Soundproof Walls

HOW do soundproof walls actually work?

I tried to find out by typing my question in the google search but it kept comming up with, how TO make soundproof walls ect... I actually want to know the science and physics of how and why soundproof walls work.

Public Comments

1. There's a lot of information on sound proofing on wikipedia: http://en.wikipedia.org/wiki/Soundproofing Hope it helps.
2. Typically, you start with a hard heavy surface that is going to take a lot of energy to get vibrating and chose a material that once vibrating will dampen the vibrations (like sheet rock and not steel plate). Behind this (and perhaps on the face) you mount absorbent material that tends not to vibrate - cloth or foam or insulation - that can't stand on its own. If possible you build the two sides of the wall so they are not connected, for example alternating 2x4 studs to make a wall 6 or 8 or 10 inches thick and mount the bottom and top of the studs with rubber isolation so vibrations hitting one wall are not carried through to the other wall. And all the gaps are sealed with soft material that does not pass air vibrations. In practice, it is possible to reduce the sound level of a room sharply simply by putting one or two barriers in place. When I close my bedroom door completely, the sound from my wife reading recordings is reduced over the door being even slightly open. If two doors are closed in the sound path, like the two in the alcove between the bedroom and the dining room the sound is lowered even more because as sound hits a surface it has to expend energy getting that vibrating and then when the vibrations are carried to the other side, they start as multiple point sources which radiate as the square of the distance. So if I am 20 feet from a sound source and hearing noise level X, then at a door 10 feet from the source, the sound level would be 4X when open. But if the door is closed, then first there is the loss getting through the door, suppose half is lost although it is probably more. But then it radiates by the square law, reducing sharply