Sound absorber for sound waves

The broadband surface-like absorber of the invention is used, in particular, for absorbing troublesome airborne noise in the acoustic frequency range. The broadband surface-like absorber, which operates on the Helmholtz resonator principle, is distinguished by a checkerwork of irregular construction, the webs of which are aligned with their narrow sides perpendicular to the principal surface of the perforated plate and the side edges of which are connected in a sound-pressure-resistant and fluidtight manner on the sound side to the rear side of the perforated plate and, on the rear side, to an extended-area cavity boundary aligned with the same orientation as the perforated plate to form differently tuned chamber resonators.

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 .lambda./2, .lambda. 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 allows either the overall area of the
 surface-like absorber to be reduced or, as an alternative, an audible
 improvement in the performance of the absorber under comparable test
 conditions while retaining identical overall areas.
 For series production of the broadband surface-like absorbers of the
 invention, the checkerwork and the rear cavity base are preferably of
 one-piece construction, being made, in particular, of a thermoplastic
 elastomer or a flexible plastic, while the perforated plate is made of a
 laminated plastic material, likewise resilient or plastically flexible,
 with a thickness in a range between 0.5, in particular 1.5 mm and up to 5
 mm, in particular up to 3 mm. In this arrangement, the rear part in the
 form of a cup-shaped trough and the perforated plate are preferably bonded
 to one another.
 It is also perfectly possible for a surface-like absorber constructed in
 this way to be produced as yard ware that can be cut to size and to be
 sold in ready made-up form both for commercial purposes and for home use.
 More specifically, however, such broadband surface-like absorbers are
 carefully tuned and used as insulating material for the series production
 of motor vehicles and, in this context, in particular, for lining the
 interior of the motor vehicle, especially for the interior lining of a
 steel roof as a so-called "headliner", for lining the engine compartment
 or as an aeroacoustic cladding for the underbody of the motor vehicle.
 By virtue of their adaptability to an extremely wide variety of use
 situations, the surface-like absorbers according to the invention are also
 suitable for use as surface-like absorbers for walls and ceilings in the
 construction of buildings and in connection with sound barriers for
 emission control.

DETAILED DESCRIPTION OF THE INVENTION
 As can be seen most clearly in FIG. 2, the embodiment example shown in
 FIGS. 1 to 3 of a broadband surface-like absorber 1 with the features of
 the invention comprises a checkerwork 2, a perforated plate 3 and a
 surface-like rear cavity boundary 4 on the other side of the checkerwork.
 The checkerwork 2 and the rear cavity boundary 4 are produced in one piece
 from a comparatively stiff, flexible thermoplastic elastomer and, overall,
 form a trough-shaped or jig-like structure divided into chambers. The
 surface-like upper edges 5 of the individual webs of the checkerwork 2
 geometrically define a continuous surface, preferably a plane or a surface
 with only a slight curvature. In this plane, the perforated plate 3, which
 is designed as a strong plastic sheet, is bonded to the upper edges 5 of
 the checkerwork 2 in a fluidtight manner and in a manner resistant to
 sound pressure. More particularly, this bond with the rear side 6 of the
 perforated plate 3, the said rear side facing away from the sound chamber
 to be damped, is made by welding or, as in the embodiment example
 described here, by adhesive bonding.
 While, in the embodiment example shown in FIG. 2, an integrally formed
 perforated plate 3 is adhesively bonded to the rear trough structure 2, 4,
 it is also possible, in the manner shown in FIG. 1, for the perforated
 plate to be made up of individual parts which are each applied separately
 or, if appropriate, in the form of repeating groups, to the individual
 resonator chambers 7 (FIG. 2).
 Whether the perforated plate is in one piece, as illustrated in FIG. 2, or
 made up of a plurality of individual parts, as shown in FIG. 1, this
 perforated plate has through-openings 8 in all cases. The volume of these
 openings 8 is filled with the fluid which surrounds the surface-like
 absorber and in which the sound waves to be damped propagate. The mass of
 the fluid enclosed in the volume of the hole, almost always air,
 corresponds to the oscillatory mass which, together with the volume of
 fluid enclosed in the resonator chambers 7, said volume acting as a
 spring, forms the Helmholtz resonator.
 FIG. 2 gives a schematic representation, by way of example, of three
 differently tuned Helmholtz chamber resonators 7. This figure shows
 specifically how tuning can be performed. Thus, in the case of formation
 of the rear cavity boundary 4 in one piece with the checkerwork 2, the
 volume of the chambers can be determined either by varying the cross
 section of the individual resonator chambers 7 or by varying their depths,
 measured from the underside 6 of the perforated plate 3 to the bottom
 surface 9 of the chambers, the bottom surface 9 of the individual
 resonator chambers 7 corresponding to the inner surface of the rear cavity
 boundary 4 of the surface-like absorber, said inner surface facing towards
 sound. The depth of the individual resonator chambers 7 is here varied by
 varying the thickness of the rearward cavity boundary 4 from chamber to
 chamber.
 Each of the resonator chambers 7 can be assigned two or more or even a
 multiplicity of openings 8 in the schematically depicted manner. While the
 axial eight of the individual openings 8 in the embodiment example shown
 in FIG. 2 is uniformly determined for all the resonator chambers 7 by the
 thickness of the sheet used for the perforated plate, the assembled
 perforated plate of the type shown in FIG. 1 has the advantage that the
 axial height of the individual openings 8 can also be varied from chamber
 to chamber, thus allowing the third spatial dimension, the Z axis as it
 were, to be used for tuning the oscillatory mass to achieve differentiated
 tuning of the absorption per unit area.
 While the structure of the broadband surface-like absorber shown in FIG. 1
 is obviously particularly suitable for experimental purposes or for
 special production runs, the embodiment shown in FIG. 2 is suitable
 especially for series production.
 When the surface-like absorber in accordance with the invention is designed
 as yard ware for DIY enthusiasts and tradesmen, it is also possible to use
 a perforated-plate sheet provided uniformly or at random with
 through-openings and for it to be applied and joined to the underlying
 structure in a more or less random manner in a continuous production
 process. This makes it possible to achieve good distribution of the
 resonance absorption while also achieving good to average absorption
 performance. A broadband surface-like absorber of this kind, especially
 one produced by a continuous process, is distinguished by a broad, varied
 range of applications and the ability to be cut to size.
 In areas of application where higher requirements apply both to the
 absorption performance and the spectral distribution of the absorption
 characteristics, on the other hand, for example to allow acoustic styling
 of noises within passenger cells of motor vehicles, very careful
 structural definition of the absorption characteristics of the
 surface-like absorber is required.
 For applications on which higher requirements are made, the perforated
 plates or perforated-plate sections are associated very accurately with
 the individual preformed resonator chamber volumes. Here, the trough-like
 structure is formed with a predetermined number of groups of resonator
 chambers 7, each with the same chamber volume, in such a way that these
 resonator chambers are distributed randomly and as homogeneously as
 possible and, as far as possible, without forming sequential patterns over
 the surface of the surface-like absorber, more specifically with the
 proviso that the spacing between each resonator chamber in each group and
 the adjacent resonator chamber in the direction of the main surface of the
 surface-like absorber is less than .lambda./2, where ".lambda." is the
 wavelength of the main resonant frequency, more precisely the mean
 resonant frequency, of the resonance band, which is narrow relative to the
 broadband absorption of the surface-like absorber overall. Such a
 distribution of the individual resonator chambers, which is indicated
 schematically in FIG. 1, prevents the formation of standing waves over the
 entire broad band of absorption of the surface-like absorber over the
 entire surface of the surface-like absorber, however this surface may be
 configured.
 In a broadband surface-like absorber constructed in this way, the
 individual chambers of each group preferably have an absorption band width
 in a range of about 100 Hz to 300 Hz, preferably a bandwidth of 200 Hz to
 300 Hz, their bandwidths overlapping with those of respective groups of
 resonator chambers of higher and lower frequencies, with a frequency width
 preferably of the order of about 50 Hz. Such finely tuned broadband
 surface-like absorbers not only allow comprehensive general absorption of
 troublesome noise in motor-vehicle construction, for example, but also
 "acoustic styling" for the individual types of motor vehicle, an
 application of growing significance in motor-vehicle construction. The
 absorption profiles to be achieved in each particular case can be achieved
 in a highly accurate manner by computer-assisted simulation without the
 need for empirical trials.
 Such flexible configuration and adjustability of the absorption
 characteristics of the broadband surface-like absorber of the invention in
 conjunction with the mechanical/structural flexibility with which these
 surface-like absorbers can be produced opens up new areas of application
 in many fields of applications engineering for the broadband surface-like
 absorbers of the invention, particularly, for instance, in the field of
 motor-vehicle construction, building construction and, more generally, of
 environmental noise protection. In the field of motor-vehicle
 construction, special mention may be made of the use of surface-like
 absorbers to insulate the passenger cells of passenger vehicles against
 the emission of structure-borne noise into the interior and as
 aeroacoustic underbody cladding for vehicles.
 The use of the absorber for lining the roof of passenger vehicles is
 illustrated schematically in FIG. 3 as an example of a use of the
 broadband surface-like absorber in accordance with the invention.
 A structural flexible broadband surface-like absorber in accordance with
 the invention is connected over its entire area with the inner surface of
 the roof panel of a motor vehicle, preferably being welded or adhesively
 bonded to it, in the manner shown in FIG. 3. Connecting it to the steel
 roof structure of the motor vehicle in this way ensures a high degree of
 stiffening and stabilization for the resonator chamber structure and the
 checkerwork despite the flexible design of the overall structure of the
 surface-like absorber. At the same time, the perforated plate of the
 broadband surface-like absorber with its opening diameters of 1 to 3 mm at
 the maximum, said perforated plate facing the interior of the passenger
 cell of the motor vehicle, offers free scope to the designer in its
 capacity as a headliner without this design activity affecting the
 technical functionality of the broadband surface-like absorber. Here,
 therefore, technical and artistic design can be applied freely and
 independently of one another to the same design element. Technically, the
 result in all cases is acoustic calming of the passenger cell of a
 standard that cannot be obtained, for example, with the prior art known
 from German Offenlegungsschrift DE 37 29 765 A1 already cited at the
 outset.
 Significantly better results than those that can currently be obtained with
 conventional rubber underbody coating can also be achieved with the
 broadband surface-like absorber when it is used for underbody insulation
 in passenger vehicle construction.