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. 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.

computer-generated layer structure, consisting of open foam covered by a nonwoven on measured and computed acoustic absorption of a two-layer fiber absorber, failure of the simple model of Delany & Bazley