Principles of Sound Absorbers

Abstract

This chapter introduces the principles of sound absorbers and the factors that affect acoustic absorption. The basic wave phenomena related to sound propagation such as acoustic reflection, absorption and scattering are introduced first and then the corresponding parameters such as transmission loss, absorption coefficient, scattering coefficient and flow resistance are explained. Sound absorbers can be made from porous materials or resonant structures, and the main mechanisms for sound absorption are acoustic impedance matching on the absorbers’ boundary and acoustic energy dissipation inside the absorbers. Porous absorbers are materials where sound propagation occurs in a network of interconnected pores so that viscous and thermal effects cause acoustic energy to be dissipated. Resonant absorbers have two common forms: membrane/panel absorbers and Helmholtz absorbers. The main difference between porous absorbers and resonant absorbers is that the former is effective for broadband from mid to high frequency while the latter is usually only effective in a narrow tunable low frequency band. The applications of sound absorbers are focused on the discussion of reverberation time control and sound pressure level control in rooms. Finally, specific discussions are given to acoustic textiles and its recent developments. The contents of this chapter are focused on the fundamental aspects of sound absorption and absorbers to serve as an introduction to students and designers with basic knowledge in mathematics and physics.