Fibrous porous materials and flow resistivity modeling

Porous materials are used to dissipate acoustical energy in a variety of environments (e.g., architectural and industrial spaces, air handling systems). In general, three types of porous materials are used: foams (e.g., polyurethane, aluminum, polyimide), consolidated materials (e.g., ceiling tiles, plasters, ceramics), and fibrous materials (e.g., fiber glass, rock wool, Kevlar^® 29). Foams have a net-like structure of non-uniform “fiber” size and may have “window panes” in the structure which can form “blind alleys” and divert the flow. Consolidated materials tend to be constructed from a range of particle/fiber material sizes and held together with a binding material. Some tiles have either a perforated or fissured face surface to allow the sound to enter the tile. Fibrous materials tend to consist of arrays of cylindrical fibers, which may or may not have some larger inclusions and/or binding material, but have no “blind alleys”. This paper concentrates on fibrous absorbers and contains a review of five theoretical/empirical flow-resistivity based, acoustical models. These models are compared to experimental results of tests on air-saturated, fibrous, porous materials at several porosities and flow resistivities. Comparisons include flow resistivity, propagation parameters, characteristic impedance, and impedance of finite samples.