Multi layer sound-proofing structure

A multi layer sound-proofing structure for absorbing sounds having frequencies of from about 100-4000 Hz, comprising:

• a first layer (2) comprising a light aggregate and a binder, having a bulk density of from 0.1 to 2.0 g/cm.sup.3 and void fraction as continuous void of from 15 to 60%;
• a second layer (4) on said first layer (2), said first layer having at least a major surface uncovered by said second layer so as to serve as an incident surface for sounds;
• a third layer (3) on said second layer (4) with said second layer (4) interposed between said first and third layers, said third layer comprising a light aggregate and a binder, each of which is the same as or different from those of the first layer and having a bulk density of from 0.1 to 2.0 g/cm.sup.3 and void fraction as continuous void of from 15 to 60%;
• said second layer (4) being of a material having a quality to insulate relatively high frequency sounds and to permit penetration of relatively low frequency sounds;
• and
• a fourth layer (1) of a material having a high sound-insulating ability, and covering all free surfaces of the layered structure of said first, second and third layers except for said uncovered major surface of said first layer (2) such that said uncovered major surface of said first layer at least substantially defines the direct incident surface for sounds impinging on said sound-proofing structure.