Acoustics – Sound-modifying means – Sound absorbing panels
Reexamination Certificate
1999-12-21
2001-09-18
Nappi, Robert E. (Department: 2837)
Acoustics
Sound-modifying means
Sound absorbing panels
C181S286000, C181S293000, C181S295000
Reexamination Certificate
active
06290022
ABSTRACT:
CROSS-REFERENCE TO RELATED INVENTIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a surface-like absorber for troublesome sound waves, in particular for troublesome airborne sound waves.
2. Description of Related Art
The use of Helmholtz resonators with a very wide variety of dimensions for damping airborne noise is known from a very wide variety of areas in industry, for example for building applications from German Offenlegungsschrift DE 195 22 363 A1 and, for the area of motor-vehicle construction, from German Offenlegungsschrift DE 196 15 917 A1, DE 196 13 875 A1 or DE 37 29 765 A1.
The disadvantage common to these known Helmholtz absorbers, which act as broadband absorbers, in some cases by design, in others more or less on the empirical level and in other cases unintentionally and unwittingly, is that they are large and bulky.
Taking this prior art as a starting point, the object on which the invention is based is to provide a surface-like sheet absorber for sound waves, in particular airborne sound waves, which can be tuned in a specifically intended manner and can be used more widely and in a more flexible manner for an extremely wide variety of applications but especially in motor-vehicle construction, without having to be adapted beforehand to predetermined installation conditions.
BRIEF SUMMARY OF THE INVENTION
The invention achieves this object by means of a surface-like sheet absorber with the features stated in claim
1
.
The essential idea of the invention is based, first of all, on providing a large-area sound absorber, in particular airborne sound absorber, which is capable of damping sound waves from the space surrounding it in a wide, tuneable frequency range by Helmholtz resonance without being tied from the outset to geometrical configurations or dimensions determined by the application. To be more precise, the invention thus provides a broadband surface-like sheet absorber, the sound absorption characteristics of which are, as it were, an adjustable surface property which is both independent of the intended application of the absorber and, especially, independent of the external shape and external dimensions of the absorber. As regards independence from the external dimensions of the surface-like absorber, it should, of course, be noted that there is a minimum total surface area of the absorber required for a response in a predetermined broad frequency band. This minimum surface area must be at least large enough to include the number of resonators tuned in this way required to cover the specified frequency band with at least a minimum overlap between the bands of the individual resonators.
As already mentioned above, another special distinguishing feature of the broadband surface-like sheet absorber is its modular construction. A particular component frequency of the broad band to be absorbed, more precisely a particular narrow frequency band with a width preferably in a range between about 100 Hz and 300 Hz, specifically with a width in a range of 200 Hz to 300 Hz, is therefore not implemented merely by a single resonator chamber as in the applications known from German Offenlegungsschrift DE 196 13 875 A1 or DE 196 15 917 A1 but by a plurality or multiplicity of smaller identical resonator chambers distributed over the entire surface of the broadband surface-like absorber.
The simplest way of obtaining a surface-like sheet absorber of this kind is by attaching, preferably welding or adhesively bonding, in a fluidtight manner, a perforated plate to an extended-area trough provided with a chamber structure by a checkerwork, more precisely on the upper edges of said trough, which lie at least essentially in one plane, the sequence of cup-like depressions formed in the trough by the checkerwork being associated in such a way with the holes formed in the perforated plate that each of the chambers is associated with a precise, precalculated number and distribution of holes, i.e. resonance openings, which, for their part, have an opening area, preferably a circular opening area, and height which is in each case configured to match the absorption distribution curve of the associated resonator chamber or determine and generate the shape of these absorption characteristic curves of the individual resonator chambers.
The distribution of the in each case identically tuned resonator chambers over the surface of the broadband surface-like absorber preferably corresponds to as homogeneous a random distribution as possible, the formation of sequential distributions preferably being completely avoided. However, this can and should not necessarily exclude the formation of relatively large repeats in practice in the case of absorbers with relatively large overall areas. Overall, however, care should preferably be taken to ensure that the spacing between the individual identically tuned chamber resonators in the principal plane of the surface-like absorber is never greater than &lgr;/2, &lgr; being the principal wavelength or “rated wavelength” of the resonance absorption of the respective chamber resonator. This measure makes it possible to prevent the formation of standing waves of this narrow frequency band or this interference wave on the surface of the broadband surface-like absorber.
According to a further refinement of the invention, the overall structure of the broadband surface-like absorber is not rigid but is flexurally elastic or flexible in order, in this way, to allow it to be adapted over a wide area to non-planar surfaces where it is used. This is achieved by the choice of suitable plastics for the production of the surface-like absorber. When the broadband surface-like absorber is constructed in this way, however, care should be taken to ensure that the chamber structure does not become so soft that it no longer forms a stable resonant frequency, i.e. can no longer couple to the interference waves to be absorbed.
A further refinement of the invention envisages that tuning of the individual chamber resonators in respect of predetermination of the chamber volumes is performed not just by changing the basic area of the chamber in the direction of the principal plane of the surface-like absorber but also by a tuning adjustment of the chamber depth calculated from the underside of the perforated plate closing off the chamber on the sound side to the bottom surface remote from the sound. In this refinement of the adaptation of the chamber volume, a bottom thickness which is segmented in accordance with the checkerwork structure and changes abruptly and hence increased strength is achieved for the rear wall of the surface-like absorber. In addition, this feature makes possible a more flexible configuration of the individual sequences of chambers in the surface-like absorber.
In a surface-like absorber constructed in the manner described above, the absorption frequencies or the narrow absorption frequency bands of the individual groups of chamber resonators are preferably tuned in such a way that, when they absorb adjacent frequency ranges, they overlap one another over a width of around 50 Hz. As the absorption curves measured on test absorbers show, such an overlap bandwidth in a range of around 50 Hz to 10 kHz is sufficient to make the broad band of a surface-like absorber constructed in this way to appear as a closed broad band without gaps in the absorption. However, this does not mean that the surface-like absorber must always be constructed in this way. If it is important, for the purpose of sound-deadening a motor vehicle for example, to damp a specific frequency range around 50 Hz, on the one hand, and a specific frequency range between about 600 and 1 kHz, on the other hand, it is not necessary to provide the surface-like absorber with resonators which also damp the intermediate range, that is to say the range between 100 Hz and 600 Hz in the example chosen here. This allow
Gartner Udo
Wolf Anton
Wolf Franz Josef
Martin Edgardo San
Nappi Robert E.
WOCO Franz-Josef Wolf & Co.
Yeager Arthur G.
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