Absorption Characteristics of Periodically Perforated Suspended Ceilings

Sammanfattning: The sound absorption characteristics of resonant absorbers, implemented as perforated suspended inner ceilings attached with thin porous layers, are investigated. Several existing theories model the principal absorption mechanisms, but the interaction with the outer sound field influences the absorption in ways that can be modelled with new approaches. The effect of arranging the absorbing surfaces in various patterns is modelled with a variational model. Periodic effects, depending on correspondence of projected wavelength and the dimensions of the periodic cells for certain incidence angles and frequencies, is modelled with an approach that formerly has been used on periodically reinforced floors. The model presupposes reflection from absorbing points and is therefore intended for small perforations. In this way an efficient mathematical treatment is gained at the expense of generality. This model is then extended to incorporate the backing field, which makes it possible to handle lateral waves within the absorber and therefore non-locally reacting absorbers. In order to evaluate the presented theories, the static flow resistance, R, of the compound absorber (thin porous layer and rigid facing) has to be known. Since R of a compound absorber in many cases is hard to estimate from a R-value measured on separate samples of the porous layer, a new device for measurements of R. on compound absorbers was implemented. The device, which has a simple and robust construction, is intended for measurements on standard tiles of perforated plaster boards attached with a thin porous layer, but, by changing measurement interface, measurements can also be performed on the sole thin porous layer.