EP4083996A1

EP4083996A1 - A soundproofing wall comprising a plate-type acoustic metamaterial

Info

Publication number: EP4083996A1
Application number: EP21171395.3A
Authority: EP
Inventors: Felix Dr.-Ing. Langfeldt; Wolfgang Proft. Dr.-Ing. Gleine
Original assignee: Hochschule fuer Angewandte Wissenschaften Hamburg
Current assignee: Hochschule fuer Angewandte Wissenschaften Hamburg
Priority date: 2021-04-30
Filing date: 2021-04-30
Publication date: 2022-11-02
Also published as: WO2022229371A1

Abstract

A soundproofing wall for isolating a first room on one side of the soundproofing wall from a sound source on the other side of the soundproofing wall, the soundproofing wall comprising a plate-type acoustic metamaterial with a baseplate and a plurality of masses arranged on the baseplate wherein at least one of the masses is strip-shaped.

Description

The invention relates to a soundproofing wall for isolating a first room on one side of the soundproofing wall from a sound source on the other side of the soundproofing wall. Such soundproofing walls find application in various fields, for example to isolate an interior of a vehicle, a vessel or an aircraft from an external sound source, to encapsulate a noise generating machine from its surroundings, within buildings, etc.
The efficiency of a soundproofing wall can be characterised in terms of the sound transmission loss (STL) which describes the loss of intensity of an incident acoustic wave having passed the soundproofing wall. For homogeneous walls, the sound transmission loss is determined by the mass-law for frequencies below the coincidence frequency, leading to the well-known problem that efficient soundproofing of low frequencies requires walls having a large mass.
In 2008, the so-called membrane-type acoustic metamaterials (MAM) were introduced as a new class of acoustic metamaterials with lightweight properties and, at the same time, frequency bands in the low-frequency range with remarkably large sound transmission loss, considerable larger than in accordance with the corresponding mass-law. However, one major challenge for the application of MAM in practical noise problems is the pretension of the membrane. The pretension generates the spring stiffness of the unit cell and is therefore critical in determining the vibroacoustic properties of the metamaterial. In practice, the pretension of a membrane can be difficult to control as it can be subject to relaxation effects in the membrane material or thermal variations. Also, MAM require a relatively heavy frame in order to sustain the pretension and this frame can significantly impair the lightweight properties of MAM
A solution to this problem is to use a plate instead of a membrane as the base material to which the masses are attached. In the case of such plate-type acoustic metamaterials (PAM), the spring stiffness is generated by the bending stiffness of a baseplate and a pretension is not required. Several different designs of PAM have been investigated, for example PAM with a frame and no masses in the unit cells, PAM with added masses or double-layer PAM with air cavities and orifices. One particular PAM design, which has particularly promising properties for applications with strong weight limitations, consists of a baseplate with a periodic array of rigid masses attached to the baseplate and no frame structure at all.
Although these PAM exhibit the same low-frequency sound reduction properties as MAM and do not require a pretensioned membrane and frame structure, the large number of masses (typically hundreds or thousands of masses per square meter) lead to a level of complexity which drives the manufacturing cost of such metamaterials to prohibitively high levels, in particular when large surfaces need to be lined with PAM for low-frequency sound insulation.
Departing therefrom, it is an object of the invention to provide an improved soundproofing wall based on a plate-type acoustic metamaterial enabling industrial manufacturing at reasonable costs.
This object is solved by the soundproofing wall with the features of claim 1. The soundproofing wall is for isolating a first room on one side of the soundproofing wall from a sound source on the other side of the soundproofing wall and comprises a plate-type acoustic metamaterial with a baseplate and a plurality of masses arranged on the baseplate, wherein at least one of the masses is strip-shaped.
The term plate-type acoustic metamaterial means that, in contrast to a membrane-type acoustic metamaterial, the vibroacoustic properties of the metamaterial (of the combination of the baseplate and the masses) do not rely on a membrane that must be held under tension, but rather on the elastic properties of the "unsupported" baseplate itself. No tensioning of the baseplate is required. The soundproofing wall therefore does not require a frame or any other support structure that provides a pretension of the baseplate. However, the soundproofing wall may nevertheless comprise a frame or support structure to which the baseplate is fastened, in order to maintain the plate-type acoustic metamaterial in place.
In addition to the plate-type acoustic metamaterial, the soundproofing wall may comprise any number of further structural or acoustic elements in layered or any other suitable form, such as one or more layers of a sound absorbing material, acoustic metamaterials, structural wall elements such as bricks, structural steel work, concrete wall elements, wood, sheet metal, fibre-reinforced plastics, and so on. In any event, the soundproofing wall by means of the plate-type acoustic metamaterial provides improved sound insulation.
In accordance with the invention, at least one of the masses of the plate-type acoustic metamaterial is strip-shaped. In particular, a plurality of the masses, a majority of the masses or all of the masses may be strip-shaped. The inventors realised that the manufacturing costs of a plate-type acoustic metamaterial can be greatly reduced when a strip-shaped mass is used instead of a plurality of point-like masses. Although the at least one strip-shaped mass covers an elongated section of the baseplate and imparts additional stiffness to the metamaterial, and thereby alters the possible vibrational motion of the metamaterial, it was found that a comparable frequency response can be achieved, in particular including resonances and anti-resonances similar to a plate-type acoustic metamaterial with point-like masses. For this reason, using at least one strip-shaped mass is an economically viable alternative to a larger number of point-shaped masses.
In an aspect of the invention, the baseplate has a stiffness. It is this stiffness that leads to the vibroacoustic properties of the metamaterial. The stiffness can be selected and adapted in relation to the at least one strip-shaped mass such that a desired frequency response of the plate-type acoustic metamaterial is achieved. It is noted that for an efficient sound transmission loss in a low-frequency range, such as for example 20 Hz to 100 Hz, the stiffness of the baseplate may be relatively low. This is true in particular when a lightweight overall structure is desired, including relatively lightweight masses. The stiffness of the baseplate may be tailored by selecting a material for the baseplate (or a combination of materials) having a certain Young's modulus, and/or by selecting a suitable thickness of the baseplate. In practice, for example various polymeric materials may be used, ranging from thin foils to thicker, solid or foamed layers. These or any other suitable materials may be combined with an adjacent layer of another material, thereby forming a sandwich-like baseplate having the desired stiffness. For example, the baseplate may comprise a thin layer of an impermeable material, e.g. a polymeric foil, and an adjacent, preferably thicker layer of another material, for example an absorber material such as a glass or mineral wool, the adjacent layer imparting stiffness and/or elasticity to the thin layer.
In an aspect of the invention, the at least one strip-shaped mass has a length and a width, wherein the length is at least three times the width. Of course, the length to width-ratio of the at least one strip-shaped mass may be larger, such as at least 5, at least 10, or at least 20. Another parameter to quantify the length of the at least one strip-shaped mass is the ratio of the length to a distance between the strip-shaped mass and an adjacent mass. This ratio may be at least 5, at least 10 or at least 20.
In an aspect of the invention, the at least one strip-shaped mass has a length extending in a length-direction, wherein the length of the at least one strip-shaped mass extends over at least 50% of a dimension of the baseplate measured in the length-direction. In particular the length of the at least one strip-shaped mass may extend over at least 80% of said dimension, or over the entirety of said dimension. This may be an advantage when manufacturing the soundproofing wall, in particular because an endless plate-type acoustic metamaterial may be produced and stored as a roll, for example.
In an aspect of the invention, the ratio µ between a surface mass density of the at least one strip-shaped mass to a surface mass density of the baseplate is 3.5 or more. The ratio µ may also be 5.0 or more, or 10 or more. The ratio µ can be calculated from the specific density ρ_M and the height h_M of the at least one strip-shaped mass and the specific density ρ_b and the height h_b of the baseplate as follows: $µ = \frac{ρ_{M} h_{M}}{ρ_{b} h_{b}}$
It was found that a ratio µ of 3.5 or more leads to a favourable sound transmission loss in a relatively broad frequency range. As an upper limit, one may consider a value of about µ = 40,000. For practical/structural reasons, however, the inventors found a suitable upper limit to be around µ = 500.
In an aspect of the invention, the ratio between a bending stiffness D_M of the at least one strip-shaped mass to a bending stiffness D_b of the baseplate is in a range of about 50 to about 500. The bending stiffness may be increased as desired by using stiffer materials for and/or larger heights/thicknesses of the masses/the baseplate. When the at least one strip-shaped mass has a high bending stiffness, it will move as a rigid body, and the vibroacoustic properties of the metamaterial will be dominated by the deformability of the baseplate. This may be assumed as long as the bending stiffness of the at least one strip-shaped mass is about 1,000 times larger than the bending stiffness of the baseplate. When the bending stiffness of the at least one strip-shaped mass is reduced, the first anti-resonance of the plate-type acoustic metamaterial shifts towards lower frequencies. When using a ratio in the claimed range, an additional anti-resonance appears at a frequency below the first anti-resonance, which leads to a broadening of the frequency range with high sound transmission loss, and/or to a second peak in sound transmission loss at a lower frequency. Both effects can be beneficial when aiming for an efficient soundproofing for a specific noise problem.
In an aspect of the invention, the at least one strip-shaped mass consists of a material having a higher mass density and/or a higher stiffness than a material of the baseplate. Both measures help to obtain a desired ratio µ of surface mass densities and/or a desired ratio D_M / D_b of bending stiffnesses in a compact/thin package.
In an aspect of the invention, a plurality of the masses are strip-shaped and arranged in a regular pattern. The regular pattern may include a simple repetition of strip-shaped masses arranged in parallel, but may also be more complex, as will be shown in some of the embodiments. For example, the regular pattern may include a geometric structure such as a square, a rectangle, or a spiral. Each geometric structure may be formed by one or by several strip-shaped masses. Each geometric structure may comprise additional/internal structures or patterns. When using a spiral, for example, a width of the at least one strip-shaped mass forming the spiral may increase from a centre of the spiral towards an outer area of the spiral. A plurality of geometric structures, each formed by one or more strip-shaped masses, may be arranged on the baseplate in a regular pattern. It was found that regular patterns from strip-shaped masses make it easy to predict and tailor the vibroacoustic characteristics of the plate-type acoustic metamaterial as desired and in a consistent manner for the entire soundproofing wall.
In an aspect of the invention, the plate-type acoustic metamaterial comprises a plurality of unit cells each including at least one of the strip-shaped masses and an adjacent, strip-shaped baseplate region in which none of the masses is present. Unit cells arranged in a regular pattern also help to achieve predictable, consistent vibroacoustic characteristics.
In an aspect of the invention, the baseplate has a thickness h_b and a longitudinal wave velocity c_b and two of the strip-shaped masses are arranged in a distance /, wherein the materials used for the strip-shaped masses and the baseplate and their dimensions are selected such that the ratio between h_b times c_b and l² is 30 per second or more. Written as a formula, this reads as follows: $\frac{h_{b} c_{b}}{l^{2}} \geq 30 \frac{1}{s}$
Based on the characteristics of the baseplate material, the longitudinal wave velocity c_b of the baseplate can be calculated as follows: $c_{b} = \sqrt{\frac{E_{b}}{ρ_{b} (1 - v_{b}^{2})}}$
with Young's modulus E_b , specific density ρ_b and Poisson's ratio v_b of the baseplate. When operating within the region defined by the above inequation, the lowest anti-resonance of the plate-type acoustic metamaterial is expected to fall in a frequency range suitable for many noise problems. As an upper limit, most practical noise problems will benefit from selecting the ratio between h_b times c_b and l² at or below 15,000 per second, at or below 7,500 per second or even at or below 1,500 per second.
In an aspect of the invention, each of the unit cells includes at least two of the strip-shaped masses. This makes it possible do design the plate-type acoustic metamaterial such it comprises one or more peaks in sound transmission loss at different, selected frequencies.
In an aspect of the invention, the at least two strip-shaped masses have different surface mass densities. This may be achieved by using different dimensions and/or materials having different mass densities for the at least two strip-shaped masses. This offers additional options for selecting suitable frequency responses of the soundproofing wall.
In an aspect, the plate-type acoustic metamaterial has a surface mass density of 5 kg/m² or less. While a common, solid soundproofing wall with this surface mass density will have relatively low performance in the low frequency range, as predicted by the mass-law, the inventive soundproofing wall can perform much better. At the same time, the inventive soundproofing wall has an overall weight low enough to be suitable for weight-sensitive applications, such as, for example, for soundproofing of an aircraft passenger cabin. In particular, the inventive soundproofing wall may be integrated in a cabin wall of an aircraft.
In an aspect of the invention, the at least one strip-shaped mass is attached to the baseplate. In particular, the at least one strip-shaped mass may be adhered or welded to the baseplate. Manufacturing the plate-type acoustic metamaterial in this way can be done with proven techniques and offers great flexibility with regard to the selection of materials. No matter how the at least one strip-shaped mass is connected to the baseplate, it may consist of a single layer or of more than one layer, such as, for example two, three or more layers. These layers may consist of different materials and/or may have different structures so that they have different viscosities, stiffnesses, densities and/or thicknesses. In particular the at least one strip-shaped mass may comprise a bottom layer of an adhesive, arranged between the baseplate and at least one upper layer.
In an aspect of the invention, the at least one strip-shaped mass comprises a first mass layer, a second mass layer and an elastic layer arranged between the first mass layer and the second mass layer. By means of this layered construction, the at least one strip-shaped mass itself forms a system of two elastically coupled masses that can vibrate on its own. This allows for additional anti-resonances of the plate-type acoustic metamaterial, which may be helpful for obtaining a broad frequency range of high transmission loss. The elastic layer may extend over the entire length and/or over the entire width of the first and second mass layers, or it may cover selected connection regions or connection points thereof only, for example arranged in a certain pattern, such as in strips running in the crosswise or lengthwise direction of the first and second mass layers. This can help achieving a lower effective spring constant with a given elastic material, and thereby a lower anti-resonance frequency. In particular, the connection regions or points may be distributed over a length direction of the mass layers such that longitudinal sections of the mass layers arranged between neighbouring connection regions remain unconnected. This allows for additional bending vibration modes of the first mass layer and/or the second mass layer and related, additional anti-resonances. The first mass layer may be connected to the baseplate by any suitable means, such as an adhesive layer. However, the first mass layer can also be formed integrally with the baseplate and from the same material as the baseplate, for example when a thickened strip of the baseplate forms the first mass layer. It is understood that the concept may be extended by adding one, two or more additional pairs of an elastic layer and a mass layer.
In an aspect of the invention, the elastic layer comprises a pressurised gas. In this aspect, variations of the distance between the two mass layers translate into pressure changes within the elastic layer. Suitable examples of such elastic layers include various foamed materials as well as larger gas volumes encapsulated in a container, such as a hose or closed profile. Elastic layers comprising a pressurised gas are relatively lightweight, cost efficient and can provide a desired degree of elasticity with low damping.
In an aspect of the invention, a material of the first mass layer and a material of the second mass layer are selected such that these layers are pulled towards one another by a magnetic force. For example, one of these layers may comprise a ferromagnetic material and the other one may comprise a permanent magnet. The pulling force may be helpful for holding the two mass layers and the elastic layer together. However, the various layers may be interconnected by any other means as well, such as by additional adhesive layers. Moreover, the magnetic force leads to a desired level of compression of the elastic layer. When combined with an elastic layer comprising a pressurised gas, in particular, vibroacoustic properties largely independent of temperature changes can be obtained.
In an aspect of the invention, the baseplate and the at least one strip-shaped mass are integrally formed from the same material. In particular, this can be done by machining a plate to remove material to form grooves in the area where no masses shall be present or by recasting the material of a plate. These manufacturing techniques are well-suited for mass production, in particular in endless lengths. Various polymeric materials, in particular, have favourable characteristics as to stiffness and weight, well suited for achieving surface mass density and bending stiffness ratios in accordance with the above mentioned, preferred ranges.
In the following, the invention is explained in greater detail based on embodiments shown in drawings. The figures show:

Fig. 1: a soundproofing wall in a schematic, perspective view;
Fig. 2: the soundproofing wall of Fig. 1 in cross section;
Fig. 3: a diagram illustrating the frequency response of a soundproofing wall;
Fig. 4: a diagram illustrating the influence of bending stiffness on the frequency response of a soundproofing wall;
Fig. 5: a schematic view of four different arrangements of strip-shaped masses having varying lengths;
Fig. 6: a schematic view of three different arrangements of strip-shaped masses having varying shapes;
Fig. 7: a schematic view of strip-shaped masses arranged in a pattern of squares;
Fig. 8: a schematic view of strip-shaped masses arranged in spirals;
Fig. 9: a schematic view of four different patterns of strip-shaped masses;
Fig. 10: a schematic view of strip-shaped masses arranged in a complex pattern;
Fig. 11: a schematic view of a soundproofing wall in cross-section having a first unit cell with two strip-shaped masses;
Fig. 12: a schematic view of a soundproofing wall in cross-section having a second unit cell with two strip-shaped masses;
Fig. 13: a schematic view of a soundproofing wall in cross-section having a third unit cell with two strip-shaped masses;
Fig. 14: a schematic, perspective view of a strip-shaped mass having a layered construction on a baseplate;
Fig. 15: an illustration of the physical principles underlying the arrangement of Fig. 14.

Figure 1 shows a soundproofing wall consisting of a baseplate 10 and a plurality of strip-shaped masses 12, together forming a plate-type acoustic metamaterial. The baseplate 10 has a height h_b, a mass density ρ_b, a Young's modulus E_b , and a Poisson's ratio v_b . These quantities essentially determine the bending stiffness D_b of the baseplate 10.
The strip-shaped masses 12 have a length extending along a y-axis, and a width b along an x-axis. The strip-shaped masses have a height h_M and a mass density ρ_M . They are arranged in parallel. Between each two strip-shaped masses 12, there is a spacing l. A unit cell of the plate-type acoustic metamaterial therefore has a width a = l + b. For some of the calculations carried out by the inventors, it is assumed that the soundproofing wall extends endlessly in the x- and y-directions, as illustrated by the series of points.
In all cases, the soundproofing wall is designed for isolating a first room on one side of the soundproofing wall from a sound source on the other side of the soundproofing wall. In Fig. 2, the soundproofing wall of Fig. 1 is shown in cross-section. An incident acoustic wave 14 is illustrated coming from the bottom of the figure, i.e. from a sound source on that side of the soundproofing wall. The figure illustrates a transversal displacement w(x,y) along the z-direction of the masses 12 and the baseplate 10 due to the incident acoustic wave 14. It can be seen that a deformation of the baseplate 10 is larger than a deformation of the strip-shaped masses, which have a larger bending stiffness than the baseplate 10.
Figure 3 shows a diagram illustrating the sound transmission loss TL measured in dB for a frequency range from 100 Hz to 1,000 Hz for a soundproofing wall as shown in Figs. 1 and 2 as compared to a mass-equivalent homogeneous plate. The sound transmission loss of the inventive soundproofing wall calculated in a mathematical model is depicted as a solid line. It shows a broad peak at about 250 Hz. The calculation is compared to a measurement with a sample having an overall size of 1 m x 1.2 m, depicted by the small circles. Both the calculations as well as the experiment were based on a baseplate of PET having a thickness h_b of 0.1 mm, a mass density ρ_b of 1,400 kg/m³, a Young's modulus E_b of 3500 MPa, and a Poisson's ratio v_b of 0.4. The strip-shaped masses 12 were made from strips of polyvinyl-chloride foam (PVC-F) with a height h_M of 5 mm and a width b of 50 mm, arranged in distances l of 20 mm, leading to a unit cell width a of 70 mm. PVC-F had a mass density ρ_M of 460 kg/m³. The total surface mass density of the plate-type acoustic metamaterial is 1.8 kg/m². One can see that the calculations and the measurement are in good agreement.
As compared to the results for the mass-equivalent homogeneous plate, depicted as a dashed line (calculated) and as little squares (measured), the sound transmission loss of the soundproofing wall is much larger (about 20 dB) in the relevant frequency range.
Fig. 4 illustrates the influence of the ratio D_M /D_b of the bending stiffnesses of the strip-shaped masses to the bending stiffness of the baseplate on the frequency response. For large ratios, such as 43,000 and 500, the sound transmission loss shows only one prominent peak shifting to lower frequencies with decreasing ratio. For smaller ratios, such as 100 and 50, the curve shows an additional second peak at a lower frequency.
Figures 5 to 10 show various examples of how the strip-shaped masses may be arranged. Only the masses 12 themselves are shown in a schematic top view. In the four examples of Fig. 5, the strip-shaped masses are arranged in parallel rows, but have different lengths.
Figure 6 illustrates that the strip-shaped masses 12 may have widths and spacings that vary over their length.
Figure 7 includes a regular pattern of nested squares, each formed by one strip-shaped mass 12. In the middle of each set of nested squares, the arrangement includes one smaller square-mass 16, i.e. an additional mass that is not strip-shaped.
In the example of Fig. 8, a regular pattern of spirals is shown, each spiral formed by a strip-shaped mass 12.
Fig. 9 shows four examples of arrangements of strip-shaped masses 12. In the first example to the left of the figure, a single strip-shaped mass 12 forms a spiral, wherein a width of the strip-shaped mass 12 is increasing from the centre towards an outer region of the spiral. The second example shows a spiral formed by a plurality of strip-shaped masses 12. The third example shows a spiral formed by a plurality of strip-shaped masses 12 as well, wherein these are interconnected to each other by narrower sections 18. The fourth example, to the right of the figure, shows a plurality of straight, strip-shaped masses 12 arranged in a rectangular pattern.
Figure 10 shows another regular pattern of strip-shaped masses 12 of a more complex arrangement, including a plurality of straight strip-shaped masses 12 of different lengths as well as a plurality of strip-shaped masses 12 forming spirals and nested squares.
Figure 11 shows another soundproofing wall in cross section, with unit cells of a width a including two strip-shaped masses 12 each. The dimensions of the strip-shaped masses 12 are identical, but their spacing varies.
Figure 12 shows yet another soundproofing wall in cross section, with unit cells of a width a including two strip-shaped masses 12 each. The dimensions of the strip-shaped masses 12 vary, wherein one of the two strip-shaped masses 12 has a larger width than the other one, while both have the same height.
Figure 13 shows yet another soundproofing wall in cross section, with unit cells of a width a including two strip-shaped masses 12 each. The dimensions of the strip-shaped masses 12 vary, wherein one of the two strip-shaped masses 12 has a larger height than the other one, while both have the same width.
Figure 14 shows a unit cell of yet another soundproofing wall, partly in cross section. The unit cell includes only one strip-shaped masse 12, which consists of four layers. These include a first mass layer 22 fastened on the baseplate 10 by means of an adhesive layer 20, a second mass layer 26 and an elastic layer 24 arranged between the first mass layer 22 and the second mass layer 26. The first mass layer 22 comprises a ferromagnetic material, the second mass layer 26 is a permanent magnet. The elastic layer 24 comprises a pressurised gas, in particular a foamed material. It may be attached to both mass layers 22, 26 by additional adhesive layers (not shown). The magnetic force between the mass layers 22, 26 leads to a certain level of compression of the elastic layer 24.
Figure 15 illustrates the "working principle" of the layered strip-shaped mass 12 of Fig. 14. The elastic layer 24 acts like a spring connecting the mass layers 22, 26. The adhesive layer 20 has elastic properties as well, leading to an elastic coupling between the baseplate 10 and the system of the two elastically coupled mass layers 22, 26.

List of reference numerals

10: baseplate
12: strip-shaped mass
14: incident acoustic wave
16: square mass
18: narrower section
20: adhesive layer
22: first mass layer
24: elastic layer
26: second mass layer

Claims

A soundproofing wall for isolating a first room on one side of the soundproofing wall from a sound source on the other side of the soundproofing wall, the soundproofing wall comprising a plate-type acoustic metamaterial with a baseplate (10) and a plurality of masses (12, 16) arranged on the baseplate (10), characterised in that at least one of the masses (12) is strip-shaped.
The soundproofing wall of claim 1, characterised in that the baseplate (10) has a stiffness.
The soundproofing wall of any of claim 1 or 2, characterised in that the ratio (µ) between a surface mass density of the at least one strip-shaped mass (12) to a surface mass density of the baseplate (10) is 3.5 or more.
The soundproofing wall of any of the claims 1 to 3, characterised in that the ratio between a bending stiffness D_M of the at least one strip-shaped mass (12) to a bending stiffness D_b of the baseplate (10) is in a range of about 50 to about 500.
The soundproofing wall of any of the claims 1 to 4, characterised in that the at least one strip-shaped mass (12) consists of a material having a higher mass density and/or a higher stiffness than a material of the baseplate (10).
The soundproofing wall of any of the claims 1 to 5, characterised in that a plurality of the masses (12) are strip-shaped and arranged in a regular pattern.
The soundproofing wall of any of claim 6, characterised in that the regular pattern comprises a plurality of unit cells each including at least one of the strip-shaped masses (12) and an adjacent, strip-shaped baseplate region in which none of the masses (12) is present.
The soundproofing wall of any of the claims 1 to 7, characterised in that the baseplate (10) has a thickness h_b and a longitudinal wave velocity c_b and two of the strip-shaped masses (12) are arranged in a distance l, wherein the materials used for the strip-shaped masses (12) and the baseplate (10) and their dimensions are selected such that the ratio between h_b times c_b and l ² is 30 per second or more.
The soundproofing wall of any of the claims 7 to 9, characterised in that each of the unit cells includes at least two of the strip-shaped masses (12), the at least two strip-shaped masses (12) in particular having different surface mass densities.
The soundproofing wall of any of the claims 1 to 9, characterised in that the plate-type acoustic metamaterial has a surface mass density of 5 kg/m² or less.
The soundproofing wall of any of the claims 1 to 10, characterised in that the at least one strip-shaped mass (12) is attached to the baseplate (10).
The soundproofing wall of any of the claims 1 to 11, characterised in that the at least one strip-shaped mass (12) comprises a first mass layer (22), a second mass layer (26) and an elastic layer (24) arranged between the first mass layer (22) and the second mass layer (26).
The soundproofing wall of claim 12, characterised in that the elastic layer (24) comprises a pressurised gas.
The soundproofing wall of claim 12 or 13, characterised in that a material of the first mass layer (22) and a material of the second mass layer (26) are selected such that these layers are pulled towards one another by a magnetic force.
The soundproofing wall of any of the claims 1 to 11, characterised in that the baseplate (10) and the at least one strip-shaped mass (12) are integrally formed from the same material.