To understand why some building spaces are acoustically better than others--and specifically how masonry can be used to limit or enhance sound transmission--designers must first understand how sound is transmitted from one building space to another. When sound strikes a wall, some of it is reflected, some absorbed, and some transmitted, depending on the materials and construction of the wall assembly. Acoustic performance of a wall assembly is characterized by either an STC (sound transmission class) rating or an NRC (noise reduction coefficient) rating. The STC of a wall assembly is a measure of its resistance to sound transmission or its ability to insulate a building space from sound on the opposite side. The NRC characterizes an assembly's ability to absorb sound. Sound absorption is important in determining how sound travels within a building space. In many cases where masonry is used for ease of maintenance and other reasons, acoustical performance also is a concern. Although acoustical characteristics may be overlooked as a particular advantage, the use of masonry walls in restrooms, kitchens, hallways, and other high-noise areas insulates surrounding spaces from unwanted noise.