Micro-perforated plates (MPP’s) are metal sheets which are perforated with sub-millimeter sized holes. These small holes generate viscous friction when a sound wave passes through. The viscous friction converts acoustic energy into heat energy.
Around the MPP layer, there are several back cavities. These cavities have a resonator function, and let the molecules ‘breathe’ through the MPP layer. The resonance frequencies of the different chambers are tuned to achieve a maximal noise reduction in separate frequency bands, tuned to the dominating spectral components of the noise source.