Sound proofing is any mechanism used to reduce the conduction of sound through a barrier or into a soundproofed chamber. There are several different basic approaches to reducing sound; increasing space in which the sound dissipates, using massive barriers to absorb the energy of the sound waves, using dampening structures, or using active antinoise sound generators.
The use of space to dissipate sound can be quite straightforward. The energy density of sound waves decrease as they spread out, so increasing the distance between the soundproofed space and the source of noise will result in a smaller amount of sound reaching it. Using mass to absorb sound is also quite straightforward, with part of the sound energy being used to vibrate the massive barrier.
Dampening is the process by which sonic vibrations are converted into heat. This can be achieved in several ways. For example, lead is both heavy and soft, with the softness allowing it to dampen the noise rather than transmitting it through. Making a noise transfer through different layers of material with different densities also helps dampen noise. This is the reason why foam is such a good sound dampening substance; the sound waves are forced to travel through multiple foam cells and foam cell walls as it passes through the foam piece. Improperly done, however, dampening can make things worse — like the string holding chimes help them ring by isolating the vibration instead of dampening it.
Sound proofing affects sound in two different ways: noise reduction and noise absorption. Noise reduction simply blocks the passage of sound waves through the use of mass and space — it reduces the amount of energy that penetrates. Noise absorption, on the other hand, operates by transforming the sound wave. Noise absorption involves suppressing echoes, reverberation, and reflection. The shape of the soundproofed chamber and the dampening characteristics of the materials it is made out of are important in noise absorption.
Antinoise generators for active noise control are a relatively modern innovation.