A microstructure material design for low frequency sound absorption

The acoustic properties of an air-saturated porous material depend on its microstructure and the thickness of the sample. Thick samples of conventional acoustic materials are required to achieve good absorption at low frequencies. This study suggests a new micro-structure for the design of low-frequency resonant acoustic absorbers. A perforated material is studied, in which the main perforations are connected to a collection of periodically spaced very thin annular dead-end pores with respect to the lateral size, these absorbers are called multi-pancake materials. It is shown, that at low frequencies, the periodic array of annular dead-end pores increases the effective compressibility without modifying the effective dynamic density. Due to this effect, the first sound absorption peak appears at much lower frequency, compared to that of the structure without dead-end pores. A transfer matrix approach is proposed to model and optimize the absorber. Prototypes have been 3D printed and tested for sound absorption and transmission loss. This design allowed to design materials capable of producing absorption peaks at a few hundred Hz and constituted of a stacking of 10-20 annular dead-end pores, each dead-end having a thickness of the order of 1 mm or less so that the overall material thickness was of a few cm. A good agreement between the data and the model predictions is demonstrated.