Acoustics – Sound-modifying means – Sound absorbing panels
Reexamination Certificate
2002-02-11
2004-11-23
Fletcher, Marlon T. (Department: 2837)
Acoustics
Sound-modifying means
Sound absorbing panels
C181S293000
Reexamination Certificate
active
06820720
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lining or shape element for means of transportation. In the prior art, absorbers made of fibrous materials are known which are employed as non-woven components, In addition to natural fibers, polymeric fibers, inorganic or metallic fibers may be employed as said fibers. Bonding to the components is effected by bonding through intrinsic as well as extrinsic fibers and/or through a binder which may be of polymeric or even inorganic nature.
2. Description of the Related Art
Further, absorbers made of open-pore foams of a polymeric, organic as well as inorganic material are known. The optimum absorption effect of such materials is within a frequency range of from 2000 to 10,000 Hz.
From the various fields of technology, the use of Helmholtz resonators having a wide variety of dimensions is known for the damping of air-borne sound; in the field of motor vehicle construction, reference may be made representatively to the German Offenlegungsschriften DE 196 15 917 A, DE 196 13 875 A and DE 37 29 765 A. These absorbers are very space-intensive and only conditionally suitable for motor vehicle construction.
DE 197 54 107 C1 and the prior art references cited therein, the full disclosure of which is included herein by reference, examine the absorptive behavior of microperforated components. The document mentioned examines baffle structures built of layers of microperforated plastic sheets pending from a ceiling or a roof as so-called compact absorbers. The microperforated plastic sheets are suitable for absorbing very effectively on one side or both sides thereof sound waves impinging from the space in a perpendicular, oblique or glancing direction, especially at higher frequencies.
If additionally, for every microperforated sheet, a further non-perforated sheet is spanned in parallel at a distance of some centimeters, the absorption of the acoustically active sheet is not improved. This is explained by the fact that the latter sheet needs neither a reverberant back wall nor a gap of any shape as an air cushion in between for displaying its damping activity. In contrast, if the microperforated sheets are replaced by non-perforated sheets of the same material, only a very low absorption remains. It is well-known that sound absorption arises for a resonator arrangement where a mass-spring effect can occur. A physical explanation for the sound absorption of the microperforated sheets is not considered possible by the inventors of the mentioned document.
Further, there are known microperforated sheets and plates which can filter out defined frequencies as a function of the pore diameter, the distance of the sheet from the wall, the surface area which is open due to the bores and the sheet thickness. DE 197 54 107 A describes such a sound absorber for the field of rooms. Here, the effect is seen that a high absorption can be achieved only over a relatively narrow frequency range. A broadening of the absorption range is always connected with a considerable loss of absorption and finally gets completely ineffective.
In a two-layer arrangement in which two sheets are arranged behind one another, two frequency ranges can be absorbed. Thus, a broadening is possible without reaching the broadness necessary in the automobile field.
Further, there are known structures where a sound-impermeable plastic sheet is provided with perforations in order to achieve an absorptive effect from the underlying absorber, see for example DE 30 18 072 A, DE 41 23 593 A.
DE 41 19 783 A describes a sound-absorbing element which consists of two perforated plates provided on top of one another, arranged at a distance from the substrate element and from each other. Between the two perforated plates, there is a foam plate. The perforated proportion of the sound-facing side is 50%, and that of the side facing away from the sound is 20% and thus clearly above the values considered necessary here.
It has been the object of the invention to provide an absorber which has an absorptive effect throughout the range of frequencies interesting to the automobile industry (from 500 Hz to 5000 Hz) and does not need much building space.
SUMMARY OF THE INVENTION
According to the invention, this object is achieved by providing a layered structure of microperforated absorbers and a non-woven material and/or foam absorber and/or air gap in such a way that a non-woven material and/or foam absorber and/or air gap alternates with a microperforated plate/sheet (microperforated sheet absorber). Preferably, the absorbers have such a structure that one or two microperforated plates/sheets are stacked in connection with two or three non-woven and/or foam absorbers or one or two gaps.
Due to the perforated design of the microperforated absorbers and the thickness of the non-woven and/or foam absorbers and/or air-gaps, a coupling of the two kinds of absorbers is achieved. When the coupling of the individual absorber types is considered, a distinction must be made in terms of which type is provided on the surface in the direction of sound entry.
If a non-woven and/or foam absorber and/or air gap is positioned on the sound, facing side, the properties of the composite (microperforated plate/sheet—non-woven and/or foam absorber and/or air gap) are determined by the non-woven and/or foam absorber alone from a defined thickness. Depending on the kind of non-woven and/or foam absorber and/or air gap employed, this thickness is from 20 to 40 mm.
With decreasing thickness (from 40 mm to 0.3 mm) of the non-woven and/or foam absorber and/or air gap, the microperforated absorber increasingly contributes to the effect. By selecting the thickness of the non-woven and/or foam absorber and/or air gap, the effects of the two absorbers can be matched to one another. The same applies to a subsequent ply of microperforated absorber and non-woven and/or foam absorber and/or air gap for multilayered absorbers.
If the microperforated sheet absorber is positioned on the sound-facing side, coupling of the two types of absorbers can be effected primarily through the open surface area. With increasing open surface area, the absorption behavior is increasingly determined by the non-woven and/or foam absorber and/or air gap. For an open surface area of about 40%, the non-woven and/or foam absorber and/or air gap reaches its full activity without the microperforated absorber completely losing its activity.
According to the invention, one or more microperforated sheet absorbers are layered with an open surface area of preferably up to 4%. On the surface on the sound-facing side and/or between the plates or between the wall and the last microperforated sheet resonator, there is a foam and/or non-woven absorber or air.
The absorption behavior of the absorber which represents the surface of the sound-facing side dominates the absorption behavior of the overall system.
Therefore, in the case where a foam and/or non-woven absorber is positioned on the surface, of course, the thickness of the absorber also plays a role. When a foam and/or non-woven absorber as used in the automobile field is positioned in the first ply towards the sound impingement, the effect of a microperforated sheet absorber positioned in the second layer as seen from the sound source is reduced from a thickness of about 7 mm of the foam and/or non-woven absorber to such a high extent that its effect can hardly be detected any more. In this case, the property picture of the foam
on-woven absorber dominates.
On the other hand, when the microperforated sheet absorber is positioned on the surface of the sound-facing side, it will dominate the property picture of the overall system. In this case, the effect of the foam and/or non-woven absorber and/or air gap is decidedly low, and its use is of no practical importance.
When the microperforated sheet absorber is positioned before the foam and/or non-woven absorber and/or air gap as seen from the sound source, the inclusion of additional open surface areas in the microperforated s
Chuong Dinh-Quyen
Fuchs Helmut V.
Nicolai Norbert
Clion Irland Ltd.
Fletcher Marlon T.
Venable LLP
Warren David S.
Wiseman Thomas G.
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