Snow as a paradigm for acoustic absorbers

In some respects snow is an ideal sound absorber. It is acoustically highly efficient, at least when the snow layer is fresh and thick enough, does not cost anything und disposes itself by melting into a non-hazardous fluid, which is just another stage in the never ending recycling process of H2O. Such considerations motivated a study on the acoustic properties of snow and an attempt to imitate nature by designing new sound absorbing materials. In addition to measuring the absorption in an impedance tube, the structure of several types of snow was determined by X-ray tomography with a resolution of 0.01 mm. These experimental data provide an excellent basis both for numerical calculations of snow properties and for correlating geometric parameters with the parameters of theoretical absorption models. Understanding how the geometric structure of a porous material relates to sound absorption is mandatory for a systematic and successful design of artificial absorbers. The measured absorption of the snow samples can be successfully described by Wilson’s model or the Johnson-Champoux-Allard model. However, the determination of the model parameters from the geometric structure is not yet completely perfect. On the other hand, the creation of artificial porous structures which have to meet also numerous economic and ecological requirements can be a difficult technological challenge. The efforts in this direction resulted in an eco-friendly artificial-snow sound absorber made exclusively from polyethylene.