Many of the materials that are used to muffle sound are made from porous substances, such as cork or foam rubber. Mineral wool is a sound-proofing material made from inorganic fibers that can be shaped to fit into areas between walls to provide a greater degree of sound reduction in a room. Thicker and denser materials tend to muffle sound better than lighter materials, and rougher surface areas will absorb more sound than smooth surfaces, which will reflect sound waves instead.
Many of the sound-proofing methods used in professional applications, such as recording studios, make use of porous, open-cell materials. The degree of sound absorption of these materials depends upon several factors, such as the size of the cells, the degree of porosity, material thickness and density. Cork is a popular material for sound proofing because the air that is contained within each cork cell enables it to absorb sound well. Overall, most of the porous materials work well in dampening the mid-range to high-frequency spectrum of sound, but the low frequency sounds tend to be those that require extra measures to dampen effectively.
When a substance absorbs sound, it is converting part of the energy of the sound wave to a small amount of heat energy within itself. The energy absorbed and converted to heat lessens, or attenuates, the degree of the sound-wave energy that is either transmitted or reflected from the intervening material. Because different materials have varying sound-absorption characteristics, the most effective choice of material is one that is best matched to the frequency range of the sounds that need to be reduced.