WO2005035950A1

WO2005035950A1 - Sound absorber

Info

Publication number: WO2005035950A1
Application number: PCT/DE2004/002269
Authority: WO
Inventors: Jörn HÜBELT; Cris Kostmann; Ulf Waag; Günter Stephani; Gerd Lotze
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2003-10-06
Filing date: 2004-10-06
Publication date: 2005-04-21
Also published as: EP1673521B1; DE502004007914D1; EP1673521A1; ATE405730T1; DE10347226B3

Abstract

The invention relates to a sound absorber that can be used to absorb sound waves transmitted by fluids. The aim of the invention is to obtain a high maximum absorption at individual frequencies with high amplitudes, and absorption of sound waves in a spread frequency spectrum, in a simple manner. To this end, the inventive sound absorber is formed, at least in certain areas, from hollow balls that are interconnected in a material fit and/or hollow cones in bulk. Furthermore, regions having different sound speeds, and a characteristic impedance and/or thickness, are adjacently arranged in the flow direction of a fluid, in a laterally successive manner or in relation to sound wave incidence angles.

Description

sound absorber

The invention relates to sound absorbers that are suitable for absorbing sound waves that are transmitted with fluids. It can be used for a wide variety of applications, with adaptation to different frequency spectra possible. It can be used, for example, on gas compressors. However, it can also be used on containers or parts of containers that contain liquids (e.g. oil sumps of internal combustion engines).

In addition to mineral fibers, which have hitherto been used predominantly for the absorption of sound waves, DE 199 49 271 A1 has pointed out possibilities for the use of metallic cavity structures using metallic hollow spheres. For a broadband absorption effect in silencers with such structures, it is pointed out that with such a structure, differently dimensioned cavities can be layered. For example, larger cavities should be arranged closer to an exhaust gas stream as carriers of the sound waves than smaller cavities. As an alternative, however, it is also pointed out that differently dimensioned cavities can also be homogeneously mixed within the cavity structure.

However, it has been shown that the broadband effect of these silencers can be increased in this form, but this increase has limits. So that a reduction in the sound level can be achieved to a still limited extent and in a restricted frequency band.

Furthermore, such structures can only be produced at great expense and expense. In addition, there are limits to the design, so that many designs are difficult or impossible to implement. A modular design of such silencers to adapt to different conditions (excitation frequency spectra) is not possible. >

It is therefore an object of the invention to propose a possibility for the absorption of sound waves in which a high maximum absorption at individual frequencies with high amplitudes can be achieved in a simple manner with the simultaneous absorption of sound waves in a broadened frequency spectrum.

According to the invention, this object is achieved with a sound absorber which has the features of claim 1. Advantageous configurations and further training fertilize the invention can be achieved with the features indicated in the subordinate claims.

The sound absorber according to the invention is essentially formed from hollow spheres, the actual shells of which are formed from an inorganic material, such as a metal, a metal alloy or also a ceramic. There are areas in which such hollow spheres are integrally connected. The connections can have been formed by means of an adhesion promoter, by soldering or combined. The cohesive connections can preferably be formed punctually between adjacent hollow spheres, so that free spaces then remain between adjacent hollow spheres.

Alone or in addition, however, hollow spheres can also be present as a loose bed on a sound absorber according to the invention.

Such a sound absorber is characterized in that, with regard to the achievable sound absorption, different areas are arranged laterally one after the other in relation to the flow direction of a fluid. Such areas can also be arranged next to one another, in which case the sound waves can strike the surfaces of these differently designed areas. Angles of incidence between 0 and 90 ° are possible

Sound waves should hit the surfaces of at least several areas at the same time. The latter is especially true for stationary media.

These areas can differ from one another as a result of sound speeds, thicknesses and / or cha- distinguish characteristic impedance.

The respective width of these areas should be smaller than the smallest sound wavelength, preferably smaller than λ / 4.

It is advantageous to provide areas formed from hollow spheres which, with preferably the same or different hollow spheres (diameter, shell thickness, porosity, material), have different thicknesses in the direction of propagation of sound waves within the sound absorber. This can be a regular, but preferably an irregularly changing arrangement of such areas.

For example, a surface of the sound absorber may have surface contouring as a result of the different thicknesses of regions. Such a surface can be wave-shaped, sawtooth-shaped, meandering. Irregular elevations and corresponding depressions, with different heights and depths, can preferably be present on such a surface contour.

The surface of a sound absorber opposite a contoured surface can also be contoured, but can also be designed as a flat surface. In this context, a flat surface is also to be understood as a surface whose surface properties, the respective hollow spheres are taken into account, that is to say a certain degree of roughness is permissible.

If two oppositely arranged surfaces with a surface contouring, as explained in more detail above, are connected to a sound provided that the respective fluid flows on these two surfaces, or a fluid flows along these surfaces, or sound waves strike the sound absorber on these surfaces. In this case, a two-sided sound absorption effect can be achieved.

With such a surface contour, it can also be achieved that the sound waves hit the respective surfaces with different angles of incidence.

However, there is also the possibility of designing the surface in such a way that regions are each inclined at the same or different angle with respect to the direction of flow of a fluid, the angle of incidence of sound waves or in relation to a plane. The sound can thus strike a larger surface more directly with a larger angle of incidence and, at the same time, locally different thicknesses can be formed in the lateral direction of a sound absorber in such an area.

In addition to the angles of inclination of such inclined surface regions, their respective surface can also be varied by such regions being of different widths.

The areas of a sound absorber can also be formed laterally one after the other from different hollow spheres. This applies to the outside diameter, shell thickness, porosity, materials and / or the respective material connection.

This fact can also apply to the respective individual areas so that different composites of hollow spheres can also be present in layers in one area. In this case, a layer area can also be formed from a loose bed of hollow spheres. The hollow spheres which are not connected to one another should then be covered at least from one side by means of a layer region which is formed from hollow spheres which are integrally connected to one another for stabilization.

A sound absorber can also consist of several their sound absorption effect different individual elements arranged one after the other are formed, which can be connected to each other but need not.

In this case, it is advantageous to use annular, truncated cone-shaped and / or with an inner cone, through which the respective gas can flow in the longitudinal direction of the sound absorber. Such internally hollow elements can, however, also have an inner convex or concave surface, which the respective gas can flow past.

Such elements can have different wall thicknesses, cone or cone angles, different widths. But they can also have different widths.

With such individual elements, sound absorbers can be assembled modularly and adapted for different applications.

For example, there is the possibility of circular elements with predefined inside and outside diameters and to combine different sound velocities and characteristic impedances with one another by inserting at least two such rings into one another to form a region of a sound absorber, so that one forms an inner ring and the other an outer ring. The rings have a different hollow sphere structure, thus forming an interface with different speeds of sound on both sides.

By lining up such differently composed areas of different individual elements, a sound absorber tuned to a certain frequency spectrum can be made available in a modular manner, which is certainly advantageous for individual use or small series or retrofitting.

However, there is also the possibility of arranging individual elements at a distance from one another, so that cavities are present which form quasi “gaps”, which in turn can be dimensioned in such a way that sound waves with suitable wavelengths also partially enter such a “gap” can and only hit an effective surface of the sound absorber in the "gap area".

Openings can also be formed on a sound absorber or a plurality of areas of a sound absorber, through which sound waves pass and where appropriate can strike a correspondingly arranged reflecting element.

In analog form, however, a reflecting element can also be formed and can be arranged discretely to one another at this “gaps” or openings. The dimensioning, shape and arrangement of such cavities ("gaps") or openings can be optimized taking into account selected sound wavelengths.

Such an optimization is also possible for the sound absorber as a whole, so that a maximum at least increased sound absorption can be achieved for several specific sound wavelengths, which are particularly critical in the respective frequency spectrum of the gas flow.

Appropriate interferences can be exploited for this purpose. So that for selected wavelengths taking into account the respective speed of sound in a region of the sound absorber or a gap between the sound absorber and a reflecting element arranged at a distance from it, the path of the sound waves through such a region, a layer region of such a region or the gap between the sound absorber and the reflecting element is taken into account as an integral multiple of λ / 4 of such selected sound wavelengths.

However, as already indicated, at least one sound reflecting element can also be present on a sound absorber according to the invention.

Such an element can be integrated in the sound absorber, so that it is enclosed on both sides by hollow spheres. In this case, a reflecting element can have a flat surface and the hollow spheres can be layered in different thicknesses in regions. However, there is also the possibility of using a reflective element designed in a different form in this form. For example, a stepped, corrugated, sawtooth-shaped or meandering element can be used, so that locally different thicknesses / spacings from hollow spheres to outer surfaces that are in contact with the respective gas flow result.

A reflective element can, however, also be arranged at a distance from the actual sound absorber on one side, which is preferably not in contact with the gas flow. In this case, a gap can be formed between the sound absorber and the reflecting element. The respective gap distance does not have to be constant, rather it is often favorable to provide for a changing gap distance between the reflecting element and the sound absorber. This can be achieved by a corresponding shape of a reflecting element and / or the surface of the sound absorber opposite the reflecting element, the shapes already mentioned elsewhere also being suitable here.

However, reflecting elements can also be a plurality of individual parts arranged discretely to one another.

A reflection effect can also be achieved by means of interfaces within a sound absorber. Such interfaces separate layer areas of the sound absorber with higher from layer areas with significantly lower sound velocity. Such interfaces can have different distances from the outer surfaces of the sound absorber, in particular from surfaces that are in contact with the fluid in the lateral direction.

Interferences can also be used advantageously in the case of deliberately caused reflections of sound waves by means as explained in advance.

On a sound absorber according to the invention, however, one or more areas can also have been mechanically compressed. For this purpose, a compressive force can be exerted locally and a compression of hollow spheres can be achieved.

For this purpose alone or in addition, surfaces of areas can also be machined, as a result of which surface contours are deliberately formed and / or shells of hollow spheres are partially removed.

In the case of the sound absorbers according to the invention, different sound velocities and characteristic impedances can be set in specific areas by the respective hollow sphere structures. In addition to the variation options already mentioned, the completely spherical spherical shape can also be replaced by a rounded cylindrical or elliptical shape, at least within areas of a sound absorber. Another parameter is the specific surface within areas, which in addition to the cavity volume, the wall thickness of the shells, the porosity, the formation of the surface of the balls in a more or less roughened form can also play a role.

The invention will be explained in more detail below with the aid of examples. Show:

Figure 1 shows an example of a sound absorber;

Figure 2 shows an example with an integrated reflective element;

Figure 3a shows an example with differently designed areas;

Figure 3b an example with differently designed areas with a different orientation and

Figure 4 shows an example with a contoured surface.

For the following explanation of the figures, it should be preceded that they have been shown in a highly schematic manner.

In the example shown in FIG. 1, a plurality of regions 1.1 to 1N were used on a sound absorber and arranged one after the other in the direction of flow of the gas.

Here it has a flat, relatively smooth surface, which sound waves hit at an obliquely inclined angle.

The individual areas 1.1 to IN are each shown in part in white and in the other part in black. The two differently colored parts each form layer areas of one of the areas 1.1 to 1N, each with a different speed of sound and characteristic impedance. The thicknesses of the layer areas are in the Ranges 1.1 to lN different. So that in each area 1.1 to 1N the path covers the sound waves through the layer areas shown in white until it strikes the interface with the respective black layer area is of different sizes.

If the "black" layer areas are configured so that the characteristic impedance is greater than the "white" layer areas, the interface has a sound-reflecting effect.

However, the layer regions shown in black can also be compact, that is, they cannot be formed from hollow spheres, so that they can also represent a reflective element.

The widths of the individual areas 1.1 to 1N seen in the direction of flow can also be of different sizes, which cannot be readily derived from the illustration in FIG. 1.

In the example of a sound absorber shown in FIG. 2, a reflecting element 2 has been integrated into this.

A fluid can flow along two opposing surfaces and sound waves can fall on the surfaces again, as indicated by the arrows, at an obliquely inclined angle.

The reflective element 2 is shaped here in such a way that in the different areas 1.1 to IN there are different distances between the outer surfaces, which the sound waves directly hit, and the reflective element 2. Accordingly, the sound travels a different long path within the areas 1.1 to 1N until it is reflected, so that, assuming a constant speed of sound in all areas 1.1 to 1N, a differently long time for the sound waves to travel through an entire layer area or to reflect back Sound waves reach the surface again.

FIGS. 3a and 3b show examples with areas 1.1 to 1N, each of which also have more than two layer areas. The number, the arrangement and the thickness of the layer areas in the areas 1.1 to 1N are different.

The areas 1.1 to 1N have layer areas with different sound velocities. These layer areas can be formed from mutually different hollow spheres or hollow sphere composites. The size and arrangement of the actual cavities in the layer areas can be of different sizes. The hollow spheres are firmly bonded to one another in regions 1.1 to 1N or hollow spheres are present as a loose bed in layer regions.

In the examples shown in FIGS. 3a and 3b, the arrangement of the regions 1.1 to IN with the respective layer regions has been chosen to be the same. Only the alignment of the areas 1.1 to 1N at an obliquely inclined angle against the direction of incidence of sound waves represents a difference that leads to an extension of the possible path for sound waves through such an area leads to the opposite surface. In addition, at interfaces suitable for sound wave reflection at adjacent regions 1.1 to 1N, such reflections can be used for increased absorption.

The example shown in FIG. 4 uses areas 1.1 to 1N, which are arranged on a flat, flat carrier, which represents a reflecting element 2. The regions 1.1 to 1N formed from hollow spheres each have different thicknesses, so that there is a contoured surface on which the sound waves strike. The sound waves must then cover different distances in the areas 1.1 to 1N until they reach the reflecting surface of the reflecting element 2.

As shown here, reflective element 2 and regions 1.1 to 1N can be directly connected to one another or be in touching contact.

In a form not shown, however, there can also be a gap between the reflecting element 2 and the corresponding surface of the regions 1.1 to 1N, which represents a constant distance. However, as already mentioned in the general part of the description, there is also the possibility of designing the reflective element 2 or the surfaces of the areas 1.1 to 1N facing it, such that the distances are locally differentiated.

FIG. 4 also indicates a possibility for an advantageous design of sound absorbers in the areas 1.6 and 1.8. There the surfaces hit the sound waves are oriented at an obliquely inclined angle. This means that the surface hits the sound waves and the angle of incidence is increased. The refraction and reflection of the sound waves incident there change in particular through an enlarged angle of incidence.

Such inclined surfaces can be present in all areas 1.1 to 1N or only in a few selected areas.

Claims

claims

1. Sound absorber, which is formed at least in regions from cohesively connected hollow spheres and / or a loose bed of hollow spheres, characterized in that regions (1.1 to 1N) on the sound absorber with a differing speed of sound, characteristic impedance and / or thickness in Flow direction of a fluid, laterally one after the other or arranged next to each other in relation to sound wave incidence angles.

2. Sound absorber according to claim 1, characterized in that regions formed from hollow spheres (1.1 to lN) have a flat surface, impinge on the sound waves and regulate the thickness of regions formed from hollow spheres (1.1 to lN) in the direction of propagation of the sound waves within the sound absorber - or changes irregularly.

3. Sound absorber according to claim 1, characterized in that a sound wave reflecting element (2) is present.

4. Sound absorber according to claim 3, characterized in that the reflective element (2) is arranged within the sound absorber and is shaped such that the distances to (r) surface (s) on the sound waves vary.

5. Sound absorber according to claim 3, characterized in that the reflective element (2) in a distance from the sound absorber forming a gap is arranged.

6. Sound absorber according to claim 5, characterized in that the reflecting element (2) and / or the respective surface of the sound absorber is shaped such that the distances are varied in the lateral direction.

7. Sound absorber according to one of the preceding claims, characterized in that the surfaces of areas (1.1 to l.N) are oriented at an obliquely inclined angle with respect to sound wave angles of incidence.

8. Sound absorber according to one of the preceding claims, characterized in that the series (1.1 to l.N) are each formed from hollow spheres with different diameter fractions.

9. Sound absorber according to one of the preceding claims, characterized in that the series (1.1 to l.N) are designed as annular elements arranged one after the other.

10. Sound absorber according to one of the preceding claims, characterized in that cavities are present between and / or at regions (1.1 to 1N).

11. Sound absorber according to one of the preceding claims, characterized in that a reflective element (2) is formed from a plurality of individual elements and / or openings are formed in the reflective element (2).

12. Sound absorber according to one of the preceding claims, that the areas (1.1 to 1N) as frustum-shaped, frustoconical, annular elements through which a gas flow can flow, elements which are provided with an inner cone and / or which are concave or convex in the direction of the gas flow , are trained.

13. Sound absorber according to claim 10 or 11, characterized in that regions (1.1 to l.N) are formed from at least two different elements.

14. Sound absorber according to one of claims 10 to 13, characterized in that it is formed from a plurality of individual elements that are modularly composed.

15. Sound absorber according to one of the preceding claims, characterized in that the thicknesses and / or widths of areas (1.1 to l.N) have been set taking into account selected wavelengths of a sound wave spectrum.

16. Sound absorber according to one of the preceding claims, characterized in that the respective distances of a reflecting element (2) have been set taking into account selected wavelengths of a sound wave spectrum.

17. Sound absorber according to claim 12 or 13, characterized in that the thicknesses, widths and / or distances cause interference for selected wavelengths.

18. Sound absorber according to one of the preceding claims, characterized in that regions (1.1 to l.N) are formed from at least two different layer regions in the direction of propagation of sound waves within the sound absorber.

19. Sound absorber according to claim 15, characterized in that at least one layer area is designed as a loose fill of hollow spheres.

20. Sound absorber according to claim 15 or 16, characterized in that the individual layer areas of an area (1.1 to l.N) have different thicknesses.

21. Sound absorber according to one of claims 15 to 17, that the individual layer regions are formed from different hollow spheres.

22. Sound absorber according to claim 18, characterized in that hollow spheres with different outer diameters, different shell thicknesses, porosity of the shells and / or different hollow sphere composites are contained in the individual layer regions.

23. Sound absorber according to one of claims 18 to 22, characterized in that sound-reflecting interfaces are present at layer areas.

24. Sound absorber according to one of the preceding claims, characterized in that at least one area (1.1 to 1.N) is mechanically post-compressed and / or a surface contour is formed by machining.