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Fiber orientation |
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computer-generated layer structure, consisting of open foam covered by a nonwoven on |
Design simulation of sound absorbing materials Acoustically effective materials are applied for noise reduction - technically speaking: for the reduction of the sound pressure level - in many areas today. Among these are porous materials such as fiber absorbers or pore-elastic foams. A series of simulation programs for the computation of the acoustic effectiveness of pore-elastic absorbers in components or assembly groups is already available today. These programs, for example AutoSEA2, LMS Sysnoise, or Actran, describe the pore-elastic materials via effective material parameters. Interestingly enough, up to now there has been no software which determines the required effective material parameters by simulation, so that a connection between the microscopic structure and the acoustic properties can be identified. These properties are still determined by complex measurements of blank parts and prototypes, which is very time-consuming and expensive. | |||
measured and computed acoustic absorption of a two-layer fiber absorber, failure of the simple model of Delany & Bazley |
For several years now, the Fraunhofer
ITWM has developed methods for a
drastic reduction of these high expenses with respect to time and money which are connected to the development of new absorber materials. It is of
central importance that, instead of being measured, all the material parameters determining the acoustic properties of the material are completely computed instead. The basis is a stochastic model which represents the
microstructure of the material realistically. In the case of highly porous absorbers, the flow resistance of the microstructure is determined on the basis
of the solution of the Stokes equation;
an effective model provides the conclusions with respect to the acoustic properties. For highly porous absorbers, all
the effects of airborne noise absorption
can be accounted for by the flow resistance. In the case of a lower porosity (e.
g., pressed nonwovens or open foams),
the flow resistance alone does not suffice, so that further parameters, such
as tortuosity or characteristic lengths,
must also be determined. In expert language, this means that the simple model of Delany & Bazley is substituted by
the model of Allard & Johnson. We
have nevertheless developed methods
for a direct computation of all acoustic material parameters also for those
materials which cannot be defined as
highly porous. Currently, we are expanding our method also for noise absorbing materials where a part of the noise energy is dissipated in the solid body. This requires, among others, the computation of the structure elastic material parameters. The advantage of our method com- pared to all the other methods currently available for pore-elastic absorbers is that one can do completely without the production of any blank parts or prototypes. |
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