VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

4.3. MEASURING THE ACOUSTIC ABSORPTION 71Material 125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hzparquet flooring on concrete 0.04 0.04 0.07 0.06 0.06 0.07carpet on concrete 0.02 0.06 0.14 0.37 0.60 0.65Brick 0.03 0.03 0.03 0.04 0.05 0.07Concrete - coarse 0.36 0.44 0.31 0.29 0.39 0.25Concrete - painted 0.10 0.05 0.06 0.07 0.09 0.08Curtain 0.03 0.04 0.11 0.17 0.24 0.35Window glass 0.35 0.25 0.18 0.12 0.07 0.04Plaster 0.013 0.015 0.02 0.03 0.04 0.05Marble/tile 0.01 0.01 0.01 0.01 0.02 0.02Glasswool (5cm) 0.22 0.82 0.99 0.99 0.99 0.99Water 0.008 0.008 0.013 0.015 0.020 0.025Person 0.25 0.35 0.42 0.46 0.5 0.5Table 4.1: Absorption coefficients of some materials3. All wave propagation directions have the same probability. The shapeof the space is not of any importance, and is basically random andirregular ; however the dimensions are approximately of the same orderof magnitude.4. If a source of constant level acts in this space, a diffuse sound field willbe built up in this space after a certain amount of time. In doing so,the energy density in all points of the field will be constant.This theory neglects therefore :The direct field in the immediate vicinity of the sound source.Certainsideeffectsintheimmediatevicinityoftheabsorbingmaterials.Possible interferences and diffractions.Conclusion : In practice we assume that this theory isapplicable onlarge,possibly irregular shaped rooms, in which acoustic absorbing materials arepresent to a limited extent (the theory is not valid for an anechoic room).Consider a space with volume V, and wall surface S. Suppose that adiffuse acoustic field prevails in the space. We consider a sound source inthis space, of which the walls have a mean absorption coefficient a. Whenthe source is disabled, the intensity is I 0 . The sound dies out, i.e. theintensity will decrease due to multiple reflections and absorptions, so that