EP0962013B1 - Lambda/4 absorber with adjustable band width - Google Patents

Description

The present. The invention relates to a λ / 4 absorber Absorption of sound, such as that of machines, in particular of vehicles, is produced with a variety of tubular λ / 4 resonators, the mouth of which is a sound reflecting Adjacent area.

The aspiration of the modern vehicle and machine industry is it from the machines resp. Vehicles Reduce or completely eliminate noise. to Sound absorption is essentially made up of mats today Fiber insulation materials or open-cell foams are used the sources of noise are laid or in their immediate Environment. However, their use is in heavily polluting environments these open-pored materials quickly with oil, water or Enrich dust and thereby its sound absorbing Lose effectiveness.

It is also known to sound absorbers from a variety differently dimensioned Helmholtz resonators build. Such Helmholtz absorbers are in the Practice not enforced for various reasons. In particular such Helmholtz absorbers are difficult to dimension and / or fabricate and unsuitable to to be used in heavily polluting environments can.

Sound absorbers have therefore already been proposed been made up of a variety of tubular resonators consist. These tubular resonators can be of this type be installed so that there is any contamination or Can not get wet in it. Furthermore distinguish these tubular resonators in their acoustic functioning of Helmholtz resonators and are known to the person skilled in the art under the name λ / 4 resonators. This difference is essentially in the same time apparent mass and compressibility of the Air in the resonator and can be recognized in particular be that with the λ / 4 resonators the resonance frequency is determined directly by the standing wave, its wavelength a quarter of the length of the tubular resonator is while the acoustic functioning and resonance of Helmholtz resonators using a spring-mass system must be described and determined.

GB 2 038 410 is an acoustically effective lining for Aircraft engines are known in which a variety of Helmholtz resonators can be combined with λ / 4 resonators. These resonators will packed as tightly as possible to achieve the highest possible absorption. The Openings of these resonators are with a perforated sheet or Non-woven fabric (with a relatively high acoustic resistance) covered to the acoustic coupling to the external sound field improve.

DE 94 08 118 discloses a sound absorber with a plurality of tubular recesses or channels, which are in a porous Absorber material are used. The openings of the individual λ / 4 tubes are covered with a porous foam, a nonwoven or a thin film. In a preferred embodiment, the cavities are with another sound-absorbing material completely or partially filled.

With practical Different versions of the Helmholtz resonators can be used Assumptions used to predict the resonance frequency not be realized. So can For example, the walls of the Helmholtz resonators are not so stiff be built that these are under the pressure fluctuations do not deform at resonance, or the mass of the Air in the neck area of the Helmholtz resonators is not exactly be determined. The advantages of λ / 4 resonators over the Helmholtz resonators are essentially in the more precise predictability of the absorption effect, their lower risk of contamination and their simpler See dimensioning and fabrication.

Such a λ / 4 absorber is described, for example, in WO 96/23294 and comprises a multiplicity of tubular resonators, the sound openings of which adjoin a surface, in such a way that the interaction zones (in which the incident sound wave and the standing waves formed in the individual resonators are located Waves destructively interfering) of the individual resonator openings are distributed as far as possible and at the same time do not substantially overlap. Such λ / 4 resonators basically absorb in a narrow frequency range around their resonance frequency f ₀ . The width of this frequency range depends on the quality factor Q of the resonators, respectively. on the size of the energy losses that occur during resonance. As described in this WO 96/23294, λ / 4 absorbers can be embedded in any dense, reverberant material, such as, for example, metal, plastic, ceramic or glass. In the practical application of these absorbers, especially if a broader frequency band is to be achieved by a large number of resonators of different lengths, it is important to be able to influence the energy losses in a simple manner. In certain embodiments, for example in deep-drawn semitubes, which are completed with a plate, the energy losses are very small, ie the Q factor and the terminating impedance are very high. This leads to undesirably narrow resonance absorption curves.

It is therefore the aim of the present invention, acoustically highly effective λ / 4 absorbers with adjustable bandwidth in easy to manufacture.

According to the invention, this is achieved by a λ / 4 absorber with the features of claim 1. In particular, provision is made to increase the sound impedance Z _mouth in the mouth region of the λ / 4 resonators by means of a perforated head part, and possibly also to generate additional energy losses in the base region through the use of soft and / or heat-exchanging material, so as to reduce Z _T. This can therefore be achieved by providing a heat sink with a large contact area with the air in the bottom region of the resonators, where the pressure fluctuations are very large. Such a heat sink is formed by any material which can absorb and dissipate heat from the temperature fluctuations in the air caused by pressure fluctuations. Those skilled in the field of noise protection are familiar with such materials. Another practical possibility is seen in the use of a plug made of closed-pore viscoelastic foam.
Another possibility is to bring about energy losses in the mouth area by installing a - low - air flow resistance, for example a "grid". In the embodiment of a deep-drawn film with a cover plate, such a “grid” can be produced by not removing the end to be opened, but rather only perforating it.

It turns out to be surprising that due to the disability the air flow in the mouth area of the λ / 4 absorber does not fundamentally affect their absorbency but that this means absorption with greater Bandwidth of the resonance frequency response can be achieved can.

The present invention thus allows efficient for the first time λ / 4 absorber industrial, i.e. inexpensive to manufacture. In addition, the present invention also enables Construction of multifrequency absorbers in a simple way, by forming a wider resonant frequency band several differently dimensioned λ / 4 resonators with increased sound energy loss in the muzzle according to the invention and floor area can be combined.

Fig. 1:

Prinzipschema zur Funktionsweise der λ/4-Resonatoren;

Fig. 2a:

Diagramm zum Absorptionsverhalten des erfindungsgemässen λ/4-Absorbers;

Fig. 2b:

Diagramm zum Absorptionsverhalten des erfindungsgemässen Multifrequenz-Absorbers;

Fig. 3a:

Ansicht einer ersten Ausführungsform eines Resonators mit geschlitztem Kopfteil für den erfindungsgemässen λ/4-Absorber;

Fig. 3b:

Ansicht einer zweiten Ausführungsform eines Resonators mit gelochtem Kopfteil für den erfindungsgemässen λ/4-Absorber;

Fig. 3c:

Ansicht einer weiteren Ausführungsform eines Resonators mit wärmetauschendem Material im Bodenteil für den erfindungsgemässen λ/4-Absorber;

Fig. 3d:

Ansicht einer besonderen Ausführungsform eines Resonators bei dem Mündungsbereich und Bodenbereich gegeneinander geneigt sind;

Fig. 4:

Querschnitt einer praktischen Ausführungsform des erfindungsgemässen λ/4-Absorbers.

The invention will be explained in more detail below with the aid of the figures and with the aid of exemplary embodiments. Show:

Fig. 1:

Principle scheme for the functioning of the λ / 4 resonators;

Fig. 2a:

Diagram of the absorption behavior of the λ / 4 absorber according to the invention;

Fig. 2b:

Diagram of the absorption behavior of the multifrequency absorber according to the invention;

Fig. 3a:

View of a first embodiment of a resonator with slotted head part for the λ / 4 absorber according to the invention;

3b:

View of a second embodiment of a resonator with a perforated head part for the λ / 4 absorber according to the invention;

3c:

View of a further embodiment of a resonator with heat-exchanging material in the base part for the λ / 4 absorber according to the invention;

Fig. 3d:

View of a special embodiment of a resonator in which the mouth region and the base region are inclined towards one another;

Fig. 4:

Cross section of a practical embodiment of the λ / 4 absorber according to the invention.

The principle of operation of the λ / 4 absorber 1 according to the invention will be explained in more detail with reference to FIG. 1. It can be seen from this figure that the opening of the λ / 4 resonator 2 lies in a sound-reflecting surface A. In the following, Z _o is used to denote the characteristic impedance of the air. The sound impedance in the floor area 3 is referred to below as Z _T and, in this simplified model, encompasses all sound energy losses inside the resonator (where Z _{T is} proportional to the quality factor Q). For a predefined length 1 and a predefined cross-sectional area S _{2 of} the λ / 4 resonator 2, an interaction zone S ₁ is formed on the reflecting surface A, in which the incident sound wave destructively interferes with the standing wave formed in the resonator 2. This interaction zone S ₁ is also known as an "equivalent absorption area". With 100% absorption, the sound impedance in the region of the interaction zone S ₁ will essentially correspond to the characteristic impedance Z _{o of} the air. If one also assumes that in the case of 100% absorption in the mouth region 4 of the λ / 4 resonator 2, the sound pressure and the particle flow are continuous, the following simple equation can be established: S 1 / S 2 = Z T / Z O ,

As shown in the aforementioned WO 96/23294, this applies not only to resonators standing perpendicular to the surface, but also just as well to resonators installed on or in this surface. If this equation is not met, there is no 100% absorption, ie there is a residual reflection which is dominated either by reflections on the reflecting surface A or by reflections on the resonator base 3. If you want to construct an absorber 1 with a high absorption capacity, S ₁ , S ₂ and Z _T cannot be freely selected and must be coordinated with one another. In addition, the desired bandwidth of the frequency response determines the value of Z _T. It is therefore important to be able to set Z _T and thus the energy losses in the resonator in the desired manner. According to the invention, this can be achieved by using soft, ie viscoelastic, closed-pore foams or other heat-exchanging materials in the bottom region of the λ / 4 resonators, it being possible to choose all materials which lead to energy dissipation in the event of high pressure fluctuations.

If one takes, for example, a resonator 2 for which the area ratio S ₁ / S ₂ = 25, then an impedance ratio Z _T / Z _o = 25 results for 100% absorption. Since Z _o corresponds to the characteristic impedance of the air, that is has a value of approx. 400 Ns / m ³ , the required sound impedance Z _T in the floor area is approx. 25 * 400 Ns / m ³ . Unfortunately, such high impedance values are difficult to achieve today.

The present invention also makes use of the knowledge that the following relationship applies to the resonance frequency for the impedance ratio Z _T / Z _o in the bottom region 3 and the impedance ratio Z _o / Z _mouth in the mouth region 4: Z T / Z O = Z O / Z Münd

This leads to the surprising insight that instead of increasing the energy losses in the bottom area 3 of the λ / 4 resonator 2, the energy losses in the mouth area 4 of the same can be increased as well.
For the above example, in which S ₁ / S ₂ = 25 has been selected, this results in an impedance ratio Z _o / Z _mouth = 25, respectively. Z _mouth = 1/25 * Z _o = 1/25 * 400 Ns / m ³ . This value corresponds approximately to the flow resistance, respectively. the sound impedance of a coarse-mesh grille (fly screen) and can thus be implemented in a simple manner, ie industrially.

Basically, however, you could at any point on the resonator by installing suitable air flow resistors bring about the desired energy dissipation.

These considerations can be confirmed by experimental measurements, as shown in FIG. 2a. Curve C in FIG. 2a shows the frequency response of an λ / 4 absorber with a depth of 84 mm and an inner diameter of 14 mm and an area ratio of S ₁ / S ₂ = 50, which has no means for increasing the sound energy loss. The frequency response, respectively. the absorption characteristic of this resonator has a bandwidth B _C of only 5.1%.

Curve D in FIG. 2a shows the frequency response of an acoustically optimized λ / 4 absorber according to the invention. With this absorber, the area ratio S ₁ / S ₂ = 25 and the absorption characteristic has a bandwidth B _D of approximately 11%.

These curves make it clear that through the change the air flow resistance, respectively. the sound impedance in the Mouth area 4 and / or in the bottom area 3 of the resonator 2 the frequency response width B can be influenced and almost 100% absorption can be realized at the same time.

FIG. 2b clearly shows the absorption behavior of the multifrequency absorber according to the invention. When using conventional λ / 4 resonators (narrowband absorbers) with different resonance frequencies, an absorption behavior is shown, as shown by curve V. The curve V results from the sum of the absorption characteristics S ₁ , S ₂ and S ₃ generated by the individual narrowband absorbers. This curve V shows the disadvantages of the multifrequency absorbers created with conventional narrowband absorbers. This curve V follows the frequency response of the individual narrowband absorbers and drops sharply between the corresponding resonance frequencies f ₁ , f ₂ and f ₃ , ie shows poor absorption in this intermediate range. In contrast, with the λ / 4 absorbers according to the invention it is possible to create a broad absorption band W with a constantly high absorption capacity. It is clear from FIG. 2b that the λ / 4 absorbers according to the invention have a larger bandwidth B than the conventional narrowband absorbers. In the case of multifrequency absorbers, this leads to significant overlaps in the absorption characteristics T ₁ , T ₂ and T _{3 of} the individual λ / 4 absorbers in the areas lying between the individual resonance frequencies f ₁ , f ₂ and f ₃ . These overlaps mean that the sum W of the absorptions T ₁ , T ₂ and T ₃ generated by the individual absorbers according to the invention also leads to an almost 100% absorption in the range between the resonance frequencies f ₁ , f ₂ and f ₃ . The curve W shows this clearly. This also makes it clear that multi-frequency absorbers with any absorption characteristic can be created with the λ / 4 absorbers according to the invention.

Figures 3a, 3b, 3c and 3d show embodiments of the λ / 4 absorber according to the invention. From Figure 3a it can be seen that the resonator 2 has a head part 5, in which has a variety of perforations, in particular Slots 6 is introduced. In addition to such a headboard 5 of such, can according to the invention a soft or in the bottom region 3 of the resonator 2 heat-exchanging material 7 may be attached (FIGS. 3a, 3c). In a further embodiment of the grid-like Headboard 5 can instead of slit-like perforations 6 holes 8 can also be provided (Figure 3b). The geometrical Design of the resonator 2, the choice of heat exchanging Materials 7 and the shape, dimensioning and Number of perforations 6, 8 are in the range of ordinary professional action. In Figure 3d is a special one Embodiment shown, in which the λ / 4 resonator as a body with two trapezoidal side surfaces is trained. This leads to the mouth area 4 and the floor area 3, respectively Opening area and the bottom surface inclined towards each other are. Thus, for example, the area of the Bottom area 3 are enlarged and thus its dissipative Effectiveness can be influenced. The same way range 4 are inclined. As a suitable energy-dissipating Materials 7 are materials to be considered which has a large heat capacity relative to air and a have as large a surface as possible, such as open-pore Foam with small cells, cotton-like fibers, granular marerial or porous ceramic material. As soft Materials come in closed pore, viscoelastic Foams or other materials in question at high Pressure fluctuations dissipate energy.

Figure 4 shows another industrially in a simple manner Realizable multi-frequency absorber 9 with a variety different sized resonators 2. In one preferred embodiment, this one from one Nonwoven fabric or foam carrier layer 10, in which tubular depressions 11 are formed. These tubular depressions 11 can be with a Adhesive layer 12 to be coated on the one hand, the pores of the To close the carrier layer 10 in this area, and on the other hand, a cover film 13 on this carrier layer 10 to fix. The holes 8 or slots according to the invention 6 can be introduced into this cover sheet 13. For certain applications is also provided, the molded one Carrier layer 10 instead of being provided with a cover film 11, to be attached to a solid outer skin, e.g. a bonnet and the perforations 8, 6 in the deformed area 14 to mount the carrier layer 10.

The λ / 4 absorbers according to the invention can be simplified Manufacture wise industrially. In particular can these are extruded in a known manner, for example as extruded Plates with tube-like depressions, which are covered with a second plate become. Depending on the area of application, these can be according to the invention Absorber also with the help of deep-drawing or Manufacture injection molding technology. In another form of production can directly corrugated material into which the perforations according to the invention introduced will be used.

It is understood that for the particular applications that λ / 4 resonators according to the invention in a suitable manner can be dimensioned and / or differently dimensioned λ / 4 resonators to form a broadband absorber can be combined with each other. It goes without saying also that the resonators according to the invention, individually, in Groups with similar resonators (monofrequency absorbers) or in groups with different dimensions Resonators (multifrequency absorbers) manufactured and used can be. Of course, the inventive Absorber also with conventional fibrous or foamed absorbers can be combined and in particular be coordinated so that this in the area of the absorption drop are effective against low frequencies. Your preferred application is in land and aircraft seen just like transformers, generators, Gearboxes or other machines of any kind.

Applications in vehicle construction are particularly in the case of absorption components on bonnets, end walls and wheel arches, in particular on the engine side, on roof linings, door panels respectively. Door hollow bodies and trunk lids, in Delivery van or truck, in the loading area, on the roof or on the walls. It goes without saying that these absorbers also Building construction or road construction can be used, in particular on the walls and ceilings of living or working spaces, in factories, sports halls, tunnels or on sound screens along roads or train paths.

Claims (10)

1. A λ/4-absorber for the absorption of sound, as is generated by machines, in particular by vehicles, comprising a plurality of tubular λ/4-resonators (2) whose openings border on a sound reflecting surface (A), whereby the openings of the individual λ/4-absorbers (2) are spaced from each other in such a manner that the interaction zones of the individual resonator openings, in which zones the impinging sound wave destructively interferes with the standing sound wave formed in the resonator, are distributed to cover the sound reflecting surface as far as possible and at the same time do not significantly overlap each other, characterized in that for adjusting the band width (B) of the resonant frequency response (C, D) of the individual λ/4-resonators (2), these are provided with means for changing the sound energy loss in the opening region (4), which means have a low air flow resistance approximately equivalent to that of a wide-meshed grid.
2. A λ/4-absorber according to claim 1, characterized in that the means (6) for changing the energy loss in the opening region (4) comprise a head part (5) with a multitude of slot-shaped perforations (6).
3. A λ/4-absorber according to claim 1, characterized in that the means (6) for changing the energy loss in the opening region (4) comprise a head part (5) with a multitude of hole-shaped perforations (6).
4. A λ/4-absorber according to claim 1, characterized in that the means (6, 8) for changing the energy loss in the opening region (4) comprise a grid-like head part (5).
5. A λ/4-absorber according to one of claims 2 to 4, characterized in that the head part (5) is an integrated part of the λ/4-absorber (2).
6. A λ/4-absorber according to claim 1, characterized in that the further means (7) for changing the energy loss are provided in the floor region (3) of the λ/4-resonator (2).
7. A λ/4-absorber according to claim 6, characterized in that the means (7) for changing the energy loss in the floor region (3) comprise a soft and/or heat exchanging material provided in the floor part of the resonator (2).
8. λ/4-absorber according to one of claims 1 to 7, characterized in that the opening region (4) and the floor region (3) are tilted towards each other.
9. The use of a λ/4-absorber according to one of claims 1 to 8 as a multifrequency absorber.
10. The use of a λ/4-absorber according to one of claims 1 to 8 as a monofrequency absorber.

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