Abstract

The acoustic efficiency of components used in the automotive industry for noise insulation can be characterized by their absorption coefficient. It can be measured either based on the normal acoustic incidence or a diffuse field. The diffuse field approach is more realistic for the car interior since the sound field inside the car is diffuse. Automotive OEMs and suppliers are usually applying experimental approaches to determine the absorption coefficient of a given porous material utilizing setups that act as small reverberant rooms called ?alpha cabins?. This paper discusses a methodology based on a frequency-domain Finite Element (FE) method to simulate an alpha cabin in the range of 400-10000 Hz and determine the absorption coefficient in a computationally efficient way. To overcome computational challenges emanating from the size of the cabin, an innovative three-step approach is proposed. Below the Schroeder frequency (~1500 Hz), where the alpha cabin is not large enough to be considered as reverberant, the absorption coefficient is determined in the same way as in the experimental approach. This involves the reconstruction of the time-domain signal from the response spectrum on a series of microphones for estimating the sound-decay values inside the cabin (RT60). For the mid-range frequencies, between 1500 and 5000 Hz, energetical quantities calculated via the finite element method in the fluid and porous parts of the setup are used to determine the RT60 values without the need of time-domain reconstruction. Finally, at high frequencies, between 5000 and 10000 Hz, the model is scaled so as to reduce its size while still keeping the cabin sufficiently diffuse, and the same post-processing method as in the mid-frequency range is used. The results from this methodology are compared with previously published results in order to evaluate the viability of the method, which provides the possibility to reduce expensive experimental work.