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ACOUSTIC ABSORPTION AND REVERBERATION TIME
This chapter briefly outlines the basics of absorption and reverberation time. More explanations are given in Chapter 6.
On entering a space, one has probably experienced a feeling like “it sounds dead” or “it sounds lively.” This has often been a problem for performers when deciding on how much volume they should turn out, as different places of similar capacity may turn out to sound differently. More to the point, one has probably experienced trouble with the understanding of speech messages in a lively space. All these feelings are linked to the reverberation of the space (the longer the reverberation, the livelier the room will be perceived).
A professor, W. Sabine, found out that according to the number of cushions brought by his student inside the rather uncomfortable lecture theatre, the space sounded more or less lively. This eventually led to the notion of absorption quantity.
An absorptive material is a porous or fibrous material in which the vibrations of air are turned into heat when scrapping against the walls of the cavities. The efficiency of such a material is characterized by its absorption coefficient, theoretically ranging from 0 (reflective) to 1 (absorptive). In practice, the absorption coefficient value often appears as greater than 1 in commercial leaflets due to reverberant room measurement techniques (cf. Section 18.104.22.168). Measurements are carried out in a reverberant room (i.e., in a diffuse field where sound is supposed to be incident from all directions), typically on a 10 m2 (according to standard ISO 354 ) or 6 m2 (according to standard ASTM C423 ) room. It also is possible to assess the absorption coefficient under normal incidence using a wave tube according to ISO  or ASTM [63, 64] standards, but the relevant absorption coefficient value under normal incidence does differ from the one in the diffuse field, and picking up the pieces is not a task for beginners .
One may care to note that an absorptive material may be hidden by a cladding, but only as long as it is not airproofed. This is a significant difference compared to thermal insulation, where one can keep the thermal insulation performance using plasterboard in front of the insulation.
Often there is confusion caused by architects and contractors indifferently calling insulation a thermal insulation material or an absorptive material. While some materials actually manage to do both (e.g., a mineral wool), most of the time they are associated with a water barrier made of an airproof foil: This means they are no longer absorptive in the middle- and high-frequency range.
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