Acoustic enclosure and method for leveling an acoustic enclosure

Abstract

------ Acoustic Enclosure and Method for Leveling an Acoustic Enclosure The present invention relates to an acoustic enclosure (20) comprising a loudspeaker (21) mounted flush on a front face of the enclosure (20), characterized in that at least one Helmholtz resonator (26) is formed on the front face of the enclosure (20) between each lateral edge of the front face and the loudspeaker (21), the at least one Helmholtz resonator (26) being dimensioned to have a band gap to block the propagation, in the plane of the front face of the enclosure (20), of acoustic waves in the useful frequency band of the loudspeaker (21). Figure to be published with the abstract: Figure 6a

Description

Title of the invention: Acoustic enclosure and method for leveling an acoustic enclosure

[0001] The present invention relates to the audio field, and more particularly to a loudspeaker and a method for leveling a loudspeaker.

[0002] When designing a sound enclosure, two of the most important quality criteria are radiation efficiency, defined as the ratio between the acoustic power radiated and the electrical power supplied to the enclosure, and directivity, which describes how the radiated energy is distributed in space.

[0003] Maximizing radiation efficiency is important because it reduces the electrical consumption of the speaker, minimizes wear on mechanical parts, and reduces the risk of distortion for the same playing power.

[0004] Regarding directivity, the objective is to obtain a unidirectional radiation lobe, concentrated towards the front of the speaker, and constant with frequency. This ensures faithful and homogeneous sound reproduction throughout the listening area, and also maximizes the amplitude of the direct sound relative to that of the first reflections off the walls of the room in which the speaker is placed.

[0005] One of the main difficulties preventing the achievement of these radiation properties is the diffraction of sound around the enclosure. When sound is emitted by the loudspeaker, some of the radiated energy is sent parallel to the front face of the enclosure. These waves are then diffracted by the edges of the enclosure, which act as secondary sources and reflect sound in all directions. The interaction between the direct waves emitted by the loudspeaker and the diffracted waves emitted by the edges creates constructive and destructive interference, resulting in acoustic radiation that is highly dependent on the listening angle and frequency. Furthermore, the acoustic waves that are sent to the sides and rear of the enclosure increase the amplitude of the reflected sound in the room compared to the direct sound, which is detrimental to faithful and immersive audio reproduction.Furthermore, diffraction at the edges represents an effective loss in radiation efficiency, as some of the radiated acoustic energy is not returned to the listening area.

[0006] These phenomena are more pronounced at frequencies where the wavelength X is of the same order of magnitude as the size of the enclosure, typically between 2 kHz (X = 17 cm) and 12 kHz (X = 3 cm) for an enclosure with sides of 10 cm to 20 cm. At higher frequencies, the loudspeaker becomes very directional so that the acoustic waves are directed towards the front of the enclosure without undergoing diffraction at the edges. At At lower frequencies, the wavelength is very large compared to the dimensions of the enclosure and the diffraction around it is less significant.

[0007] A loudspeaker can also be recessed. This is the case, for example, in some recording studios or movie theaters, where it is preferable to recess the speakers into the walls of the room to save space. Another example is that of motor vehicles, where the loudspeakers are recessed into the doors or the front panel of the vehicle. Even if these configurations do not present diffraction problems at the edges, controlling lateral radiation remains an important issue when it comes to controlling the directivity characteristics and radiation efficiency of the loudspeaker. Indeed, just as in the previous case (non-recessed), lateral radiation generates a loss of radiation efficiency and increases the amplitude of the first reflections compared to the direct sound.Furthermore, having control over the direction and width of the radiation lobe is advantageous in these applications in order to better concentrate acoustic energy in a smaller listening area and thus obtain a more faithful and immersive audio reproduction.

[0008] In order to solve these problems, there are several solutions.

[0009] A first approach is a passive one, which consists of using waveguides or acoustic horns. An example of such a solution is described in US patent US4071112A. According to this approach, the acoustic impedance between the loudspeaker and the free space is matched. This makes it possible, in particular, to increase acoustic radiation and achieve uniform directivity over a wide listening area. However, waveguides have the disadvantage of being very bulky, and their design is generally very expensive and complex to implement. An inefficient horn design can exhibit acoustic resonances in the waveguide or mechanical resonances (vibrational modes) that can significantly distort the acoustic response of the loudspeaker.

[0010] Another passive solution involves using enclosures with rounded shapes, preferably spherical, which minimizes diffraction. While this solution reduces diffraction effects, its effectiveness remains limited, and rounded shapes increase the complexity and production costs of the enclosure compared to simpler parallelepiped shapes.

[0011] Finally, still in the category of passive solutions, acoustic metamaterials have been implemented to solve the problem of interaction between the direct waves emitted by the loudspeaker of a speaker enclosure and the diffracted waves emitted by the edges of the enclosure.

[0012] These passive solutions based on acoustic metamaterials offer new possibilities for controlling the radiation of a soundproof enclosure. In the sense Broadly speaking, acoustic metamaterials are defined as structured materials where the presence of resonances produces acoustic properties that cannot be obtained with conventional materials. Typically, metamaterials are composed of slot- or channel-type resonator arrays, Helmholtz resonators (also referred to by their acronym RH in this application), or membrane resonators. Excitation of these resonators yields remarkable properties, such as strong sub-wavelength absorption, excellent acoustic transmission, a negative bulk modulus, or sound insulation superior to that predicted by the mass law for a conventional material.

[0013] International PCT application WO2023198967A1 discloses a grid composed of a metamaterial containing several parallel guides and one of whose walls contains Helmholtz resonators.

[0014] Helmholtz resonators allow the speed of sound to be controlled independently in each waveguide, thus forming a kind of acoustic lens. This grille must be placed in front of the loudspeaker in order to direct the wave emitted by the loudspeaker in a desired direction.

[0015] Although this solution allows for control of directivity, this occurs within a limited frequency band, and the presence of the metamaterial in front of the loudspeaker reduces its overall radiation efficiency. Furthermore, internal resonances in the grille can cause significant variations in the system's frequency response, as well as phase distortions.

[0016] US patent application US20230018951A1 discloses the use of a A metamaterial composed of quarter-wave channels arranged on the surface of the enclosure. This invention exploits resonances within the channels to absorb acoustic waves and thus prevent diffraction at the edges. The drawback of this solution is that absorption occurs only at the channel resonance frequencies, which appear at wavelengths X = 4*LC, where Lc is the channel length. To achieve good absorption across the entire frequency range of interest, it would be necessary to design a very large number of channels of varying sizes with perfectly matched losses. Furthermore, the numerous resonances involved can introduce significant phase distortions across the entire frequency range of interest, preventing high-fidelity audio reproduction.Finally, a last drawback of this solution is that diffraction control is based on a dissipative mechanism, which implies a loss in radiation efficiency.

[0017] Another approach is an active approach, by which one

[0018] seeks to control directivity by using a loudspeaker array. Such an approach is described, for example, in US patents US7826622B2 and US7684574B2. The loudspeaker array is driven by a signal processing system that modifies the signal injected into each speaker, thus achieving the desired acoustic radiation. However, implementing such a control system and using multiple speakers adds considerable cost and complexity to the speaker design compared to passive solutions.

[0019] The present invention aims to overcome the drawbacks of prior art devices and relates to an acoustic enclosure comprising a loudspeaker having a useful frequency band between a first frequency fa and a second frequency fb, with fb>fa, the loudspeaker being mounted flush on a front face of the enclosure, characterized by the fact that at least one Helmholtz resonator is formed on the front face of the enclosure between each lateral edge of the front face and the loudspeaker, the at least one Helmholtz resonator being dimensioned to have a first resonance frequency f0 less than or equal to fa and a second resonance frequency fi equal to or greater than fb.

[0020] In the case of a conventional, non-recessed speaker, the lateral edges of the front face correspond to the lateral edges of the speaker. In the case of a recessed speaker, the front face is understood to be the surface into which the speaker is recessed, the lateral edges then corresponding to the edges of this surface into which the speaker is recessed.

[0021] The frequencies f0 and fi are included in the band of frequencies audible to the human ear, preferably but not limited to 20 Hz to 20 kHz.

[0022] The arrangement of at least one Helmholtz resonator is henceforth called a metasurface, because it represents a two-dimensional metamaterial.

[0023] Between frequencies f0 and fb, at least one Helmholtz resonator behaves like a mass-spring system. In this regime, the normal acoustic velocity in the vicinity of at least one Helmholtz resonator is high compared to that of the enclosure wall, which is typically close to zero. This property blocks the propagation of sound in the plane of the front face of the enclosure without involving any dissipative mechanism, thus concentrating the radiated energy forward.

[0024] The effect is therefore a control of the directivity and an increase in the efficiency of radiation towards the front of the enclosure: a narrower radiation lobe is obtained, which considerably reduces diffraction effects, while controlling the group delay of the acoustic waves radiated by the enclosure. In the case of non-recessed enclosures, the creation of diffracted waves generated by the edges of the enclosure is thus avoided.

[0025] In the absence of at least one Helmholtz resonator, the enclosure surface represents an acoustically rigid boundary (i.e., with zero normal velocity). The waves emitted by the loudspeaker can then propagate freely in the plane parallel to the front face of the enclosure, generating diffraction when these waves reach the edges. Conversely, when at least one Helmholtz resonator is located on the front face of the enclosure, an acoustically flexible surface (i.e., with high normal velocity) is created between frequencies f0 and fb, which prevents the formation of laterally propagating acoustic waves. From the point of view of the dispersion pattern of waves propagating parallel to the plane of the front face of the enclosure, this behavior is manifested by a band gap between f0 and fb, which prevents the propagation of waves between f0 and fb.

[0026] Unlike other approaches, notably US patent 20230018951A1, the present invention does not involve any dissipative mechanism, which makes it possible to concentrate the radiated energy towards the front of the enclosure.

[0027] The effect of at least one Helmholtz resonator is to control directivity and increase radiation efficiency. This effect is more pronounced when the front face of the loudspeaker is completely covered with resonators, but a single Helmholtz resonator is sufficient to obtain a significant effect.

[0028] Another advantage over existing approaches is that the metasurface is completely passive, and it operates between two resonances, which limits phase distortions.

[0029] According to one embodiment, several Helmholtz resonators are formed between the loudspeaker and each lateral edge of the front panel. This reduces the surface area of ​​the enclosure between the loudspeaker and the edges of the enclosure over which waves can propagate.

[0030] Advantageously, Helmholtz resonators are provided on any path of passage of an acoustic wave on the front surface of the enclosure between the loudspeaker and the lateral edges of the enclosure.

[0031] According to one embodiment, the Helmholtz resonators are arranged on the front face of the enclosure in one or more arrangements among a line, preferably parallel to the lateral edges of the enclosure, several lines, preferably parallel to the lateral edges of the enclosure, all or part of a circle, preferably concentric around the loudspeaker, all or part of several circles, preferably concentric around the loudspeaker, all or part of a polygon surrounding the loudspeaker, all or part of several polygons surrounding the loudspeaker, preferably concentric, all or part of a closed curved line surrounding the loudspeaker, all or part of several closed curved lines surrounding the loudspeaker.

[0032] Advantageously, Helmholtz resonators are arranged on either side of the loudspeaker, between the loudspeaker and each lateral edge of the enclosure, over a height corresponding to the maximum height of the loudspeaker, which is the height of the loudspeaker projected orthogonally onto a lateral edge of the enclosure. Of course, other Helmholtz resonators can be provided over a height greater than the maximum height of the loudspeaker, or even surround all or part of the loudspeaker.

[0033] According to one embodiment, the Helmholtz resonators are of the same dimensions.

[0034] According to one embodiment, the Helmholtz resonators are of different dimensions, preferably arranged in order of increasing or decreasing size from the loudspeaker to the considered lateral edge of the front face of the enclosure. If the cavities of the Helmholtz resonators have a decreasing size from the loudspeaker to the lateral edges of the enclosure, the radiation lobe is wider compared to the case where all the cavities have the same dimensions, whereas if the cavities of the Helmholtz resonators have an increasing size from the loudspeaker to the lateral edges of the enclosure, the radiation lobe is narrower compared to the case where all the cavities have the same dimensions.

[0035] According to one embodiment, at least one Helmholtz resonator is formed in an acoustic metasurface integrated into the front face of the enclosure. The entire front face of the enclosure may also be made of an acoustic metasurface.

[0036] According to one embodiment, the front face of the enclosure is projecting, having vertical ridges on either side of the loudspeaker extending vertically from the edge of the loudspeaker to the corresponding lateral edge of the front face of the enclosure. The surface bearing at least one Helmholtz resonator is thus inclined relative to a completely flat front enclosure surface. When the acoustic metasurface is inclined, the radiation lobe widens or narrows compared to the case where the front face of the enclosure is flat, depending on whether the metasurface has a convex or concave shape, respectively. If the front metasurface is inclined on only one side of the loudspeaker, the radiation lobe is directed towards that same side.

[0037] According to one embodiment, at least one Helmholtz resonator has a neck and a cavity, the cavity having a shape such as square, rectangular, round, oval, or polygonal. It is understood that the invention is not limited to these shapes, and that any resonator shape with a neck (acoustic mass) and a cavity (acoustic spring) that constitutes a Helmholtz resonator falls within the scope of the present invention.

[0038] According to one embodiment, the metasurface is obtained by 3D printing. However, any other method of obtaining the metasurface is envisaged within the scope of the present invention.

[0039] The invention also relates to a method for upgrading an enclosure comprising a loudspeaker having a useful frequency band between a first frequency fa and a second frequency fb, with fb>fa, the loudspeaker being mounted flush on a front face of the enclosure, characterized in that it comprises the formation of at least one Helmholtz resonator on the front face of the enclosure between each lateral edge of the front face and the loudspeaker, the at least one Helmholtz resonator being dimensioned to have a first resonance frequency f0 less than or equal to fa and a second resonance frequency fi equal to or greater than fb.

[0040] The at least one Helmholtz resonator can take any form, in particular can be in the form of one or more straight or curved bands of resonator assemblies, arranged on all or part of the periphery of the loudspeaker.

[0041] To better illustrate the object of the present invention, several embodiments, given by way of illustration and not limitation, will now be described, in connection with the attached drawings.

[0042] On these drawings:

[0043] [Fig.l] is a schematic top view of an acoustic enclosure according to the prior art.

[0044] [Fig.2] is a schematic top view of a sound enclosure according to a mode of the realization of the invention.

[0045] [Fig.3a] is a diagram representing the dispersion relation of acoustic waves propagating in the plane parallel to the front face of the enclosure as a function of frequency for an acoustic enclosure according to the state of the art.

[0046] [Fig.3b] is a diagram representing the dispersion relation of acoustic waves propagating in the plane parallel to the front face of the enclosure as a function of frequency for an acoustic enclosure according to the invention.

[0047] [Fig.4a] is a schematic cross-sectional view of a metasurface according to a first mode of the realization of the present invention.

[0048] [Fig.4b] is a figure analogous to [Fig.4a] of a metasurface according to a second embodiment of the present invention.

[0049] [Fig. 5] is a partial cross-sectional view of a front face of a loudspeaker enclosure according to a method of embodiment of the invention.

[0050] [Fig.6a] is a schematic view of an acoustic enclosure according to a mode of realization of the invention.

[0051] [Fig.6b] is a schematic view of a sound enclosure according to another mode of realization of the invention.

[0052] [Fig.6c] is a schematic view of a sound enclosure according to another mode of realization of the invention.

[0053] [Fig.6d] is a schematic view of an acoustic enclosure according to another embodiment of the invention.

[0054] [Fig.6e] is a schematic view of a sound enclosure according to another mode of realization of the invention.

[0055] [Fig.6f] is a schematic view of an acoustic enclosure according to another embodiment of the invention.

[0056] [Fig.6g] is a schematic view of a sound enclosure according to another mode of realization of the invention.

[0057] [Fig.6h] is a schematic view of an acoustic enclosure according to another embodiment of the invention.

[0058] [Fig.6i] is a schematic view of an acoustic enclosure according to another embodiment of the invention.

[0059] [Fig.6j] is a schematic view of an acoustic enclosure according to another embodiment of the invention.

[0060] [Fig.6k] is a schematic view of a sound enclosure according to another mode of realization of the invention.

[0061] [Fig.61] is a schematic view of a sound enclosure according to another mode of realization of the invention.

[0062] [Fig.7] is a schematic view illustrating the influence of the inclination of the metasurface on the radiation lobe of the acoustic enclosure.

[0063] [Fig.8] is a schematic view illustrating the influence of the size of the resonators of Helmholtz in the metasurface on the radiation lobe of the acoustic enclosure.

[0064] [Fig.9] is a schematic view of a sound enclosure according to another mode of realization of the invention.

[0065] [Fig. 10] is a schematic view of an acoustic enclosure according to another embodiment of the invention.

[0066] [Fig. 11] is a schematic view of an acoustic enclosure according to another embodiment of the invention.

[0067] [Fig. 12a] represents a sound pressure level at 1 m, as a function of angle and frequency, of a conventional circular tweeter test enclosure.

[0068] [Fig. 12b] represents a sound pressure level at 1 m, as a function of angle and frequency, of a circular tweeter enclosure with test metasurface according to the invention.

[0069] [Fig. 12c] represents the gain of the circular tweeter enclosure with metasurface according to the invention.

[0070] [Fig. 12d] represents the directivity diagrams of the test circular tweeter enclosure, conventional (dashed line) and of the invention (solid line), at 5100 Hz.

[0071] [Fig. 13a] represents a sound pressure level at 1 m from a conventional test ribbon tweeter enclosure.

[0072] [Fig. 13b] represents a sound pressure level at 1 m, as a function of angle and frequency, of a ribbon tweeter enclosure with test metasurface according to the invention.

[0073] [Fig. 13c] represents the gain of the ribbon tweeter enclosure with metasurface according to the invention.

[0074] [Fig. 13d] represents the directivity diagrams of the conventional (dashed line) and invention (solid line) test ribbon tweeter enclosure at 5100 Hz.

[0075] If we refer to [Fig. 1], we can see that an acoustic enclosure 1 has been represented according to the prior art, viewed from above.

[0076] The enclosure 1 is schematically represented as a parallelepiped, with a front face 2 carrying a loudspeaker 3, whose diaphragm is flush with the front face 2 and whose motor and chassis are positioned in the enclosure 1.

[0077] The loudspeaker 3 generates a direct acoustic wave Od (lobe) propagating towards the front of the enclosure 1, this direct acoustic wave Od generating diffracted waves Os on the lateral edges, respectively 4 and 5, of the front face 2 of the enclosure 1.

[0078] By convention, the plane of the front face 2 of the enclosure 1 will be the xz plane (x being the direction from one lateral edge to the other of the enclosure 1, z being the height direction of the enclosure 1), y then being the depth direction of the enclosure 1. These conventions will be kept for the description of the enclosures according to the invention in the rest of the description.

[0079] In [Fig.3a], which schematically represents the dispersion relation of acoustic waves propagating in the plane parallel to the front face 2 of the enclosure 1 (plane xz), the waves emitted by the loudspeaker 3 can propagate freely in the plane parallel to the front face 2 of the enclosure 1 at the speed of sound in a free field (C0=343 m / s), which generates diffraction (diffracted waves Os) when these waves arrive at the lateral edges 4, 5 of the front face 2. In this [Fig.3a], ko is the acoustic wave number in the plane xz.

[0080] In [Fig.2], a top view of an acoustic enclosure 10 according to the invention, also schematically represented as a parallelepiped, with a front face 12 carrying a loudspeaker 13, is shown in a manner analogous to [Fig.1].

[0081] Unlike enclosure 1 of [Fig.1], metasurfaces 16, described in more detail below, are arranged between the loudspeaker 13 and each of the lateral edges 14, 15.

[0082] As schematically represented in [Fig.2], the effect of the metasurfaces 16 is to suppress the diffracted Os waves present in the enclosures 1 of the prior art, to obtain better directivity of the lobe L generated by the enclosure 10 and better radiation efficiency of the enclosure 10 according to the invention.

[0083] Thus, as shown in [Fig. 3b], it can be observed that when the metasurfaces 16 are located on the front face 12 of the enclosure 10, a frequency band (designated as the band gap) is created between the frequencies f0 and fi, which prevents the formation of acoustic waves in the xz plane parallel to the front face 12 of the enclosure 10. For frequencies within this band gap, the energy radiated by the loudspeaker 13 is concentrated towards the front of the enclosure 10, and diffraction phenomena are strongly attenuated, or even completely eliminated. In this regime, a narrower radiation lobe L is obtained, which considerably reduces diffraction effects.

[0084] The metasurfaces 16 thus prevent the formation of diffracted waves by creating a band gap in the xz plane, so that the energy radiated by the loudspeaker 13 is concentrated mainly at the front of the enclosure 10. This non-dissipative way of controlling diffraction represents a significant advantage over the absorbing channel-based approach of US patent 20230018951A1, as it allows for a considerable increase in the radiation efficiency of the enclosure 10. Furthermore, since the diffraction control takes place between two resonances, the metasurfaces 16 do not introduce strong phase distortions over the entire frequency band of interest fa-fb of the loudspeaker, with fa <fb.

[0085] The band gap is defined between a first resonance frequency f0 and a second resonance frequency fi of the Helmholtz resonators formed in the metasurfaces 16, the frequency range of interest of the loudspeaker (or useful band) fa-fb being contained within the band gap of the metasurface, f0-fb

[0086] The metasurfaces 16 will now be described in more detail with reference to Figures 4a and 4b, which represent two non-limiting embodiments of the metasurfaces 16.

[0087] The metasurface 16 of [Fig. 4a] is an acoustic metasurface and comprises several Helmholtz resonators 17 formed from a solid material, each Helmholtz resonator 17 comprising a cavity 18 and a neck 19 opening to the outside to form an acoustic resonator. The Helmholtz resonators 17 can, for example, be formed by 3D printing, although other manufacturing methods could be considered.

[0088] In the embodiment of [Fig. 4a], the cavities 18 are square in shape. We can define wn as the width of the neck 19, hn as the height of the neck 19, wc as the width of the cavity 18, hc as the height of the cavity 18.

[0089] In [Fig. 4b], a metasurface 16' is shown according to a second embodiment of the invention. The metasurface 16' is an acoustic metasurface and comprises several Helmholtz resonators 17' formed in a solid material, each Helmholtz resonator 17' comprising a cavity 18' and a neck 19' opening to the outside to form an acoustic resonator.

[0090] In the embodiment of [Fig. 4b], the cavities 18' are round. We can define wn as the width of the neck 19', hn as the height of the neck 19', and rc as the radius of the cavity 18'.

[0091] Any other shape can be envisaged for the Helmholtz resonators formed in the metasurfaces 16, for example rectangular, polygonal, provided that the Helmholtz resonators form a band gap for the desired frequency band.

[0092] The frequencies f0 and fi of the band gap can be defined according to the geometry of each Helmholtz resonator and chosen independently to obtain wide band gaps, the band gap being delimited between the fundamental frequency of the resonator f0 and the frequency fi of the first higher-order radiating (non-trapped) mode of the resonator.

[0093] Since the geometry of the resonator is extruded in the z direction, the fundamental frequency of the resonator is given by:

[0094] [Math.l] f = âi. EK" JQ 2it\ h„Ac

[0095] where wn and hn are as defined above for a square cavity, and Ac is the cross-section of the cavity. For example, for a cavity with a rectangular cross-section, Ac = hc * wc, or Ac = ir * rc² for a cavity with a circular cross-section.

[0096] On the other hand, the frequency fi of the first higher-order radiating (non-trapped) mode of the resonator depends mainly on the dimensions of the rear cavity, and is given by:

[0097] [Math.2] f cn

[0098] where L = max (hc, wc) for a cavity with rectangular cross-section and where L = 2*Jt*rc for a cavity with circular cross-section.

[0099] In the particular case of a resonator with the neck centered in the middle of the cavity at x, the first mode radiating along x is given by:

[0100] [Math.3]

[0101] because the first higher-order mode in x, of frequency:

[0102] [Math.4] f __ Ci) J —

[0103] is non-radiating (trapped) and therefore does not participate in the response of the enclosure.

[0104] From the expressions for f0 and fb, a design strategy for the metasurface 16 or 16' can be defined by fixing the dimensions of the cavity 18, 18' to establish the high frequency fi of the band gap. For example, for a high frequency of 20 kHz corresponding to the limit of human hearing, the maximum dimension L of the cavity 18, 18' should not exceed approximately 85 mm.

[0105] In practice, in the case of a rectangular cavity, it is appropriate to choose L = hc = wc, because this gives the maximum cavity section and therefore a minimum frequency f0.

[0106] Once the dimensions of the cavity are fixed, the dimensions of the neck are chosen according to the low frequency f0 of the band gap that one wishes to obtain. This is possible by choosing the width and height of the neck 19, 19', wn and hn respectively, independently of the dimensions of the cavity 18, 18' which determine the value of fb. This provides a great deal of flexibility to separate the frequencies f0 and fi and thus obtain a very wide operating band of the metasurface 16, 16'.

[0107] It should be noted that the metasurface 16, 16' is all the more effective as the number of resonators 17, 17' is high and as it completely covers the front face 12 of the enclosure 10.

[0108] However, the metasurface 16, 16' shows a reduction in diffraction on the edges 14, 15 even when the number of resonators 17, 17' is small.

[0109] Figure 5 represents a metasurface 16, seen in cross-section, in which one can see that the resonators 17 are arranged concentrically around a loudspeaker (not shown) intended to be placed at the center of the metasurface 16.

[0110] The face of the metasurface 16 on the left in [Fig. 5] represents the face of the metasurface 16 facing outwards from the enclosure 10 in use, with the collars 19 therefore facing outwards from the enclosure 10 on the front face 12, in order to block acoustic waves at frequencies located in the band gap. The cavities 18 are located within the thickness of the metasurface 16, on the inner side of the enclosure 10.

[0111] Figures 6a to 61 represent different embodiments of an enclosure according to the invention.

[0112] In [Fig.6a], the enclosure 20 according to the invention comprises a circular tweeter 21 and a loudspeaker 22.

[0113] The acoustic metasurface consists of concentric rings 26 of Helmholtz resonators arranged around the tweeter 21 to control the radiation of the tweeter 21 in the x and z directions of the front face of the enclosure 20.

[0114] In [Fig.6b], the enclosure 30 according to the invention comprises a circular tweeter 31 and a loudspeaker 32.

[0115] The acoustic metasurface consists of concentric ring portions 36 of Helmholtz resonators arranged around the tweeter 31 to control the radiation of the tweeter 31 in the x direction of the front face of the enclosure 30.

[0116] In [Fig.6c], the enclosure 40 according to the invention comprises a circular tweeter 41 and a loudspeaker 42.

[0117] The acoustic metasurface consists of concentric ring portions 46 partitioned with Helmholtz resonators arranged around the tweeter 41 to control the radiation of the tweeter 41 in the x direction of the front face of the enclosure 40. The partitioning of the resonators 46 makes it possible to avoid internal resonances along the extrusion direction.

[0118] In [Fig.6d], the enclosure 50 according to the invention comprises a circular tweeter 51 and a loudspeaker 52.

[0119] The acoustic metasurface consists of concentric rings 56 of Helmholtz resonators arranged around the loudspeaker 52 to control the radiation of the loudspeaker 52 in the x and z directions of the front face of the enclosure 50.

[0120] In [Fig.6e], the enclosure 60 according to the invention comprises a ribbon tweeter 61 and a loudspeaker 62.

[0121] The acoustic metasurface consists of concentric rectangles 66 of Helmholtz resonators arranged around the ribbon tweeter 61 to control the radiation of the ribbon tweeter 61 in the x and z directions of the front face of the enclosure 60.

[0122] In [Fig.6f], the enclosure 70 according to the invention comprises a ribbon tweeter 71 and a loudspeaker 72.

[0123] The acoustic metasurface consists of concentric rectangular portions 76 of Helmholtz resonators arranged around the ribbon tweeter 71 to control the radiation of the ribbon tweeter 71 in the x and z directions of the front face of the enclosure 70. The partitioning of the resonators 76 makes it possible to avoid internal resonances along the extrusion direction.

[0124] In [Fig.6g], the enclosure 80 according to the invention includes a ribbon tweeter 81 close to one of the edges of the enclosure 80.

[0125] The metasurface 86 is composed of linear parallel resonators, parallel to the edges of the enclosure 80, linearly extruded to control the radiation of the ribbon tweeter 81 in the transverse x direction. In this configuration, the objective is to Place the speaker (ribbon tweeter 81) near one edge of the enclosure 80, so that only the diffraction on the opposite edge needs to be controlled. This is possible if the distance d between the edge near the ribbon tweeter 81 and the ribbon tweeter 81 is similar to or smaller than the shortest wavelength that one wishes to control.

[0126] In [Fig. 0h], the enclosure 90 according to the invention comprises a circular tweeter 91 and a loudspeaker 92.

[0127] A first acoustic metasurface consists of concentric rings 96 of Helmholtz resonators arranged around the circular tweeter 91 to control the radiation of the circular tweeter 91 in the x and z directions of the front face of the enclosure 60.

[0128] A second acoustic metasurface consists of concentric rings 96' of Helmholtz resonators arranged around the loudspeaker 92 to control the radiation of the loudspeaker 92 in the x and z directions of the front face of the enclosure 60.

[0129] In [Fig.6i], the enclosure 100 according to the invention comprises a ribbon tweeter 101 and a loudspeaker 102.

[0130] The acoustic metasurface consists of parallel lines 106 of Helmholtz resonators arranged on either side of the ribbon tweeter 101 to control the radiation of the ribbon tweeter 101 in the x direction of the front face of the enclosure 100.

[0131] In [Fig.6j], the enclosure 110 according to the invention comprises a ribbon tweeter 111 and a loudspeaker 112.

[0132] The acoustic metasurface consists of portions of parallel lines 116 of Helmholtz resonators arranged on either side of the ribbon tweeter 111 to control the radiation of the ribbon tweeter 111 in the x direction of the front face of the enclosure 110. The partitioning of the resonators 116 makes it possible to avoid internal resonances along the extrusion direction.

[0133] In [Fig.6k], the enclosure 120 according to the invention includes a circular tweeter 121 close to one of the edges of the enclosure 120.

[0134] The metasurface 126 is composed of linear parallel resonators, parallel to the edges of the enclosure 120, linearly extruded to control the radiation of the circular tweeter 121 in the transverse x direction. In this configuration, the objective is to place the loudspeaker (circular tweeter 121) near one of the edges of the enclosure 120, so that only the diffraction on the opposite edge needs to be controlled. This is possible if the distance d between the edge near the circular tweeter 121 and the circular tweeter 121 is similar to or smaller than the shortest wavelength that one wishes to control.

[0135] In [Fig. 61], the enclosure 130 comprises a circular tweeter 131 and a high- speaker 132. The metasurface 136 is composed of resonators of random sizes and orientations to control the radiation of the circular tweeter 131 and the speaker 132 in the x and z directions.

[0136] In all embodiments described in connection with Figures 6a to 61, it is preferable to partition the resonators in order to avoid acoustic resonances in the extrusion direction. The length of these partitions dp should be chosen such that the resonance frequency in the extrusion direction fp is greater than or equal to fn

[0137] [Math.5] J p Id?

[0138] These partitions are not, in principle, necessary in the embodiment of [Fig. 6a] because the asymmetric extrusion with the loudspeaker placed in the center prevents the formation of resonances in the extrusion direction. However, the metasurface in the embodiment of [Fig. 6a] can be partitioned in the extrusion direction to make it more robust against implementation defects that can generate internal modes (for example, when the tweeter or the rings are slightly off-center).

[0139] If we now refer to [Fig.7], we can see that a schematic top view of an enclosure 140 has been represented in which the loudspeaker 141 is framed by two metasurfaces 146a, 146b according to the invention, each comprising a set of RH Helmholtz resonators.

[0140] When the two metasurfaces 146a, 146b are in the plane of the front surface of the enclosure 140 (case A), a radiation lobe Li (solid line) is obtained centered on the loudspeaker 141.

[0141] When the metasurfaces 146a and 146b are inclined towards the interior of the enclosure 140 at an angle 0 (case B), a radiation lobe L2 (dashed line) is obtained, centered on the loudspeaker 141 but widened compared to case A.

[0142] Finally, if only one of the two metasurfaces, the metasurface 146b in the example shown, is inclined towards the inside of the enclosure 140 at an angle 0 (case C), we obtain a radiation lobe L3 (in dashed line) centered on the loudspeaker 141 but directed towards the side of the inclined metasurface 146b.

[0143] If the slope of the metasurface is different depending on the angle around the loudspeaker, the direction lobe is directed in the direction in which the angle 0 is greatest, which allows the radiation lobe to be directed in a given direction.

[0144] The steeper the slope, the wider the radiation lobe, and vice versa. Conversely, if the outer part is higher than the central part, a narrower radiation lobe is obtained.

[0145] It is therefore possible to play on the width of the radiation lobe and its orientation by playing on the inclination of the metasurfaces framing the loudspeaker.

[0146] If we now refer to [Fig.8], we can see that a schematic top view of an enclosure 150 has been represented in which the loudspeaker 151 is framed by two metasurfaces 156a, 156b according to the invention.

[0147] In case A, the metasurfaces 156a and 156b comprise identical RH Helmholtz resonators. A radiation lobe Li (solid line) is obtained, centered on the loudspeaker 151.

[0148] In case B, the metasurfaces 156a and 156b comprise RH Helmholtz resonators of decreasing sizes from the loudspeaker side 151 to the enclosure edge side 140.

[0149] A radiation lobe L2 (in dashed line) is obtained centered on the loudspeaker 151 but widened compared to case A.

[0150] In case C, the metasurfaces 156a and 156b comprise RH Helmholtz resonators of increasing sizes from the loudspeaker side 151 to the enclosure edge side 140.

[0151] A radiation lobe L3 (in dotted line) is obtained centered on the loudspeaker 151 but narrowed compared to case A.

[0152] It is of course possible to combine both an inclination of the metasurfaces and a variation of the dimensions of the Helmholtz resonators inside the metasurfaces to influence the shape of the radiation lobe: narrowing, widening, lateral deviation.

[0153] Referring to [Fig.9], we can see that a top view of an enclosure 160 has been schematically represented according to another embodiment of the invention.

[0154] In this embodiment, the loudspeaker 161 framed by the metasurfaces 166 comprising RH Helmholtz resonators is recessed relative to the front face of the enclosure 160 on which the metasurfaces 166 are formed.

[0155] An acoustic cavity 167 of width A and depth B is thus formed in front of the loudspeaker 161. This configuration is possible with the invention provided that this acoustic cavity 167 does not create resonances. For this acoustic cavity 167 not to create resonances, the range of wavelengths of interest of the enclosure 160 must be of the same order as A and B or greater than A and B. More precisely, the cavities will not create acoustic resonance if X ≥ 2A and X ≥ 4B.

[0156] Referring to [Fig. 10], we can see that a top view of an enclosure 170 has been schematically represented according to another embodiment of the invention.

[0157] In this embodiment, the metasurfaces 176 comprising RH Helmholtz resonators and framing the loudspeaker 171 are recessed relative to the front face of the enclosure 170 on which the metasurfaces 176 are formed.

[0158] Two acoustic cavities 177 of width A and depth B are thus formed in front of the metasurfaces 177. This configuration is possible with the invention provided that these acoustic cavities 177 do not create resonances. Approximately, the cavities 177 will not create acoustic resonance if X 2* A and X 4*(B+B'), where B' is a length correction that takes into account cavity radiation.

[0159] Referring to [Fig. 11], we can see that a top view of an enclosure 180 has been schematically represented according to another embodiment of the invention.

[0160] In this embodiment, the metasurfaces 186 comprising Helmholtz RH resonators and framing the loudspeaker 181 are zigzag-shaped and form triangular acoustic cavities 187 of width A and depth B. This configuration is possible with the invention provided that these acoustic cavities 187 do not create resonances. Approximately, the cavities will not create acoustic resonance if X ≥ 2*A and X ≥ 4*(B+B'), where B' is a correction of length which takes into account the radiation from the cavity.

[0161] With these configurations, an acoustic response similar to that obtained when all the elements are placed flush with the speaker enclosure is achieved, thus avoiding amplitude and phase distortions in the speaker's response within its useful band. Alternatively, it is also possible to use these resonances to amplify the acoustic response at certain frequencies, which can be achieved, in particular, by choosing (B+B') = Xo / 4, where Xo is the wavelength of the frequency to be amplified.

[0162] The performance of a circular metasurface according to the invention, located around a 3.2 mm diameter circular tweeter, Dayton model (registered trademark) ND16FA, was measured experimentally. This corresponds to the embodiment illustrated in [Fig. 6a]. The circular tweeter is mounted on a parallelepiped enclosure with dimensions (width x height x depth) of 20 cm x 22 cm x 22 cm. The circular tweeter is centered along the width of the enclosure and is positioned 7 cm from the top edge (15 cm from the bottom edge) of the enclosure. The enclosure is placed on a rotating platform to measure its directivity in the horizontal plane. A bandpass-filtered pulse between 2 kHz and 20 kHz is injected into the loudspeaker and The pressure level is measured with an omnidirectional microphone model Umik-2 located at the same height as the circular tweeter and 1 m away from it.

[0163] The measurements are carried out in a wooden box covered on the inside with 10 cm thick melamine foam, in order to obtain anechoic conditions throughout the band of interest.

[0164] The metasurface is manufactured by 3D printing with a resin-type material and consists of four concentric ring-shaped resonators. The Helmholtz resonators have a rectangular cavity with dimensions wc = 8.5 mm and lc = 8.5 mm, and a neck with dimensions wn = 3 mm and ln = 4 mm. The walls of the resonators are 2 mm thick. These dimensions give theoretical band gap frequencies f0 = 5.6 kHz and fi = 20.1 kHz. In practice, the frequency fo is slightly lower, around 4.5 kHz, due to the typical neck length correction for a Helmholtz resonator, which was not taken into account in the calculation of f0.

[0165] Figures 12a and 12b show the pressure level in dB as a function of frequency and angle of incidence, without and with the meta-surface, respectively. The pressure level for each angle is normalized by the pressure level on the axis to disregard the tweeter's response. Without the meta-surface, secondary lobes appear at approximately +45° and -45°. Conversely, with the meta-surface, these lobes disappear, and a more uniform directivity is achieved. It can also be seen that the radiation towards the rear of the enclosure, i.e., for angles >90° and <90°, is much weaker with the meta-surface.

[0166] Fig. 12c shows the gain in dB, defined as the ratio between the pressure level with the metasurface and the pressure level without the metasurface on the tweeter axis (angle 0°).

[0167] A very substantial gain of 3dB-6dB is observed in a fairly wide frequency band, between 4.5 kHz and 7 kHz, which corresponds to the frequency band where diffraction phenomena are most markedly observed in [Fig. 12a].

[0168] Figure 12d shows the directivity at 6 kHz without and with the meta-surface. Without the meta-surface (dashed line), the radiation pattern is non-uniform with respect to the angle, with secondary lobes at +45° and -45° and dips at +30° and -30°. In contrast, with the meta-surface (solid line), a uniform unidirectional lobe is obtained, more focused towards the front of the enclosure, and with a considerably greater amplitude between +45° and -45° compared to the case without the meta-surface.

[0169] The same procedure was used to measure the performance of a linear metasurface. The aim in this case is to correct the directivity of a Dayton PTMini-6 ribbon tweeter.

[0170] Figures 13a and 13b show the pressure level in dB as a function of frequency and angle of incidence without and with the metasurface, respectively. Without the metasurface, secondary lobes appear at approximately +50° and -50°. In contrast, with the metasurface, these lobes disappear, resulting in more uniform directivity. It is also evident that the radiation towards the rear of the enclosure is significantly reduced with the metasurface.

[0171] Fig. 13c shows the gain in dB. As with the ribbon tweeter, we observe a The gain is very significant, between 2dB and 6dB in the 4.5 kHz and 7 kHz band. However, a dip in the gain is observed around 5500 Hz, which could be due to diffraction with the upper or lower edge, which is not controlled in this configuration.

[0172] Figure 13d shows the 6 kHz directivity without and with the metasurface. The behavior is quite similar to that obtained with the circular metasurface. Without the metasurface (dashed line), the radiation pattern is non-uniform with respect to the angle, with strong secondary lobes at +50° and -50° and dips at +28° and -28°. In contrast, with the metasurface (solid line), a uniform unidirectional lobe is obtained, more focused towards the front of the enclosure, and with a considerably greater amplitude between +45° and -45° compared to the case without the metasurface.

[0173] These experimental results demonstrate the effectiveness of the metasurface proposed according to the present invention in suppressing diffraction effects and thus obtaining:

[0174] - a more uniform radiation, without side lobes, and constant with the frequency,

[0175] - increased radiation efficiency of the enclosure.

[0176] Although the invention is described with several Helmholtz resonators, it is understood that the effect of the invention is observed from a Helmholtz resonator placed on the periphery of the loudspeaker, between the loudspeaker and the lateral edges of the enclosure.

[0177] Also, although the invention has been described with a conventional non-recessed enclosure, it also applies to recessed enclosures, the front face being understood as the surface on which the enclosure is recessed, the side edges corresponding to the edges of this surface on which the enclosure is recessed. The invention also relates to a method of upgrading a speaker enclosure, consisting of adding metasurfaces comprising at least one Helmholtz resonator, between the speaker of the enclosure and the lateral edges of the enclosure.

[0178] Thus, it is possible to change the front face of the enclosure to form a new front face with the metasurfaces of the invention, or to cut holes in the front face to place metasurfaces according to the invention.

Claims

Demands

1. - Acoustic enclosure (10) comprising a loudspeaker (13) having a useful frequency band between a first frequency fa and a second frequency fb, with fb>fa, the loudspeaker (13) being mounted flush on a front face (12) of the enclosure (10), characterized in that at least one Helmholtz resonator (16) is formed on the front face (12) of the enclosure (10) between each lateral edge (14, 15) of the front face (12) and the loudspeaker (13), the at least one Helmholtz resonator (16) being dimensioned to have a first resonant frequency f0 less than or equal to fa and a second resonant frequency fi equal to or greater than fb.

2. - Acoustic enclosure (10) according to claim 1, characterized in that several Helmholtz resonators (16) are formed between the loudspeaker (13) and each lateral edge (14, 15) of the front face (12).

3. - Acoustic enclosure (10) according to claim 2, characterized in that the Helmholtz resonators (16) are arranged on the front face (12) of the enclosure (10) in one or more arrangements among a line, preferably parallel to the lateral edges (14, 15) of the enclosure (10), several lines, preferably parallel to the lateral edges (14, 15) of the enclosure (10), all or part of a circle, preferably concentric around the loudspeaker (13), all or part of several circles, preferably concentric around the loudspeaker (13), all or part of a polygon surrounding the loudspeaker (13), all or part of several polygons surrounding the loudspeaker (13), preferably concentric, all or part of a closed curved line surrounding the loudspeaker, all or part of several closed curved lines surrounding the loudspeaker.

4. - Acoustic enclosure (10) according to claim 2 or claim 3, characterized in that the Helmholtz resonators (16) are of the same dimensions.

5. - Acoustic enclosure (10) according to claim 2 or claim 3, characterized in that the Helmholtz resonators (16) are of different dimensions, preferably arranged in order of increasing or decreasing size from the loudspeaker (13) towards the lateral edge (14, 15) considered of the front face (12) of the enclosure (10).

6. - Acoustic enclosure (10) according to any one of claims 1 to 5, characterized in that at least one Helmholtz resonator (16) is formed in an acoustic metasurface integrated into the front face (12) of the enclosure (10).

7. - Acoustic enclosure (10) according to any one of claims 1 to 6, characterized in that the front face (12) of the enclosure (10) is projecting, having vertical panels on either side of the loudspeaker (13) extending vertically from the edge of the loudspeaker (13) to the corresponding lateral edge (14, 15) of the front face (12) of the enclosure (10).

8. - Acoustic enclosure (10) according to any one of claims 1 to 7, characterized in that at least one Helmholtz resonator (17, 17') has a neck (19, 19') and a cavity (18, 18'), the cavity (18, 18') having a shape among square, rectangular, round, oval, polygonal.

9. - Acoustic enclosure (10) according to any one of claims 1 to 8, characterized in that the metasurface (16) is obtained by 3D printing.

10. - A method for upgrading an enclosure comprising a loudspeaker having a useful frequency band between a first frequency fa and a second frequency fb, with fb>fa, the loudspeaker being mounted flush on a front face of the enclosure, characterized in that it comprises the formation of at least one Helmholtz resonator on the front face of the enclosure between each lateral edge of the front face and the loudspeaker, the at least one Helmholtz resonator being dimensioned to have a first resonant frequency f0 less than or equal to fa and a second resonant frequency fi equal to or greater than fb.

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