Perforated Materials with Periodically Distributed Annular Cavities for Low Frequency Acoustic Absorption

A microstructure design of thin low-frequency resonant acoustic absorbers is proposed. 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 has been shown in the past that the acoustical properties of perforated materials can be described by the classical theory of porous media. With the help of this model in the rigid frame approximation, 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 absorption peak appears at much lower frequency, compared to that of the perforated 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. New materials capable of producing absorption peaks at a few hundred Hz for an overall material thickness of a few cm were designed. A good agreement between the data and the model predictions is demonstrated. New designs for broader absorption and under higher sound pressures are investigated.