Enhance sound absorption coefficient in low and mid frequency range by alternating open and closed pores

Abstract

Porous material is widely applied to absorb sound energy and secure comfort in a passenger compartment of a vehicle. Due to the limitation of design space, the thickness of porous material is constrained up to 20 mm, and a thin porous material results in low sound absorption performance, especially in low- and mid- frequency range. Therefore, sound absorption performance have to be enhanced with thin porous material in low- and mid- frequency range. In this study, we propose a new concept of porous material that consists of open and closed cells. This concept utilizes the tortuosity effect, and flow path of acoustic wave propagating in porous material are warped and elongated by alternating open and closed cells periodically. Microscopic unit cell models are constructed and sound absorption performance is calculated by using the homogenization method based on an asymptotic expansion. By compared with porous material by open cells of the same pore size and flow area, sound absorption coefficient increases up to 0.5 at 1 kHz with 10 mm thickness. Moreover, a prototype of the porous material is fabricated by using additive manufacturing of BMF microArch S240. Measured sound absorption characteristics were found to agree well with the calculated characteristics.