ABSTRACT
Typical porous absorbers are carpets, acoustic tiles, acoustic (open cell) foams, curtains, cushions, cotton and mineral wool. They are materials where sound propagation occurs in a network of interconnected pores in such a way that viscous and thermal effects cause acoustic energy to be dissipated. As discussed in Chapter 1, they are used widely to treat acoustic problems, from reducing resonances in double partition cavities to improve sound insulation, to their application to absorb noise and reduce sound levels in factories. This chapter will detail the physical processes producing the absorption and theoretical models for predicting absorption properties. The first section gives a qualitative description of the use of porous absorbers; this
will be followed by some example materials in Section 5.2. These materials range from standard well-known materials, such as mineral wool, to more recent developments, such as absorbent plaster systems. Section 5.3 and onwards then outlines the methods needed to predict the absorption from this type of absorber. The theoretical sections start by outlining how the sound propagation within a porous absorbent might be modelled in terms of characteristic acoustic parameters of the material. There are empirical and semi-empirical approaches, and both are detailed. The chapter then proceeds to show how these acoustic parameters are combined with mounting conditions, to enable the absorption coefficient and surface impedance to be predicted, which is ultimately what is required in design.
