Sound Absorber

Abstract

A sound absorber, especially for aerodynamic installations, comprising a tubular-like housing closed at each end by a cover member, each said cover member leaving free a central throughpassage opening. Two dampening systems are housed within the housing, one such dampening system extending as a longitudinal dampening device over the housing inner wall, the other dampening system being formed by partition walls located perpendicular to the throughflow direction and distributed over the length of the housing. These partition walls only cover a part of the internal compartment of the housing left free by the longitudinal dampening device.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a new and improved construction of sound absorber or sound dampening device, especially for aerodynamic installations, and comprises a tubular-like housing closed at each end by a cover member which in each case leaves free a central throughflow opening.

Sound absorbers for aerodynamic installations must be designed in consideration of their free throughflow cross-section such that the flow velocity and pressure drop are as small as possible. The effective dampening region should begin at approximately 200 Hz and should be very wide-band.

In order to fulfill these requirements it is generally necessary to provide internally of the sound absorber housing special installations. This is especially so when the cross-section of the housing is circular, oval or quadratic, i.e., only slightly rectangular.

The prior art is already familiar with constructions of sound absorbers having installations arranged internally of the housing. A tube formed of perforated sheet metal and filled with a sound-absorbing material is most often used. Such perforated sheet metal tube is preferably employed for coaxial mounting in sound absorbers of circular cross-section. Furthermore, it is known in this particular field of technology to arrange internally of a sound absorber a spiral-shaped coiled metal sheet of a length corresponding to the sound absorber, that is to say having a number of coils. With this known type of installation there is not realized any sound dampening effect at the low-frequency region and only a very slight dampening effect is obtained at the middle frequency region. At best the sheet metal spiral is effective at the high-frequency region, namely by impeding the so-called sound radiation. However, it has the additional drawback that with respect to the air throughflow there is produced a superimposed spin flow and a considerable additional pressure loss.

SUMMARY OF THE INVENTION

Accordingly, from what has been explained above it should be apparent that this particular field of technology is still in need of sound absorbers which are not associated with the aforementioned drawbacks and limitations of the state-of-the-art proposals. Therefore, it is a primary object of the present invention to provide a new and improved construction of sound absorber which is not associated with the aforementioned disadvantages of the heretofore discussed proposals and which effectively and reliably fulfills the existing need in the art.

Another and more specific object of the present invention relates to a new and improved construction of sound absorber which avoids the drawbacks of the prior art sound absorbers and, in particular, aims at the provision of special installations for the interior of the sound absorber which are capable of realizing up to the middle-frequency range, that is up to the third octave band a high sound dampening action independent of the dampening devices along the periphery and extensively independent of the parameter U/F.

According to a particular manifestation of the invention it is a further object to almost completely suppress the so-called sound radiation, especially for sound absorbers with an unfavorable U/F-parameter, by the provision of suitably designed installations arranged internally of the sound absorber.

Yet a further significant object of the present invention relates to a novel construction of sound absorber, the frequency range of which encompasses higher sound absorption of at least five octaves.

Now, in order to implement these and still further objects of the invention, which will become more readily apparent as the description proceeds, the invention contemplates accomodating two dampening systems internally of the sound absorber housing. One of the dampening systems extends in the form of a longitudinal dampening device over the inner wall of the housing and the other dampening system is formed by partition walls arranged perpendicular to the direction of throughflow and distributed over the length of the housing. The partition walls only cover a portion of the internal compartment of the housing which is left free by the longitudinal dampening device.

According to the invention both dampening systems are constructed according to the principle of a resonator. The partition walls, together with the free-remaining partial cross-sections and the partial hollow compartments located between the partition walls, form a low-pass resonator. Moreover, with the cross-section of the housing constant over the length of the sound absorber the partition walls distributed over the length of the housing are arranged at the same spacing from one another. The housing cross-section which is left free by the partition walls approximately corresponds to the throughflow surface of the connection opening of the sound absorber at the housing covers.

The partition walls can be in the form of ring-shaped or annular discs mounted in the housing and which in each instance exhibit an inner throughflow opening, or these partition walls are in the form of surface discs which are arranged at the central axis of the housing and fixed relative to the housing and fixed relative to the housing wall by supports. Finally, the partition walls can be interconnected or supported by supports extending through the lengthwise axis of the housing. For each meter of sound absorber at least five such type partition walls can be advantageously employed.

If the partition walls are constructed as ring-shaped discs then there is realized a preferred constructional manifestation of the sound absorber which acoustically strives for a low-pass like resonator dampening action and aerodynamically realizes a very favorable pressure loss.

If, on the other hand, the partition walls are designed as surface discs arranged at the center of the housing, then, the outer diameter thereof should correspond approximately to the inner diameter of the connection openings at the covers; the outer diameter can be however somewhat smaller. Not only is there thus realized a low-pass type resonator dampening action, rather there is additionally attained an almost complete suppression of the so-called sound radiation as well as considerable dampening in the high-frequency range. It should be understood that the discs are mounted coaxially with regard to the housing axis of the sound absorber.

According to the invention the edges of the partition walls confronting the sound throughpassage are bent or flexed in the direction of such sound throughpassage. At the side confronting the sound absorber outlet the partition walls are covered with a sound-absorbing material, for instance mineral wool, foamed plastic, just to mention a few suitable materials. However, it is equally possible to also fill the intermediate compartments between the successive partition walls with sound-absorbing material. At the inlet side pressure losses are avoided by an appropriately arranged inflow lining which is located at the forwardmost partition wall. Moreover, the partition walls can be molded components formed of metal, plastic, especially formed plastics or the like.

A further advantage of the invention is realized in that for the construction of the dampening system there are formed resonator inserts fabricated from plastic and which exhibit the shape of a box open at one side with throughpassage openings at the bottom or floor. These resonator inserts are advantageously arranged such that their open side confronts the housing inner wall and the partition walls.

A particularly advantageous construction of sound absorber can be realized if the housing inner wall is covered with resonator inserts arranged in a row closely following one another and which are directed inwardly by means of their perforated floor. Moreover, the resonator inserts are covered by a continuous layer of sound-absorbing material (mineral wool) which is retained by a rigid grid, sieve or the like. The inner dampening system can either consist of partition walls formed or ring-shaped discs and covered with a mineral wool layer, the partition walls merging with the longitudinal dampening device, or there can be used partition walls in the form of circular discs located at the central axis of the sound absorber housing and which, in turn, together with the sound-absorbing material arranged thereon or therebetween are enclosed by a sieve, grid or the like and supported in the center of the housing by suitable supports, struts or the like. A particularly good effect also can be then realized if the resonator inserts are covered at their open side confronting the inner wall of the housing by a plastic foil or the like.

Fabrication of the inventive sound absorber is simplified in that both dampening systems are manufactured as pre-fabricated units and each for itself or conjointly can be inserted from the head end into the tubular-like housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and objects other than those set forth above, will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:

FIG. 1 is a longitudinal sectional view through a first embodiment of inventive sound absorber;

FIG. 2 is a longitudinal sectional view through a second embodiment of inventive sound absorber;

FIG. 3 is a partial longitudinal sectional view through a sound absorber of the type shown in FIG. 1 on an enlarged scale;

FIG. 4 is a cross-sectional view of the arrangement of FIG. 3, taken substantially along the line IV--IV thereof;

FIG. 5 is a partial longitudinal sectional view of the sound absorber depicted in FIG. 2 on an enlarged scale;

FIG. 6 is a cross-sectional view of the arrangement of FIG. 5, taken substantially along the line VI--VI thereof;

FIG. 7 is a perspective view of a resonator insert;

FIG. 8 is a cross-sectional view of the arrangement of FIG. 7, taken substantially along the line VIII--VIII thereof;

FIG. 9 is a cross-sectional view of the arrangement of FIG. 7, taken substantially along the line IX--IX thereof; and

FIG. 10 is a graph explaining the mode of operation of the sound absorber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Describing now the drawings, the inventive sound absorbers or sound-dampening devices generally possess a tubular-like or pipe-like housing 1 which may possess a round, oval or quadrangular cross-section and closed at its ends by the covers or cover members 2 which, in turn, each possess a central throughflow opening 3. Each such throughflow opening 3 can be advantageously provided with a flange 3a for connection to other pipe conduits. Internally of the housing 1 there is provided a longitudinal dampening device 4 which can be constructed either according to the absorption-principle, but most preferably according to the resonator principle. The longitudinal dampening device 4 is covered and supported in the housing 1 at the inside by means of a sieve cylinder or by means of a grid, perforated sheet metal or the like, as generally indicated by reference numeral 5.

Now with the embodiment of sound absorber of the type depicted in FIG. 1 partition walls 6 are distributively arranged at a uniform spacing from one another throughout the length of housing 1. Each of these partition walls 6 exhibits a central throughflow opening 7 which is approximately equal in surface area to the connection openings 3 at the housing covers 2. At the edge of each of the throughflow openings 7 the illustrated ring-shaped discs forming such partition walls 6 are provided with flexed or bent portions 8 disposed in the direction of throughflow. As clearly seen by referring to FIG. 1 the partition walls 6 are covered with a sound-absorbing material 9, for instance mineral wool, at the region of the hollow compartment bounded by the flexed portions 8. The partition walls 6 are advantageously fixedly seated at the sieve or perforated sheet metal member 5. They form together with the longitudinal dampening device 4 an insert unit which can be pushed into the housing 1 from the head end thereof. What is decisive for the magnitude of the upper boundary frequency of the low-pass like dampening is the magnitude of the partial hollow compartment H between the partition walls in relation to the square of the radius of the throughpassage opening 7.

With the embodiment of sound absorber of the type depicted in FIG. 2 the partition walls are formed by circular discs 11 which are coaxially arranged at the same spacing from one another at the interior of the housing 1 lined with the longitudinal dampening device 4, 5. The discs 11 have marginal flexed portions 13 and are covered with a sound-absorbing material, for instance mineral wool 14, at the side confronting the outlet pr exit, as shown. The discs 11 are retained by means of struts 10 piercingly inserted through the housing 1 and supported at the lateral covers 2. At the inflow side the forwardmost strut 11 is provided with an inflow lining 12 for reducing the pressure loss.

The direction of flow is marked by arrows both in the arrangement of FIG. 1 and also in the arrangement of FIG. 2.

Now in FIGS. 3 and 4 there is illustrated an improved version of the sound absorber of the type depicted in FIG. 1. The partition walls 6 are covered and connected with one another at the inside by means of a sieve, grid, perforated sheet metal or the like, as generally indicated by the reference character 15. In this connection the space between the partition walls 6 can be also filled with a suitable sound-absorbing material 16.

In order to ensure for a good dampening action for the longitudinal dampening system there are employed resonator inserts 17, as such have been illustrated in detail in FIGS. 7 to 9. These inserts 17 entirely consist of foamed plastic, such as for instance styropor. They essentially possess a box-like configuration, and compartments 20 are formed by transverse walls 18 and longitudinally extending walls 19 and wherein the floor 21 has a somewhat larger thickness and is equipped with a multiplicity of openings 22 arranged in rows behind and adjacent one another.

In order to facilitate assembly of the resonator inserts 17 into a ring-shaped lining there are provided at one side of the floor the beads 23 or recesses 24.

From the arrangement of FIGS. 3 and 4 it will be recognized that the resonator inserts 17 are mounted in the housing 1 behind and neighboring one another with their open side facing towards the outside. A cover foil 25 covers the resonator compartments 20 at their open side.

The resonator inserts introduced as a lining into the housing 1 are finally covered with a sound-absorbing layer 26 formed of mineral wool or the like and retained by the cylindrical sieve, grid or equivalent structure, generally indicated by reference character 5.

Now in FIGS. 5 and 6 there is illustrated an improved version of sound absorber of the type shown in FIG. 2. Just as was previously the case, here also the longitudinal dampening system is formed by resonator inserts. On the other hand, the inner dampening device consists of centrally arranged circular discs 11 forming the partition walls, as such have already been used for the embodiment of sound absorber of FIG. 2. The inner sound absorber is enclosed as a unit by a sieve cylinder 27 and is retained by means of the discs 28 at the housing, and specifically at the sieve jacket 15.

With the inventive constructions of sound absorbers there is realized, among other things, the advantage that with the internally arranged resonator sound can be effectively dampened in a low-pass like manner at least up to the third octave region, that is into the middle-frequency region. Furthermore, by virtue thereof there is realized the very notable advantage that the thickness of the longitudinal dampening device can be generally considerably less than is usually the case. With the special constructional embodiment which contemplates the internal arrangement of the surface discs there is realized the additional advantage that the so-called sound radiation can be almost completely suppressed without additional manufacturing expenditure. As a result there is produced a sound absorber with the important acoustical advantage that in a frequency range of five to six octaves high dampening with approximately constant frequency course of the dampening can be realized.

For reasons of fabrication and especially for reasons of assembly it is furthermore very advantageous if the inner resonator arrangement is designed as an independent insert unit.

Finally, the graph depicted in FIG. 10 illustrates the throughput dampening mass Db plotted over the frequency f for a tested sound absorber designed according to the invention.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and parcticed within the scope of the following claims.

Claims

What is claimed is:

1. A sound absorber, especially for aerodynamic installations, comprising a housing having a tubular wall defining a flow passage there-through, said housing including an inlet end and an outlet end, each of said ends being partially closed by covers having throughflow openings defined therein, an outer dampening system defined by a plurality of resonator inserts arranged tightly in a row behind one another and extending longitudinally over the inner surface of said tubular wall, said resonator inserts having an open side and a perforated floor, said open sides facing the inner surface of said wall and said perforated floors facing inwardly of said housing and covered by a continuous layer of sound absorbing material, an inner dampening system disposed coaxially with said other dampening system and partially obstructing said flow passage, said inner dampening system including a plurality of partition members distributed over the length of said housing and having side walls extending transversely of said flow passage and facing said inlet and outlet ends, and sound absorbing material covering said side wall of each of said partition members which faces said outlet end.

2. The sound absorber as defined in claim 1, wherein each cover has an opening defining a connection opening forming a throughflow area, and wherein the cross-section area of the flow passage left unobstructed by the partition members approximately corresponds to the throughflow area of the connection opening at the housing covers.

3. The sound absorber as defined in claim 2, wherein said partition members comprise a number of ring-shaped discs inserted into the housing, each of said ring-shaped discs having an internal throughflow opening.

4. The sound absorber as defined in claim 2, wherein said partition members comprises surface discs which are arranged at the central axis of the housing, and support means for supporting such surface discs relative to the wall of the housing.

5. The sound absorber as defined in claim 1, further including support means extending in the direction of the lengthwise axis of the housing for interconnecting and positionally retaining said partition members.

6. The sound absorber as defined in claim 5, wherein said partition members have edges confronting sound throughflow, said partition member edges being flexed in the direction of sound throughflow.

7. The sound absorber as defined in claim 1, wherein said sound-absorbing material is mineral wool.

8. The sound absorber as defined in claim 1, wherein the sound-absorbing material is foamed plastic.

9. The sound absorber as defined in claim 1, wherein the partition members are spaced from one another so as to define therebetween intermediate compartments, and sound-absorbing material filling such intermediate compartments.

10. The sound absorber as defined in claim 9, wherein the partition member walls facing the inlet end of the sound absorber are provided with an inflow lining reducing pressure losses.

11. The sound absorber as defined in claim 10, wherein said partition members comprise molded components.

12. The sound absorber as defined in claim 1, wherein the partition members are in the form of ring-shaped discs which merge with support means for the resonator inserts and the sound-absorbing material.

13. The sound absorber as defined in claim 1, wherein the partition members form circular discs located substantially at the axial center of the sound absorber housing, means for enclosing said circular discs together with the sound-absorbing material, and support means for securing the circular discs at the center of the housing.

14. The sound absorber as defined in claim 1, further including a plastic foil for covering the resonator inserts at their open side confronting the inner surface of the housing wall.

15. The sound absorber as defined in claim 1, wherein both dampening systems are constructed as pre-fabricated units each of which can be individually or collectively inserted from one end of the housing into such housing.

16. The sound absorber as defined in claim 1 wherein each of said resonator inserts is defined by a box, and wherein said open side and said perforated floor comprise opposite sides of said box.