Acoustic properties of lightweight micro-perforated plate systems

The paper extends the investigation of micro-perforated plates, originally initiated by Maa, into acoustic properties by considering the coupling of such a plate with an ordinary flexible plate through an air gap. It has been found that the resulting two plate system, with one of the plates being micro-perforated, can provide potentially interesting performance while still remaining free of any classical absorption material. The analysis is limited to infinite plates, with the consideration of the cases of normal, oblique and diffuse incidence. A classical wave approach is used to model the absorption and the transmission of a micro-perforated plate backed by a simple plate. Formulae are supported by plots which illustrate the system performance in potentially applicative cases. Measurements done in the Kundt tube and in an experimental window have confirmed the validity of the developed model. By acting on the parameters of the system, i.e. the perforation rate, hole diameter, gap width and plate thickness, the absorption and transmission coefficients of the system can be tuned, within certain limits, for increased efficiency in a selected frequency range. The presence of a second flexible plate was shown to increase transmission loss, reduce the reflection effect and weaken the coincidence effect.