1.1 Sound and Acoustics Defined

In everyday conversational language, acoustics is a term that refers to the quality of enclosed spaces such as lecture and concert halls in regard to their effect on the perception of speech and music. It is supposed to be used with a verb in its plural form. The term applies also to outdoor theaters and bowls.

From the standpoint of the physical sciences and engineering, acoustics has a much broader meaning and it is usually defined as the science of waves and vibrations in matter. On the microscopic level, sound is an intermolecular collision process, and, unlike an electromagnetic wave, a material medium is required to carry a sound wave. ^[1] On the macroscopic level, acoustics deals with time dependent variations in pressure or stress, often cyclic, with the number of cycles per second, cps or Hz, being the frequency.

The frequency range extends from zero to an upper limit which, in a gas, is of the order of the intermolecular collision frequency; in normal air it is ? 10 ⁹ Hz and the upper vibration frequency in a solid is ? 10 ¹³ Hz. Thus, acoustics deals with problems ranging from earthquakes (and the vibrations induced by them) at the low-frequency end to thermal vibrations in matter on the high.

A small portion of the acoustic spectrum, ? 20 to ? 20, 000 Hz, falls in the audible range and sound is often used to...