Patent ID: 8631899

Claim:
A sound absorber having porous material for muffling sound, comprising: a first porous foamed layer comprising a first porosity; a second porous foamed layer comprising a second porosity different than the first porosity of the first porous foamed layer, the second porous foamed layer being stacked on the first porous foamed layer and forming a first bordering region therebetween and having pores of a different size than the first porous foamed layer; a third porous foamed layer comprising a third porosity different than the first porosity of the first porous foamed layer and the second porosity of the second foamed porous layer, the third porous foamed layer being stacked on the second porous foamed layer and forming a second bordering region therebetween, wherein: the first and second bordering regions are different in that they have at least one of different input impedances, different sound propagation rates, different densities, different porosities, different flow resistances and at which there is an impedance shift between two bordering regions, wherein the first and second bordering regions are designed, so that there are at least two different boundary layers between the regions having impedance shifts of different value so that sound having frequencies below 600 Hz will be absorbed by at least 50%.