Basic Linear Acoustics

Abstract

This chapter deals with the physical and mathematical aspects of sound when the disturbances are, in some sense, small. Acoustics is usually concerned with small-amplitude phenomena, and consequently a linear description is usually applicable. Disturbances are governed by the properties of the medium in which they occur, and the governing equations are the equations of continuum mechanics, which apply equally to gases, liquids, and solids. These include the mass, momentum, and energy equations, as well as thermodynamic principles. The viscosity and thermal conduction enter into the versions of these equations that apply to fluids. Fluids of typical great interest are air and sea water, and consequently this chapter includes a summary of their relevant acoustic properties. The foundation is also laid for the consideration of acoustic waves in elastic solids, suspensions, bubbly liquids, and porous media. This is a long chapter, and a great number of what one might term classical acoustics topics are included, especially topics that one might encounter in an introductory course in acoustics: the wave theory of sound, the wave equation, reflection of sound, transmission from one media to another, propagation through ducts, radiation from various types of sources, and the diffraction of sound.