Airborne noise propagates in the air. When they hit a wall, they are partially reflected by the wall, absorbed by the material, and transmitted on the other side.
To improve the acoustic performance of a wall or a partition, several solutions are possible (see diagrams).
The weakening index Rw indicates the level of sound reduction (in dB) of the wall or insulation.
For example, when a sound of 65 dB is emitted in a room and the wall shows a Rw index of 35 dB, the perception will be 30 dB in the next room.
If we simply want to avoid noise reverberation, we must double the wall with a very absorbent material (alpha absorption coefficient close to 1, surface reflection coefficient R close to 0).
But if you want a really optimal insulation: either the wall is very thick, heavy and homogeneous, or it is composed of materials enclosing a flexible material (type glass wool). This is the "mass-spring-mass" effect. For example, a 14 cm thick complex with two BA 13 gypsum boards enclosing mineral wool attenuates the sound as much as a 20 cm thick concrete wall while weighing ten times less.
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Are mineral wools dangerous?
After the asbestos business was banned in 1997, mineral wools were long suspected to be also products at risk...
Yet the International Agency for Research on Cancer (IARC) recently downgraded these products from group 2B (possible carcinogen) to group 3 (unclassifiable) because no experimentation could demonstrate that new fibers developed by industry - more small and less permanent - were harmful to humans.