Acoustical materials are a variety of foams, fabrics, metals, etc. used to quiet workplaces, homes, automobiles, and so forth to increase the comfort and safety of their inhabitants by reducing noise generated both inside and outside of those spaces. Acoustical materials are used in two major ways: as soundproofing, by which noise generated from outside a given space is blocked from entering the space; and, as sound absorbing, where noise generated within a space is reduced inside the space itself. As an example of soundproofing, a school might construct a special wall to isolate the music room from a general classroom next door. As for sound absorbing, a machine shop might install barriers to block and absorb the acoustic energy of a noisy air compressor. In this article, the focus will be on the blocking and absorbing methods, as those are most traditionally associated with acoustical materials. There are several specifications of the acoustic properties of acoustical materials that are measured and delineated by manufacturers and which are used to quantify the effectiveness of the material for handling sound or noise. The most common of these include: The absorption coefficie...