EP2203728A2

EP2203728A2 - Sound absorber

Info

Publication number: EP2203728A2
Application number: EP08841320A
Authority: EP
Inventors: Frank Zickmantel
Original assignee: SilenceResearch GmbH
Current assignee: SilenceResearch GmbH
Priority date: 2007-10-24
Filing date: 2008-10-21
Publication date: 2010-07-07
Anticipated expiration: 2028-10-21
Also published as: CN101911179A; US8631899B2; WO2009053349A3; WO2009053349A2; RU2495500C2; RU2010117344A; EP2203728B1; DE102007000568A1; US20100307866A1; BRPI0818884A2

Abstract

To provide an inexpensive, slender sound absorber, it has a plurality of porous layers or regions of different densities or different flow resistances. Of significance are the boundary surfaces between the different, porous layers, which are accompanied by changes in impedance. Homogenized and adapted flow resistance conditions should be avoided. Although the thermal frictional effect in the porous material is desired, in particular to absorb higher frequencies, according to the present invention it only forms one element of the absorptive working mechanism. In addition, the effect known in physics as refraction is used. At the boundary layer between two materials of different density or different flow resistance there is an abrupt change in impedance. This leads to a phase shift of the sound wave, and so a sound absorbing effect is made possible. By contrast with exclusively porous absorber layers with homogeneously or steadily increasing flow resistances, frequently changing transitions and porous materials having different, respectively suitable, direct impedances allow much higher degrees of sound absorption to be achieved in the range of low frequencies, in particular between 100 Hz and 500 Hz.

Description

Schαllαbsorber

The invention relates to a Schαllαbsorber with the features of

Generic term, for example, known from the document DE 24 37 947 OS.

It is known that porous materials are suitable for damping rooms. Typical building materials can be found, for example, in acoustic ceilings. Here, the adaptation ratio of a porous so-called λ / 4 - absorber according to

800 <Ξ ^* d <2400 Pa * s / m to achieve at least 80% sound absorption. One

Body moving at a speed relative to a gaseous or liquid medium undergoes a flow resistance in the form of a force opposing the direction of movement, represents the length-specific flow resistance and d the thickness of the absorber. The flow resistance of the porous absorber must therefore be selected that the sound wave can penetrate into this and the forced by the airborne sound particle motion is damped by friction in the material structure of the absorber. Excessively high flow resistances lead to reflection at the front layer of the absorber, whereas too low a penetration of the absorber without frictional losses.

Porous sound absorbers usually have a homogeneous, sound-absorbing layer. But there are also wedge-shaped structures, for example, for lining low-reflection rooms. Wedge-shaped structures are achieved by - to the space boundary surfaces - homogeneous rising flow resistance. The mixing ratio of air to fiber material, which forms the porous material, then grows steadily in the direction of the space boundary surface. The aim is to achieve uniformly high sound absorption over the entire frequency range. It is also possible to realize a wedge-shaped structure with the aid of foams in a simple manner approximately. It is known to thread fibrous or porous cubes in the wall towards increasing size and density of vertical wires. Between the individual distances are provided in this known solution.

Various foams could also be arranged in layers in order to realize a wedge-shaped structure, wherein from layer to layer in the direction of the space boundary surface the amount of material could increase and the pores in the material could decrease. From layer to layer, attention should then be paid to adapted flow conditions in order to minimize sound reflections at boundary layers and thus to approach the ideal wedge-shaped structure. The input impedances of the different layers would then be similar.

From the publications "Mechel, F. (1 995) Sound absorber volume 2, internal sound fields, structures. Hirzel Verlag Stuttgart - Leipzig "as well as" Mechel, F. (1 998) Sound absorber volume 3, applications Hirzel publishing house Stuttgart - Leipzig "is well-known how an entrance impedance of a porous sound absorber can be determined before reverberant rear wall. In particular, for the absorption of low frequencies enormous depths of the existing porous material absorber due to the long wavelengths are needed because most energy can be converted when the absorption material can intervene in the fast maximum of the sound wave at λ / 4 of Figure 1. Therefore, a considerably larger volume must already be taken into account for the technical interior design during the shell construction planning so that in an extreme case only half of the usable volume is available through the use of porous materials.

In commercial interior design, cost optimization comes first. In order to save costs, today shell heights of building floors are already reduced, so that often acoustic ceilings with insufficient slope height must be installed. this leads to inevitably to development approaches of sound absorbers, which have a large absorption coefficient up to low frequencies even with significantly reduced depth.

From the document DE 295 02 964 U l a sound absorber is known which comprises porous material consisting of fibers. The fibers may be made of plastic or metal. Porous material to be absorbed with the sound, but may also consist of other materials such as foams, as the document DE 402751 1 C l can be seen. It is essential that it is an open-pored system. The sound should be able to penetrate into the porous material and be converted here into heat.

The longer the wavelength of sound, the greater the depth of such an absorber must be in order to absorb even low frequencies successfully. In order to absorb even low frequencies, a large volume of such a sound absorber is required, as DE 402751 1 C l can be seen. It must then be used relatively thick absorber. On the one hand, this reduces the available space. On the other hand, such absorbers are relatively expensive, since a relatively large amount of material must be used.

In order to still be able to absorb broadband and, above all, low frequencies with small depths, it is proposed, according to DE 402751 1 C1, to provide a hybrid sound absorber which, in addition to a conventional, passive absorber, comprises an electronic system with which sound is attenuated. So it is operated a technically high cost, which also requires a power supply.

From the documents DE 41 1 3628 C2 and DE 2408028 Al sound absorbers emerge, the porous material having closed pores include.

In order to avoid a large volume of construction, alternatively so-called plate resonators are used. Such a plate resonator is in the Publication DE 1 021 31 07 Al described. The plate resonator known therefrom comprises a swingably mounted metal plate. The principle is based on the fact that the plate is set in motion, so that sound is converted into kinetic energy of the plate. Behind such a plate, a damping medium, such as air or other damping material is arranged. Here, the kinetic energy of the plate is converted into heat. Corresponding frequencies are absorbed in accordance with the set resonance frequency of such a plate resonator. It succeeds so to be able to absorb low frequencies despite low depth. However, such a plate resonator only absorbs certain frequencies according to the set resonance frequency. In addition, the plate resonator is relatively expensive due to the metal plate.

In order to absorb in a plate resonator in addition to low frequencies and high frequencies, plate resonators are combined, for example, with foam materials, as can be found in the document WO 96/26331 Al. The plate resonator is then adjusted so that low frequencies are filtered out. The high frequencies are filtered out by the porous material. Although in such a solution a relatively large spectrum of frequencies is absorbed. However, an additional cost of materials is required, which causes costs and increases the space requirement.

Alternatively, so-called Helmholtz resonators are used. These include a perforated plate with a volume behind. It takes a relatively large volume of air behind a perforated plate to absorb low frequencies can. A Helmholtz resonator therefore consumes a relatively large amount of space. Also, a single Helmholtz resonator can only absorb a set relatively small frequency range. A Helmholtz resonator is apparent from the document DE 891 61 79 U l or from the document EP 1 5701 38 Al. Instead of perforated plates are also used in a Helmholtz resonator plates or films with micropores, as is known from the document DE 1 01 51 474 Al. There is an additional absorption at the edges of the micropores. This improves the effect of a Helmholtz resonator.

From the document DE 7427551 U a sound absorber is known which comprises two different porous materials. One of the two porous materials is chosen so that the sound absorber is mechanically stable. The second porous material is chosen so that it is particularly inexpensive. So the manufacturing costs should be reduced. As before, this solution has the problem of having to provide a high depth, in order to absorb even low frequencies can.

It is an object of the invention to provide a low-cost sound absorber, which is able to absorb sound broadband sound despite low depth and, above all, low frequencies.

The object of the invention is achieved by a sound absorber having the features of claim 1. Advantageous embodiments emerge from the subclaims.

To achieve the object, a sound absorber is provided which has a plurality of porous layers or regions. No air gaps remain between the porous layers or areas. The transition from a porous layer to an adjacent porous layer is accompanied by an impedance discontinuity. This means that the input impedance or the input resistance of a porous region is different from the

Input impedance of an adjacent porous region so clearly different that in this way low frequencies below 600 Hz, preferably below 500 Hz are absorbed. In particular, will Sound with a frequency of less than 600 Hz at least 50%, preferably at least 80% absorbed.

In one embodiment of the invention is achieved so that at least 50% of the sound with frequencies in the particular region of interest between ca. 200 to approx. 700 Hz is absorbed, preferably at least 80%. This information applies throughout the entire frequency range mentioned. Preferably, sound is absorbed with all audible frequencies from 250 Hz to at least 80%. In particular, this succeeds even with a maximum of 1 0 cm thick, surface mounted on a wall or ceiling claimant absorber.

Apart from a housing for the porous layers or areas, the claimed absorber comprises in one embodiment, no other components such as plates and the like.

If the sound propagation velocity in a porous layer is different compared to the adjacent porous layer, then there is an impedance discontinuity.

A different sound propagation velocity in different porous layers or a different input resistance is regularly present when the densities, the flow resistances or the porosities of two porous layers or regions are different. If a porous layer differs from another porous layer only by the density, porosity or flow resistance, then the two porous layers necessarily have a different input resistance. Other parameters such as compression hardness and tensile strength of a porous layer also affect the input impedance.

The greater an impedance jump, the deeper the frequencies absorbed as a result of the impedance jump. So important are the Interfaces between the different, porous layers, which are accompanied by sudden changes in the input resistances.

Although a thermal friction effect in the porous material is desired, especially to absorb even higher frequencies. The thermal frictional effect, which forms the basis of conventional porous sound absorbers, is according to the invention but only one column of the absorptive mechanism of action. Above all, the effect known in physics as refraction is exploited. At the boundary layer between two materials of different input resistances, for example due to a different density or different flow resistances, an impedance jump occurs. This leads to a phase jump of the sound wave, so that a sound absorption effect is made possible. In frequently changing transitions and porous materials, each with suitably different input resistances, in contrast to exclusively porous layers with homogeneous or steadily increasing input resistances, significantly higher sound absorption levels in the range of low frequencies can be achieved, in particular between 200 Hz and 700 Hz.

An absorber according to the present invention thus consists of at least two, preferably at least three, porous layers or regions which are different. It is essential that the boundary layer between the layers or areas be such that they are connected to an impedance discontinuity. The impedance jumps are suitable to choose large, in order to absorb low frequencies well.

However, an impedance jump may not be so great that sound no longer passes from one material to the other material. Regularly a large impedance jump is achieved when the densities of two adjacent porous layers or regions differ widely, preferably by at least 20 kg / m ³ or when the flow resistances differ widely, preferably by at least 5 kPa s / m ² , The present invention abandons the idea of uniformly absorbing a frequency spectrum. The problem is the low frequencies. To absorb high frequencies is relatively easy and inexpensive. By or the impedance jumps can be achieved that low frequencies can be absorbed very well. The greater an impedance jump, the lower the frequencies that can be absorbed.

The provision of an impedance jump is in contradiction to the prevailing opinions known from the prior art: Accordingly, care must be taken with different porous materials to minimize differences in input impedances to minimize reflections at boundary layers, so as to achieve good absorption results.

Preferably, a sound absorber according to the present invention consists of several different porous layers or regions, so that different sized impedance jumps occur. This ensures that low frequencies are absorbed broadband. There are several different layers with boundary layers that are always the same

Show impedance jump, the absorption effect is amplified in relation to a frequency or a narrow frequency band. If there are different impedance jumps, ie impedance jumps that are different in size, the spectrum that is absorbed as a result of the impedance jumps is broadened.

It is possible, low frequencies and especially the most interesting frequencies of approx. 200 to approx. 700 Hz with a mere 1 0 cm thick system to absorb well. Moreover, since conventional porous material is provided, even higher frequencies are well absorbed by a claimed sound absorber. Overall, this achieves a broadband sound absorption, with which especially the low frequencies are absorbed even at depths of only 1 0 cm. As a particularly suitable porous material PU foams have been found that are different porous and different density. Half-closed PU foams can also be used. A semi-closed porous material has both open and closed pores. These are mainly PU foams based on polyester or polyether with variable cell structure, compression hardness, density, air permeability and tensile strength.

It is particularly preferred to use only foams but not fibrous materials for providing porous material. Foams have the advantage of having a rigid skeletal structure. Overall, such a rigid skeleton structure, it is additionally excited to vibrate. This causes additional absorption.

It is advantageous, initially porous material with a relatively high

Provide input resistance where the sound enters the absorber. Such an entry region usually comprises openings through which the sound can pass into the porous material. The entrance area may be formed by a plate or foil with holes or a perforation. This is adjacent to the material with the relatively large input resistance. Behind this, there is one or more porous areas with lower input resistance.

For example, a sound absorber for this reason at the beginning of the absorber to a semi-closed porous material. Fully open-pore materials are then spatially located behind the semi-closed porous material. The desired absorption of low frequencies is achieved particularly well.

The various porous layers or regions are preferably pressed together in the claimed sound absorber. To press the porous layers or areas together, they are housed, for example, in a suitably sized box or housing. The box or the housing is at an entrance side for Sound sealed with a porous or holey surface. The porous layers are then under pressure and thus pressed in the box.

By pressing pressure is achieved that the skeletal structures of the individual porous layers oscillate against each other. As a result, an additional sound absorption effect is achieved.

In order to further optimize the sound absorption, a box or housing is provided which is acoustically permeable not only from a front side, but also laterally, so that sound can also easily penetrate laterally into the porous material. Thus, edge diffraction effects are exploited, which additionally provide for absorption. The sound absorption is further optimized.

Particularly preferred is an embodiment of the invention, in which are provided in a box or housing front and side holes for the penetration of sound waves. In particular, in such a case, porous layers are preferably not only stacked on top of each other, but also laterally against an already existing layer system. Here again attention is paid to large impedance jumps. This ensures that sound which penetrates laterally into a box is absorbed not only due to edge absorption, but also as a result of phase jumps on boundary layers.

In one embodiment of the invention, the porous system consists of a plurality of cubes, cuboids or the like, which are adjacent to each other and one above the other. The materials of the cubes etc. are chosen such that large impedance jumps between the boundary layers in the sense of the present invention are present at least regularly. This ensures that sound that moves through the porous material is constantly confronted with large impedance jumps. Irrespective of the angle at which or from which side sound penetrates into the absorber, it always passes through boundary layers with large impedance jumps. This allows variable geometries of the absorber. Its shape can then be adapted to the shape of niches and the like.

The invention will be explained in more detail with reference to the figures.

Figure 1 is intended to illustrate why porous absorber according to the prior art must have a high overall depth in order to absorb even low frequencies satisfactorily. The dotted line a) shows the wavelength of a sound wave having a low frequency, which strikes a space boundary surface 2 after passing through a porous layer 1. The maximum speed of sound lies outside of the porous layer 1 acting as a sound absorber. The low frequency is hardly absorbed. At higher frequencies or shorter wavelengths, finally, the fast maximum 3 lies within the porous layer 1, as the dashed line b) illustrates. Sound with the wavelength b) is therefore absorbed optimally. From this it is clear that a porous absorber must be very thick or must have a large overall depth if the absorption is based solely on the porosity of the material 1 and also low frequencies are to be absorbed.

FIG. 2 shows a first embodiment. In the front entrance area for sound, there is a porous absorber layer 1a (ie a region of porous material) with large open pores. The input resistance is therefore small. Behind and to the side is a porous absorber layer 1 b with small pores. The input resistance of this absorber layer is large. Between the front layer I a and the underlying layer 1 b therefore occurs an impedance jump, with which an absorption of low frequencies below 500 Hz is achieved. In the direction of the wall behind the layer I b with the small pores again a layer I a is present, which has large pores. This is followed by a layer I c with medium-sized pores and a medium input resistance. Behind it is again a layer I b, which has small pores and which is adjacent to a wall 2. There are thus four impedance jumps in horizontal and two impedance jumps in the vertical direction. All impedance jumps cause absorption of low frequencies between 1 00 and 500 Hz. With such a structure, it is therefore possible to be able to absorb even low frequencies of 1 00 Hz to 500 Hz well.

Figure 3 shows another structure of the various aforementioned porous layers 1 a, 1 b and 1 c, which are pressed against a wall 2 by a housing, not shown. In such cases, but sufficient for attachment already a plate which is anchored, for example by means of rods in the wall. If, as in the case of FIG. 3, sound can enter through the plate, then the plate is provided with bores. The porous layers are arranged exclusively parallel to the wall 2. The entry region begins with a layer 1 b, which is provided with small pores and has a greater input resistance or input impedance compared to the layers I a and 1 c arranged behind it in the direction of the wall.

FIG. 4 shows a further possible embodiment. The various porous layers I a, I b and I c lie horizontally one above the other and are pressed against a wall 2. In this case, it is favorable if the sound (also) can enter from above and / or below in the porous layers, since then the sound is particularly reliably passed through many different boundary layers with impedance jumps. Such an embodiment is preferable when, for example, a sound absorber is to be placed behind an object such as a cabinet, since in such an arrangement, entry from the front is obstructed by the object.

Figure 5 shows an embodiment in which the absorbing region consists of a plurality of porous rectangles 1 a, 1 b and 1 c, which are arranged one above the other and next to each other so that in each

Direction a plurality of impedance jumps occurs. Regardless of which side of the sound enters, it always passes through a plurality of boundary layers, at which impedance jumps occur that lead to the absorption of low frequencies. Such a structure is also for an accommodation in niches particularly well suited. A corresponding housing, in which the porous rectangles are located, is then preferably designed such that sound can enter the housing from the front, from both sides, from above and from below. However, it can also satisfy an anchored plate again to fix the porous areas and visually shield.

Figure 6 illustrates a particularly preferred embodiment, which is arranged behind a cabinet 4. The various porous areas I a, I b and I c are vertically aligned, adjacent to a wall 2 and reach to the bottom on which the cabinet 4 stands. If sound penetrates laterally as indicated by the arrows 5 into the porous regions I a, I b or I c, then the sound passes through interfaces with impedance jumps which cause the absorption of low frequencies. If the sound penetrates from above along the arrow 6, sound does not necessarily pass through

Interfaces with impedance jumps. But the distance to reach the bottom is very long, so that then low frequencies are absorbed for this reason. With such a structure, a special case can be omitted because the porous portions can be fixed to the back of the cabinet.

The claimed sound absorber is used, for example, in modern interior design. Especially in the age of increased communication needs and high levels of telecommunications, human speech is the major source of disability at work. The optimization of the room acoustics of office administration or open-plan offices must therefore take place under the aspects of the human language spectrum.

FIG. 7 a shows the typical male and female speech spectrum of humans. It becomes clear that high sound pressure levels in the frequency range between approx. 100 and approx. 700 Hz occur, which can be extensively damped with the absorber according to the invention already at depths of 20 cm, but also of only 1 0 cm. FIG. 7b illustrates the perception of the human spectrum as a function of the masking threshold of 60 dB. Accordingly, it is especially in rooms where sound is generated by human voices as in open - plan offices or banks on it, sound with frequencies from approx. 200 Hz until at least approx. To absorb 700 Hz extensively. This is achieved by the claimed absorber and is even superior to a plate resonator in this particularly interesting frequency range.

Figure 8 shows an embodiment in which the various porous layers I a, I b, I c rest on a perforated, suspended ceiling 7, which is mounted below a ceiling 8 with suspensions 9.

Due to the slim construction depth, the sound absorber can be installed in partitions, but also on the front of furniture pieces in a particularly inconspicuous manner. It can be attached to walls or ceilings, such as behind perforated panels that are attached to the wall or ceiling and that press the various porous areas against a wall or ceiling. It can be installed in lintel areas or buildings, as its shape can be adapted very variably to the available space. It can be housed inconspicuously behind thermally functional wall or ceiling elements.

FIG. 9 shows results achieved with a sound absorber according to the invention in comparison to a plate resonator. The measurements were carried out in a reverberation room with statistical sound incidence according to DIN EN ISO 354. In statistical sound incidence, it is assumed that the sound pressure incident on a measuring microphone or on a boundary surface of all angles of incidence is the same and also independent of location.

Both sound absorbers were examined with the same dimensions and with the same spatial positioning. Also, the number and positions of the microphones for averaging the reverberation times were left the same. Consequently are relative measurement errors, for example, due to eigenmodes of the room almost impossible and a direct comparison of the sound absorbers possible.

Curve a) shows the measured result for a porous cover plate resonator, the structure of which is shown in FIG. The plate resonator shown in FIG. 10 comprises a porous covering layer 10 with a thickness of 0.03 m, a length-specific flow resistance of 4.7 kPas / m ² and a density of 20 kg / m ³ . Below the cover layer 10 is a metal plate 11 with a thickness of 0.001 m and a density of 7800 kg / m ³ . Below the metal plate is a porous layer 12 with a thickness of 0.07 m, a length-specific flow resistance of 11, 5 kPas / m ² and a density of 40 kg / m ³ arranged. The porous layer 12 adjoins a reverberant wall 13.

The other curve shown in FIG. 9 relates to a sound absorber according to the invention, the basic structure of which is shown in FIG. The sound absorber consists of five different porous foam layers 14, 15, 16, 17 and 18, which adjoin a reverberant wall 13.

Both absorbers, so both the plate resonator and the absorber according to the invention were housed in a same housing 19, which consisted of a sheet steel frame with small perforated front.

Curve b) in FIG. 9 illustrates the absorption as a function of the frequency for a sound absorber according to the invention with impedance jumps between the individual layers, the individual layers 14, 15, 16, 17 and 18 having the following properties:

14 porous layer with thickness = 0.02 m

Air permeability> 350 mmWS Density = 76 kg / m ³

Compression hardness = 9.00 kPa Tensile strength = 1 94 kPα

1 5 porous layer with thickness = 0.02 m air permeability> 350 mmWS

Density = 76 kg / m ³ compression hardness = 4, 77 kPa tensile strength = 47 kPa

1 6 porous layer with thickness = 0.02 m

Air permeability = 320 mmWS Density = 75 kg / m ³ Stiff hardness = 8, 81 kPa

Tensile strength = 21 1 kPa

1 7 porous layer with thickness = 0.02 m

Air permeability = 230 mmWS Density = 23 kg / m ³ Stiff hardness = 4, 36 kPa Tensile strength = 1 31 kPa

1 8 porous layer with thickness = 0.02 m air permeability 350 mmWS density = 75 kg / m ³

Compression hardness = 9.08 kPa tensile strength = 1 95 kPa

The air permeability represents a measure of the flow resistance. In contrast to the other layers, the layer 1 5 is not an open-pored foam, but a semi-closed one.

At very low frequencies below 1b of 1 40 Hz, the plate resonator (curve a) is still slightly superior to the sound absorber according to the invention. This changes however already from frequencies of approx. 1 50 Hz. In the area of the largest Sprachlasst, however, the absorber according to the invention is superior to the plate resonator, and in most cases very clearly. The absorber according to the invention can therefore not only cheaper compared be made for Plαttenresonαtor. It is also much better suited to absorbing in rooms such sounds caused by human speech. By the sound absorber according to the invention succeeded in absorbing the sound of more than 80%, even at low frequencies of less than 500 HZ.

Overall, sound is best absorbed in the frequency range of interest with the sound absorber according to the invention according to curve b). The cost of the inventive sound absorber according to the curves b) are compared to the plate resonator according to curve a) significantly lower, since no relatively expensive metal plate is needed.

Porous, homogeneously constructed sound absorbers with a thickness of 1 0 cm, in comparison, can not reach nearly as good absorption values as the investigated plate resonator according to curve a) and the sound absorber according to the invention according to FIG.

Claims

claims

1 . Schαllαbsorber with porous material for the insulation of sound, characterized by adjacent regions (I a, I b, 1 c) of the porous material, which differ by different input impedances, by different sound propagation velocities, different densities, different porosities and / or different flow resistance and / or where there is an impedance discontinuity between two adjacent regions (I a, I b, I c).

2. A sound absorber according to claim 1, characterized in that the densities of two adjacent areas (I a, I b, I c) of the porous material by at least 20 kilograms / cubic meter and / or the flow resistance of two adjacent areas ( I a, I b, I c) of the porous material differ by at least 5 kilopascals-seconds / square meter.

3. Sound absorber according to one of the preceding claims, characterized in that the adjoining, formed from porous material areas (I a, I b, I c) are such that there are at least two different interfaces between the areas (I a, I b , I c), which have different sized impedance jumps.

4. Sound absorber according to one of the preceding claims, characterized in that the overall depth of the sound absorber is less than 20 cm, preferably less than 1 0 cm.

5. Sound absorber according to one of the preceding claims, characterized in that the porous material is formed by foams and preferably by PU foams.

6. Schαllαbsorber according to any one of the preceding claims, characterized in that a region adjacent to the inlet region for sound in the sound absorber has a higher flow resistance in comparison to an adjacent, underlying porous area, which is located further away from the entrance area for sound.

7. Sound absorber according to one of the preceding claims, characterized in that the porous regions (I a, I b, I c) are pressed against each other.

8. Sound absorber according to one of the preceding claims, characterized by an inlet region for sound through a front side and through further lateral inlet regions for sound, wherein the inlet region is preferably formed by the front side by a perforated plate.

9. Sound absorber according to one of the preceding claims, characterized in that different regions (I a, I b, I c), which consist of porous material, are arranged one above the other and side by side.

1 0. Sound absorber according to one of the preceding claims, characterized in that from an inlet region for sound in the sound absorber into the opposite boundary surface of the sound absorber, the flow resistance does not increase steadily.

1 1. Sound absorber according to one of the preceding claims, characterized in that no air gap remains between two adjoining bordering regions (I a, I b, I c) of the porous material.

1 2. Sound absorber according to one of the preceding claims, characterized in that the porous material (I a, I b, I c) open pores and / or closed pores. 3. Schαllαbsorber according to any one of the preceding claims, characterized in that it is arranged behind a cabinet (4). 4. Sound absorber according to one of the preceding claims, characterized in that the different porous areas (I a, I b, I c) run vertically behind a piece of furniture (4) from the top of the piece of furniture to the floor and sound from above and from the Page can penetrate into these areas. 5. Sound absorber according to one of the preceding claims, characterized in that it rests on a suspended ceiling (7). 6. Sound absorber according to one of the preceding claims, characterized in that the or the existing impedance jumps are so large that sound at frequencies below 600 Hz, preferably below 500 Hz to at least 50%, preferably at least 80% is absorbed.