Little theoretical or experimental information is currently available regarding the acoustic absorption coefficients of the lightweight double panel constructions which are frequently used as walls or roofs of factories. Recent measurements have indicated that these surfaces may provide the majority of the low frequency absorption in spaces where they are used. A literature review suggested that an existing model for sound transmission through infinite double panels could be adapted to the calculation of absorption coefficients. Such a model was developed and used with material properties appropriate to asbestos panels to predict the plane wave absorption coefficient as a function of angle, from which the random incidence absorption coefficient was calculated. It has been demonstrated that absorption arises from three mechanisms: sound transmission through the structure, the mass-air-mass resonance and coincidence effects. Each of these phenomena is associated with a characteristic variation of absorption with angle of incidence and it is proposed that under appropriate conditions measurements of absorption coefficient versus angle may be used to identify particular absorption mechanisms. When realistic physical parameter values were used, the calculated absorption coefficients showed some features akin to those observed in measured results: e.g., a peak in the 125Hz 1/3 octave band. However, the overall level of the absorption coefficient was significantly under-estimated by the double infinite panel model even though the absorption coefficient was large at particular angles of incidence. Thus it appears that the infinite double panel model does not account for all significant absorption mechanisms which operate in real constructions. A more complete model which accounts for finite component panel size is described in a companion article.
Factory acoustics, Absorption coefficients, Angular variation, Double panels, Roofs
Acoustics and Noise Control
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