Acoustic Design of the Microstructure of Granular Sound Absorbers

© Fraunhofer IBP

Cylindrical granules serving to produce sound absorbing model substances

Open-celled porous materials used as sound absorbers are prevalent.The sound propagation is mainly absorbed within the air-filled pore volume. Thus, the absorbing mechanisms are virtually depending only on the geometry of the pores. Acoustically appropriate pore diameters range around 100 micrometers.

If this microstructure is specifically designed, the thus achieved materials are more efficient than common fibre fleeces and regularly structured polyurethane foams. For this purpose, an optimization method by means of computer simulation was used, being already well-established in the field of filter media development. Based on geometry models, flow and diffusion processes are calculated, thus gaining input data for classic absorber models.

Segmented computer tomographies can directly be used as geometry models. However, parameterized models, being created by structure generating programs and allowing for correlations with production parameters of material systems, like for example grain size distribution and binder content, are more expedient. Yet an essential part of the work has been consistently focusing on the introduction of the procedure: By help of model substances, geometric dimensions were deduced from image analyses, acoustical dimensions were measured and subsequently reproduced by computer simulation. For this purpose, bulks of well-defined, non-porous granules such as glas beads or cylindrical blasting abrasive were used.

Essential insights could be gained for the following two systems: loose bulks of solid granules and thin-layered granulated acoustic plaster for suspended ceilings. For the bulks, different influences like grain size distribution, sphericity and roundness of the granules were determined. In addition, practical aspects were investigated systematically, like variable packing density due to subsidence effects or vibrations. Thus, bulks with particular absorption spectra can be specifically created, for example by using recycled material frequently available in granular form. When investigating acoustic plasters, the complete layered system including the porous base plate situated underneath was examined – with some surprising results. In contrast to thicker porous absorbers, the maximum required open porosity of the plaster layer amounts to a mere 40 percent and porous aggregate does not provide any benefits. Following these investigations, the potential of materials with different porous phases is to be probed. For low frequencies, the absorber performance is expected to be good even with comparatively thin layers.