Acoustics – Sound-modifying means – Sound absorbing panels
Utility Patent
1999-08-02
2001-01-02
Dang, Khanh (Department: 2837)
Acoustics
Sound-modifying means
Sound absorbing panels
Utility Patent
active
06167985
ABSTRACT:
BACKGROUND
The present invention relates to a &lgr;/4-absorber for the absorption of sound as is produced by machines, in particular vehicles, with a multitude of tubular &lgr;/4-resonators, whose opening borders on a sound reflecting surface.
1. Field of the Invention
It is the endeavour of the modern vehicle and machine industry to reduce or completely eliminate the noises emitted from either the machines or vehicles. For sound absorption today essentially mats of fibre insulation material or open-pore foams are employed which are laid around the sources of noise or are mounted in the direct vicinity of these. Their application however is restricted in heavily contaminated environments, since these open-pore materials rapidly accumulate with oil, water or dust and thus their sound absorbing effectiveness is lost.
It is also known to construct sound absorbers of a multitude of variously dimensioned Helmholtz resonators. Such Helmholtz absorbers have not proven themselves in practice for various reasons. In particular such Helmholtz absorbers are difficult to dimension and/or to manufacture and are not suitable for use in heavily contaminated environments.
For this reason there have already been put forward sound absorbers which consist of a multitude of tubular resonators. These tubular resonators may be mounted in a manner such that occuring contaminations or moisture may not get captured the rein. Furthermore these tubular resonators differ from Helmhholz resonators in their acoustic functioning manner and are known to the expert under the name &lgr;/4-resonators.
This difference lies essentially in the simultaneously occuring mass and compressibility of the air in the resonator and in particular may be recognized in that with &lgr;/4-resonators the resonant frequency is directly determined by the standing wave, whose wavelength is a quarter of the length of the tubular resonator, whilst the acoustic functioning manner and resonance of Helmholtz resonators must be described and determined by a spring-mass system.
2. Description of the Prior Art
GB 2 038 410 describes an acoustically effective lining for airplane engines, in which a multitude of Helmholtz absorbers are combined with &lgr;/4-resonators. These resonators are grouped as tightly as possible in order to achieve a maximum in absorption capability. The sound openings of these resonators are covered with a perforated metal sheet or with a nonwoven fiber (having a relatively high acoustic resistance) in order to improve the acoustic connection to the outer sound field. Furthermore, it is suggested to acoustically connect the inidividual hollow bodies by means of perforations. This arrangement has the same effect as classical Helmholtz resonators, which all dissipate the sound energy internally.
DE 94 08 118 discloses a sound absorber with a plurality of tubular shaped recesses or channels which are inserted into a porous absorbant material. The openings of the individual &lgr;/4-tubes are covered with a porous foam, a nonwoven fiber or a thin foil. In a preferred embodiment the cavities are totally or partially filled with a further sound absorbing material.
With practical embodiments of Helmholtz resonators, many assumptions which are made for the advance calculation of the resonant frequency may not be realised. Thus for example the walls of Helmholtz resonators may not be constructed so stiffly that these do not deform on pressure fluctuations on resonance, or the mass of air in the throat region of the Helmholtz resonators may not be exactly determined. The advantages of &lgr;/4-resonators with respect to Helmholtz resonators lie thus essentially in the more exact determination in advance of the absorption effect, their lower danger of contamination and their more simple dimensioning and manufacture.
Such a &lgr;/4-absorber is for example described in WO 96/23294 and comprises a multitude of tubular resonators, whose sound openings border on a surface in such a manner that the interaction zones (in which the impinging sound wave and the standing waves formed in the individual resonators interfere destructively) of the individual resonator openings are distributed covering as much surface as possible and at the same time do not considerably overlap. Such &lgr;/4-resonators fundamentally absorb in a narrow frequency range about their resonant frequency f
o
. The width of this frequency range is dependent on the quality factor Q of the resonators or on the size of the energy losses which occur on resonance. &lgr;/4-absorbers, as is described in this WO 96/23294, may be embedded in any dense-bodied, reverberant material such as for example metal, plastic, ceramic or glass. With practical application of these absorbers, in particular when a wider frequency band is to be achieved with a multitude of resonators, it is important to be able to influence the energy losses in a simple manner. With certain embodiment forms e.g deep-drawn semi-tubes which are completed with a plate, the energy losses are very small, i.e. the Q-factor and the terminating impedance are very high. This leads to undesired narrow resonance absorption curves.
SUMMARY OF THE INVENTION
It is therefore the aim of the present invention to be able to manufacture acoustically highly effective &lgr;/4-absorbers with an adjustable band width in a simple manner.
According to the invention this is achieved by a &lgr;/4-absorber. In particular it is provided either for increasing the acoustic impedance Z
open
in the opening region of the &lgr;/4-resonators by way of a perforated head part, or to produce additional energy losses in the floor region by applying soft and/or heat-exchanging material, thus in order to reduce Z
T
. This may be achieved in that in the floor region of the resonators, where the pressure fluctuations are very large, a heat sink with a large contact surface for air is provided. Such a heat sink is produced by any material which can take up and carry off heat from the temperature fluctuations of the air produced by pressure fluctuation. The man skilled in the art in the field of noise protection has adequate knowledge of such materials. Another practical possibility is seen in the use of a stopper from closed-pore viscoelastic foam.
Another possibility lies in creating energy losses in the opening region by way of incorporating a—low—airflow resistance, e.g. a “grid”. With the embodiment form of a deep-drawn foil and cover plate, such a “grid” may be produced in that one does not remove the end to be opened, but only perforates it.
It has been surprisingly proven that by restricting the airflow in the opening region of the &lgr;/4-absorber, its absorption capability is not fundamentally reduced, but rather by way of this, an absorption with a larger band width of the frequency response may be achieved.
The present invention thus permits for the first time efficient &lgr;/4-absorbers to be manufactured industrially, i.e. inexpensively. Furthermore the present invention also permits the design of multifrequency absorbers in a simple manner in that, for forming a wider resonant frequency band, several differently dimensioned &lgr;/4-resonators with an increased sound energy loss according to the invention in the opening region and/or the floor region are combined.
In the following the invention is explained in more detail by way of the figures and with the help of embodiment examples. With this there are shown:
REFERENCES:
patent: 4160491 (1979-07-01), Matsumoto et al.
patent: 5457291 (1995-10-01), Richardson
patent: 5959265 (1999-09-01), Van Ligten
patent: 6012543 (2000-01-01), Shimpo et al.
Dang Khanh
Nath Gary M.
Nath & Associates
Novick Harold L.
Rieter Automotive (International ) AG
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