An introductory account is given of the theory of the production and propagation of sound and its interactions with solid structures. It is intended for a one-semester course on acoustics at the advanced undergraduate or graduate level, and is therefore shorter than most of the standard texts – many important applications are omitted, such as speech and musical acoustics, ultrasonic imaging, and thermoacoustics. Sound is treated as a branch of fluid mechanics, which is possible because most students embarking on an advanced course are likely to have some familiarity with fluid mechanics or be sufficiently mature to assimilate the review material provided in the text. It is also desirable because an ability to relate acoustic events to hydrodynamic phenomena provides valuable insight into acoustic principles, in particular into the role of ‘vorticity’ in the mechanics of sound production by vibrating bodies and in the scattering and diffraction of sound. Any homogeneous fluid that has kinetic energy independent of moving boundaries must possess vorticity, a quantity that propagates by convection and molecular diffusion, and that therefore undergoes relatively little displacement during a typical acoustic cycle; the existence of vorticity signifies the presence of possible sources of sound, and its production occurs when sound is dissipated.

Chapter 1 forms a stand-alone introduction to theoretical acoustics, and is suitable together with material from §§2.1, 2.3, 2.4, 3.1, 3.2, 4.1–4.3, for a ‘short course’ on acoustics and aerodynamic sound. A general discussion of the Kirchhoff integral representation and generalisations is given in Chapter 2, with particular emphasis on sound sources of various types near an acoustically ‘compact’ body or section of a larger solid boundary, including the influence of surface vibration and scattering. Chapter 3 deals with the sound radiated by a baffled piston, Kirchhoff diffraction theory, and interactions of sound with apertures and perforated screens.