The key word in this discussion of porous sound absorbers is interstices. It is simply the space between two things. If a sound wave strikes a wad of cotton batting, the sound energy sets the cotton fibers vibrating. The fiber amplitude will never be as great as the air particle amplitude of the sound wave because of frictional resistance. Some sound energy is changed to frictional heat as fibers are set in motion, restricted as this motion is. The sound penetrates more and more into the interstices of the cotton, losing more and more energy as more and more fibers are vibrated. Cotton is an excellent sound absorber that has been specified in studio treatment in Africa where it was plentiful and cheap, and because imported materials were out of the question.

Porous absorptive materials most commonly used as sound absorbers are usually fuzzy, fibrous materials in the form of boards, foams, fabrics, carpets, cushions, etc. If the fibers are too loosely packed, there will be little energy lost as heat. On the other hand, if they are packed too densely, penetration suffers and the air motion cannot generate enough friction to be effective. Between these extremes are many materials that are very good absorbers of sound. These are commonly composed of cellulose or mineral fiber. Their effectiveness depends on the thickness of the material, the airspace, and the density of the material.

The absorption efficiency of materials depending on the trapping and dissipating...