The interaction of sound with the surfaces of an enclosed space comes under the purview of room acoustics. Room acoustics in earlier times was also called architectural acoustics and building acoustics, when the science and engineering of pleasant sound was the main objective. That was the time in which interest in machine-generated noise was not very important. Reducing sound from noisy machinery inside an enclosed space is much different from modifying an enclosed space to obtain pleasant sound. I would prefer to use room acoustics with respect to machinery noise and architectural acoustics for producing the right note in an auditorium. The most important approach to changing room acoustics is the use of sound-absorbing materials, which is the subject of this chapter.