A basic understanding of acoustics is important for designers of air-conditioning systems because the system is a primary generator of sound in a building. Noise results from the operation of compressors, fans, and pumps and from flow when air or water moves through ducts and pipes or, especially, on abrupt drops in pressure. The air-conditioning system is also linked with sound and noise concerns not only because it is a generator of noise but also because pipes or ducts transmit noises from conversations or office equipment from one room to another.

This chapter seeks to provide a foundation from which to proceed in deciding how to control sound in a building through proper design or the installation of sound-absorbing devices and materials. Following an introduction to the physics of sound waves, this chapter explains the distinction between sound power level and sound pressure level. A generator of sound emits a certain magnitude of sound power, but this magnitude normally cannot be measured directly. Instead, instruments are available to measure the sound pressure level. The sound power level of the generator and the sound pressure level measured some distance from the source are related through the acoustic characteristics of the enclosure. With this understanding of power level and pressure level, the text attempts to provide an appreciation of how sound absorption affects room characteristics and how sound is transmitted and attenuated (reduced) through ducts to provide the background for further study of sound and noise control in buildings.