.g. switch, may be manually, e.g. mechanically, activated. For example, a user may actuate a mechanical switch capable of opening and/or closing one or more openings or apertures.

The control mechanism and/or an actuation mechanism, e.g. switch, may be digitally and/or electronically activated or assisted. For example, a user may activate a command, e.g. a button or switch may activate an electronic circuit capable of opening and/or closing one or more openings or apertures. The acoustic apparatus may further comprise an active sound cancellation system. By such provision, the acoustic apparatus may effectively attenuate noise at low frequencies (e.g. below 500 Hz), e.g. via the active sound cancellation system, and may also effectively attenuate noise within the human speech frequency range via the metamaterial structure provided within the acoustic apparatus. Thus, the combination of a metamaterial structure and an active sound cancellation system in the acoustic apparatus may provide sound attenuation over a broader range of frequencies than currently permitted by existing technologies.

According to a second aspect there is provided a wearable electroacoustic apparatus comprising: a housing comprising at least one opening in a wall thereof; an electroacoustic transducer provided within the housing; and a metamaterial structure provided within the housing.

Advantageously, the acoustic apparatus may be configured to provide sound attenuation in the housing in a frequency range of about 300 to about 3000 Hz.

Advantageously, the metamaterial structure may be provided between the housing and the electroacoustic transducer, e.g. between the at least one opening and the electroacoustic transducer. By providing or locating the metamaterial structure against or adjacent an inner surface of the housing wall, e.g. adjacent or near one or more openings or apertures, sound attenuation of unwanted noise may be permitted outside of the electroacoustic transducer, thus improving performance to a user.

The features described in relation to the apparatus according to the first aspect may apply in relation to the apparatus according to the second aspect, and are not repeated here merely for brevity.

According to a third aspect of the invention, there is provided the use of a metamaterial structure in a wearable electroacoustic apparatus to provide sound attenuation in a frequency range of about 300 to about 3000 Hz, wherein the apparatus comprises: a housing comprising at least one opening in a wall thereof; an electroacoustic transducer provided within the housing; and the metamaterial structure provided within the housing. The features described in relation to the apparatus according to the first aspect or second aspect may apply in relation to the use according to the third aspect, and are not repeated here merely for brevity

It will be understood that the features described in relation to any aspect may equally apply in relation to any other aspect, and are not repeated merely for brevity.

Brief Description of Drawings

Embodiments of the present disclosure will now be given by way of example only, and with reference to the accompanying drawings, which are:

Figure 1 an acoustic apparatus according to an embodiment;

Figure 2 a view of an outer side of a housing of the apparatus of Figure 1 showing metamaterials;

Figure 3 a view of an inner side of a housing of the apparatus of Figure 1 showing metamaterials;

Figure 4 a perspective view of a housing and an electroacoustic transducer of the apparatus of Figure 1 ;

Figure 5 a perspective view of a housing of the apparatus of Figure 1 ;

Figures 6-7 views of a setup in an “open field” acoustic booth to carry out testing on an acoustic apparatus according to an embodiment;

Figures 8-10 views of a “HATS” setup in an acoustic booth to carry out testing on an acoustic apparatus according to an embodiment;

Figures 11-12 graphs showing sound transmission in the apparatus of Figure 1 , according to the set-up of Figures 6-7;

Figures 13-14 graphs showing sound transmission in the apparatus of Figure 1 , according to the set-up of Figures 8-10;

Figure 15 a perspective view of metamaterials based on an array of

Helmholtz resonators according to an embodiment;

Figure 16 a perspective view of metamaterials based on membranes of thin plates according to an embodiment;

Figure 17 a perspective view of metamaterials based on an array of a combination of Helmholtz resonators and thin plates according to an embodiment;

Figures 18(a) - 18(j) graphs showing models of the resonance frequency of a thin plate, each varying two parameters with three remaining parameters being fixed; Figures 19(a) - 19(j) graphs showing models of the resonance frequency of a thin plate, each varying two parameters with three remaining parameters being fixed;

Figures 20(a) — 20(j) graphs showing models of the resonance frequency of a thin plate, each varying two parameters with three remaining parameters being fixed;

Figures 21(a) - 21 (j) graphs showing models of the resonance frequency of a thin plate, each varying two parameters with three remaining parameters being fixed;

Figures 22(a) - 22(f) graphs showing models of the resonance frequency and first overtone of a Helmholtz resonator, each varying two parameters with two remaining parameters being fixed;

Figures 23(a) - 23(f) graphs showing models of the resonance frequency and first overtone of a Helmholtz resonator, each varying two parameters with two remaining parameters being fixed;

Figure 24 a perspective view of an embodiment of a switch for use in the apparatus of Figure 1.

Detailed Description of Drawings

Referring to Figure 1 there is shown a perspective view of an acoustic apparatus, generally designated 10, according to a first embodiment. In this embodiment, the acoustic apparatus 10 is a set of headphones 10. The headphones 10 are rested on a holder 5 for convenience.

The headphones 10 include a pair of acoustic devices 15 connected together by a band 30.

The headphones 10 also have a pair of inserts 40 configured to fit in and attach to an open end of a respective housing 20. As best shown in Figure 5, each housing 20 as a generically elliptical shape, and has walls 24 defining a chamber 26 configured to encapsulate and/or surround a user’s ear.

The housing 20 defines an open end or open side configured, in use to face a user’s ear. In this embodiment, the dimensions of the housing 20 are approximately 90 mm in height, about 49 mm in width, and 73 mm in depth. However, it will be appreciated that the dimensions of the housing(s) 20 may be chosen or modified to suit particular specifications determined by a brand, a user and/or a manufacturer.

Each insert 40 includes an electroacoustic transducer in the form of a loudspeaker 42 and padding 44 configured to fit over or around a user’s ear and configured to contact and/or engage with a user’s skull and/or face so as to improve comfort and provide a degree of sound insulation from the surrounding environment by limiting noise permeation at the interface between the padding 44 and the user’s body.

Figure 4 is a perspective view of one of the acoustic devices 15 of the headphones 10, illustrating the housing 20 and the insert 40 disconnected from each and showing the location of the Ioudspeaker42.

It will be appreciated that in other embodiments, however, there may not be provided a loudspeaker, for example when the acoustic apparatus is intended to be used as an “ear muffler” rather than headphones, that is, then the apparatus is intended for noise cancellation only and not for additionally playing a sound, e.g. music.

The headphones 10 include metamaterials 50, which, in this embodiment are provided in the form of two distinct metamaterial elements 52, as best shown in Figures 2 and 3. The metamaterials elements 52 are located within the chamber 26 of the housing 20.

A person of skill in the art will appreciate that the overall shape of each metamaterial elements 52 and the number of metamaterial elements 52 may vary according to each specific application, and may be influenced by, for example, the volume and shape of the housings 20, the intended pricing for the headphones 10, and/or any target weight limit for the headphones 10.

The headphones 10 are configured to provide sound attenuation in the housings 20 in a frequency range corresponding to human speech, typically in a frequency range of about 300 to about 3000 Hz.

Advantageously, the acoustic apparatus 15 is configured to provide sound attenuation in the housing. The acoustic apparatus may be configured to provide sound attenuation in the housing in a frequency range of about 500 to about 3000 Hz, e.g. about 1000 to about 3000 Hz.

The wall 24 of housing 20 has a plurality of openings 22. The metamaterial elements 52 are disposed against or adjacent an inner surface 25 of the housing wall 24.

The metamaterial elements 52 are disposed adjacent some of the openings 22. Without wishing to be bound by theory, it is believed that sound attenuation by the acoustic metamaterial elements 52 is permitted when the associated sound waves are dissipated by the metamaterial elements 52. Therefore, it is advantageous to allow the sound to be attenuated to penetrate or enter the housing 20, e.g. chamber 26 defined by the housing 20, so as to allow the associated sound wave to interact with the metamaterial elements 52. Advantageously, in this embodiment, the metamaterial elements 52 are provided between the housing 20 (that is, between a wall 24 thereof) and a user’s ear, and more specifically between a wall 24 of the housing 20 and the loudspeakers 40. By such provision, sound attenuation may be optimised.

The openings 22 or apertures may be created as part of the manufacturing design during manufacture of the housing 20, or may be made by machining an existing housing.

Advantageously, the provision of openings 22 in the housing 20 reduces the weight of the headphones 10, and also improves breathability, thus also improving comfort to a user.

In order to control and/or actuate sound attenuation by the metamaterial structure, one or both acoustic devices 15 of the headphones 10 may be equipped with a switch 80, which is shown in Figure 24. The switch 80 is associated with one or more openings or apertures 22, and thus permit selective partial or full opening or closing of one or more openings 22. In some embodiments, one or both acoustic devices 15 of the headphones 10 may be provided a plurality of switches 80, each allowing opening or closing of one or more openings or apertures 22. By such provision, a user may selectively control activation of the sound attenuation by the acoustic device 15, and/or may control the degree or level of sound attenuation provided by the acoustic device, e.g. by the metamaterial elements 52.

Figures 6-7 depict a typical “open field” set up of an acoustic booth used to carry out testing on the acoustic device 15 shown in Figure 4.

In this set up, a microphone 61 is placed near the insert 40 on a side thereof opposite the housing 20, that is, facing the open side of the housing 20, while a loudspeaker 65 plays a sound in the audible frequency range.

The effect of the metamaterial elements 52 provided inside the housing 20 of the device 15 was tested by measuring sound captured by the microphone 61 , with and without the metamaterial elements 52 and by comparing the two results.

The associated results are shown in Figures 11 and 12, which are graphs showing the sound transmission though the acoustic device 15 containing the metamaterials 52, normalized to the response of the same acoustic device 15 without the metamaterials 52. It can be observed that the device 15 enabled sound attenuation of about 4 dB in the frequency range corresponding to audible human speech (particularly around 1000 - 2200 Hz), and enabled sound attenuation of about 4-6 dB at higher frequencies, particularly around 4000-5500 Hz and 7000-9000Hz. These results are useful in terms of identifying the correct range of and gaps; however, the level of sound attenuation within the housing was clearly affected by the open field nature of the testing environment.

Figures 8-10 depict a typical Head & Torso Simulator (‘HATS’) 70 setup in an acoustic booth to carry out testing on the apparatus 10 of Figure 1. The HATS 70 is equipped with a 3 ” microphone (not shown) and standard-size pinna, while a pink noise signal in the audible frequency range was played by a loudspeaker 65.

The effect of the metamaterial elements 52 provided inside the housing 20 of the apparatus 10 was tested by measuring sound captured by the microphone of the HATS 70, with and without the metamaterial elements 52 and by comparing the two results.

The associated results are shown in Figures 13 and 14, which are graphs showing the sound transmission though the apparatus 10 containing the metamaterials 52, normalized to the response of the same apparatus 10 without the metamaterials 52. It can be observed that the apparatus 10 enabled sound attenuation of about 10 dB in the frequency range corresponding to audible human speech (particularly around 1000 - 3000 Hz), and enabled sound attenuation of about 20 dB at higher frequencies, particularly around 5000 - 12000 Hz. These results demonstrate the efficacy of the apparatus 10 in attenuating sound in the normal human frequency range (as well as at certain higher frequencies), which is particularly useful in the context of an ear covering apparatus such as headphones.

Figures 15-17 show various types of metamaterials that may be used in the apparatus 10 and/or device 15, e.g. to prepare metamaterial elements 52.

Figure 15 is a perspective view of metamaterials 55 based on an array of Helmholtz resonators according to an embodiment. Figure 16 is a perspective view of metamaterials 58 based on membranes of thin plates according to another embodiment. Figure 17 is a perspective view of metamaterials 59 based on an array of a combination of Helmholtz resonators and thin plates according to another embodiment.

The test carried out in relation to Figures 6-14 were performed based on metamaterials 55 according to Figure 15. Each unit cell (four unit cells being depicted in Figure 15) has a cavity 56 and a neck 57. In this embodiment, the dimensions of the unit cells were as follows:

Cavity Height: 25 mm;

Cavity Width: 21 mm

- Cavity Depth: 6.5 mm

Neck Height: 1 mm

Neck Width: 4 mm

Neck Depth: 4 mm

Walls depth: 1 mm

Thus, in this embodiment, the neck 57 corresponds to the thickness of the cavity walls. However, in other embodiments, the neck 56 may protrude or extend from the cavity 56. For example, as explained below in more detail, the specific parameters of the unit cells and/or of the material used in the metamaterial elements 52 may be varied or modified in order to attenuate sound in or around a desired frequency or range of frequencies.

In Figures 18 to 21 , the effect of varying the physical properties of the material of each unit cell when using thin plates or membranes, was investigated.

Figures 18(a) - 18(j) depicts models of the resonance frequency of a thin plate, each varying two parameters with three remaining parameters being fixed. In particular, the resonance frequency of a thin plate is modelled when five parameters are varied across values. These parameters were Young’s Modulus, Density, Poisson’s Ratio, Thickness and Radius of a circular thin plate. In this visualization, each set of permutations of two varying parameters is plotted and the three remaining parameters are fixed. The fixed parameters were chosen after running an optimization algorithm that finds a set of values for the five parameters that result in a first mode of vibration in the range of speech frequencies (300 - 3000 Hz).

Specifically, the fixed parameters were:

Young's modulus: 200 x 10 ⁶ Pa (range 1 x 10 ⁶ - 1 x 10 ⁹ );

Density: 1180 Kg/m ³ (range 1000 - 3000);

Poisson's Ratio: 0.35 (range 0.3 - 0.5);

Thickness: 70 x 10 ^-6 (range 20 x 10 ^-6 - 0.5 x 10 ^-3 );

Radius: 7.5 x 10 ^-3 (range 3 x 10 ^-3 - 10 x 10 ^-3 ).

Figures 19(a) - 19(j) models the resonance frequency of a thin plate when the five parameters listed in Figures 18(a) - 18(j) are varied across values. In this visualization, each set of permutations of two varying parameters is plotted and the three remaining parameters are fixed. The fixed parameters were chosen after running an optimization algorithm that finds a set of values for the five parameters that minimizes the value of the resonance frequency fO within all the values existing in a five-dimensional matrix. The resulting set of parameters will optimize this visualization for low-frequencies solutions.

Specifically, the fixed parameters were:

Young's modulus: 1 x 10 ⁶ Pa (range 1 x 10 ⁶ - 1 x 10 ⁹ );

Density: 3000 Kg/m ³ (range 1000 - 3000);

Poisson's Ratio: 0.3 (range 0.3 - 0.5);

Thickness: 2 x 10 ^-5 (range 20 x 10 ^-6 - 0.5 x 10 ^-3 );

Radius: 10 x 10' ³ (range 3 x 10' ³ - 10 x 1 O' ³ ).

Figures 20(a) - 20(j) models the resonance frequency of a thin plate when the five parameters listed in Figures 18(a) - 18(j) are varied across values. In this visualization, each set of permutations of two varying parameters is plotted and the three remaining parameters are fixed. The fixed parameters were chosen after running an optimization algorithm that finds a set of values for the five parameters that finds the median value of the resonance frequency fO within all the values existing in a fivedimensional matrix. The resulting set of parameters will optimize this visualization for mid-frequencies solutions.

Specifically, the fixed parameters were:

Young's modulus: 8.6 x 10 ⁸ Pa (range 1 x 10 ⁶ - 1 x 10 ⁹ );

Density: 2551 Kg/m ³ (range 1000 - 3000);

Poisson's Ratio: 0.44 (range 0.3 - 0.5);

Thickness: 2.7 x 10' ⁴ (range 20 x 10' ⁶ - 0.5 x 10' ³ );

Radius: 6.1 x 10' ³ (range 3 x 10' ³ - 10 x 10' ³ )

Figures 21(a) - 21 (j) models the resonance frequency of a thin plate when the five parameters listed in Figures 18(a) - 18(j) are varied across values. In this visualization, each set of permutations of two varying parameters is plotted and the three remaining parameters are fixed. The fixed parameters are chosen after running an optimization algorithm that finds a set of values for the five parameters that maximizes the value of the resonance frequency fO within all the values existing in a five-dimensional matrix. The resulting set of parameters will optimize this visualization for high-frequencies solutions.

Specifically, the fixed parameters were:

Young's modulus: 1 x 10 ⁹ Pa (range 1 x 10 ⁶ - 1 x 10 ⁹ );

Density: 1000 Kg/m ³ (range 1000 - 3000);

Poisson's Ratio: 0.5 (range 0.3 - 0.5);

Thickness: 5 x 10' ⁴ (range 20 x 10' ⁶ - 0.5 x 10' ³ );

Radius: 3 x 10' ³ (range 3 x 10' ³ - 10 x 10' ³ ).

Thus, it can be seen from Figures 18-21 that, for metamaterials based on thin plates, the physical parameters of the material used to prepare the metamaterial elements 52 can be varied or tailored in order to attenuate sound in or around a desired frequency or range of frequencies.

In Figures 22 and 23, the effect of varying the dimensions of the unit cells when using Helmholtz resonators, was investigated.

Figures 22(a) - 22(f) shows models of the resonance frequency and first overtone of a Helmholtz resonator, each varying two parameters with two remaining parameters being fixed. These four parameters were height of the cavity, side length of the cavity, length of the neck and side length of the neck. In this visualization, each set of permutations of two varying parameters is plotted and the two remaining parameters are fixed. The upper and lower limits of the parameters values are set according to standard 3D printer constraints and to specific applications constraints, such as the space inside the housing 20 of each acoustic device 15. The values of fixed parameters were set to be the mean of possible values.

In Figures 22(a) - 22(f), the parameters were fixed by choosing the mid value in the range provided. This is generally considered to be an appropriate starting point in optimization processes. Specifically, the fixed parameters were: i. Cavity Height: 35 mm (range 20 - 50 mm); ii. Cavity Width and Depth: 18.6 mm (range 11 .7 - 25.4 mm); iii. Neck Height: 1 .5 mm (range 0.5 - 2.5 mm); iv. Neck Width and Depth: 5 mm (range 2 - 8 mm).

Specifically, Figure 22(a) varies cavity height and cavity side length, and keeps the other parameters fixed. Figure 22(b) varies cavity height and neck height, and keeps the other parameters fixed. Figure 22(c) varies cavity height and neck side length, and keeps the other parameters fixed. Figure 23(d) varies cavity side length and neck height, and keeps the other parameters fixed. Figure 23(e) varies cavity side length and neck side length, and keeps the other parameters fixed. Figure 23(f) varies neck height and neck side length, and keeps the other parameters fixed.

Figures 23(a) - 23(f) shows models of the resonance frequency and first overtone of a Helmholtz resonator, each varying two parameters with two remaining parameters being fixed. Figure 23 models the resonance frequency and first overtone of a Helmholtz resonator when the four parameters identified in Figure 22 were varied across values. In this visualization, the fixed parameters were chosen after running an optimization algorithm that maximizes the difference between the resonance and the first overtone.

By creating two four-dimensional matrices with all the possible values for fO and f1 and subtracting the two, it is possible to find the parameters that result in the maximum difference. Setting the fixed values to these parameters, allows to visualize similar solutions and eventually to choose the parameters that best optimize other constraints, such as damping, sound energy and dimensionality constraints.

In Figures 23(a) - 23(f), the fixed values were the values that scored best when running an optimization algorithm. In this optimization algorithm, all the parameters were considered together, without keeping fixed any of the others parameters. The values that scored best were the values that gave the largest difference between the fundamental frequency and the first overtone. Specifically, the fixed values for Figure 23 were as follows: i. Cavity Height: 20 mm (range 20 - 50 mm) ii. Cavity Width and Depth: 17.1 mm (range 1 1 .7 - 25.4 mm) iii. Neck Height: 2.5 mm (range 0.5 - 2.5 mm) iv. Neck Width and Depth: 2 mm (range 2 - 8 mm).

Thus, it can be seen from Figures 22 and 23 that, for metamaterials based on Helmholtz resonators, the dimensions of the unit cells used to prepare the metamaterial elements 52 can be varied or tailored in order to attenuate sound in or around a desired frequency or range of frequencies.

It will be appreciated that the embodiments of the invention hereinbefore described are given by way of example only and are not meant to limit the scope thereof in any way.