Acoustic Design of the Microstructure of Granular Sound Absorbers

Cylindrical granules
© Fraunhofer IBP
Cylindrical granules for the production of sound-absorbing model substances.

Open-celled porous materials are widely used as sound absorbers. The sound propagation is mainly absorbed in the air-filled pore volume, so the sound attenuation depends almost only on the pore geometry. Acoustically appropriate pore diameters are in the range of 100 micrometers. If this microstructure is specifically designed, more efficient materials can be built up than the common fiber fleeces and regular soft foams. For this purpose, an optimization method using computer simulation was applied, which is already well-established in the field of filter media development. Based on geometry models, flow and diffusion processes are calculated, thus obtaining input data for classic absorber models.

Segmented computer tomographies can be used directly as geometry models. However, parameterized models, which are created with structure-generating programs and allow correlations to production parameters of material systems, like for example particle size distribution and binder content, are more suitable. An essential part of the work so far has been focusing on the introduction of the process: By help of model substances, geometric quantities were deduced from image analyses, acoustical quantities were measured and subsequently reproduced by computer simulation. For this purpose, loose bulks of well-defined, non-porous granules such as glass beads or cylindrical blasting media were used.

Essential insights could be gained for two systems: loose bulks of solid granules and thin-layered granulated acoustic plaster for suspended ceilings. For the bulks, influences such as particle size distribution, sphericity and roundness of the granules were determined. In addition, practical aspects such as variable packing densities, occurring for example in the case of subsidence phenomena or vibrations, were systematically investigated. Thus, bulks with specific absorption spectra can be created, for example by using recycled material often available in granular form. When investigating acoustic plasters, the complete layered system including the porous base plate underneath was examined with surprising results. In contrast to thicker porous absorbers, the maximum required open porosity of the plaster layer is only 40 percent and porous aggregates have no advantages. Following these investigations, the potential of materials with different porous phases will now be explored. For low frequencies, good absorber properties are expected even with comparatively thin layers.