[kostix777]

One of the best sound absorption materials is fiberglass (not the dense, resin glued panels, but the cutton-like fluffy fiberglass). The method by which it works is that when the air vibrations (sound) hit the fiberglass, the fibers in it rub against each other and a lot of this vibrational energy is transformed into small amounts of heat, which "absorbs" the sound.
Say you have 2 parallel walls, be it in your room, or a simple square box, any pair of parallel faced walls, will have an inherent resonant frequency, which happens to be directly related to the wave length of the frequency in question. Lets say we have 2 parallel walls, separated by 5 meters distance.
Now, to calculate the resonant frequency of that parallelity, you need to divide the speed of sound by the separation between the walls.
At 24ºC, speed of sound is approximatelly 344 meters per second, so we have 344 / 5 = 68.8Hz.. this means that when the frequency of 68.8Hz (and its subharmonics and higher harmonic) is played in between those 2 parallel walls, it will bounce between the 2 walls longer than any other frequencies.
Now if we were to damp one of the walls with some layers of fiberglass, when the sound hits that wall, part of the sound will be transformed into heat and part of it will still hit the wall and bounce back (specially the lower frequencies).
This same method can be used to at least partially fix, damp any size resonant system.

Just my little contribution.

Best regards,
Konstantin Tokarev