Sources of sound

Music, calm speech, whispering leaves fluttering in a breeze are pleasant and desirable sounds. Noise, howling gales, explosions and screeching traffic are less so. A quantitative understanding of the sources of all such sounds can be obtained by careful analysis of the mechanical equations of motion. Most sources are very complex, frequently involving ill-defined turbulent and perhaps combusting flows and their interactions with vibrating structures, and the energy released as sound tends to be a tiny fraction of that of the structural and hydrodynamic motions. Our analysis must correctly and reliably account for this general inefficiency of sound generation, because small errors in source modelling can lead to very large errors in acoustic prediction.

In this book we shall consider only the simplest case in which the fluid can be regarded as continuous and locally homogeneous at all levels of subdivision. The motion of the fluid will be defined when the velocity and the thermodynamic state are specified for each of the fluid particles of which it may be supposed to consist. The distinctive fluid property possessed by both liquids and gases is that these fluid particles can move freely relative to one another under the influence of applied forces or other externally imposed changes at the boundaries of the fluid. Five scalar partial differential equations are required to determine these motions. They are statements of conservation of mass, momentum and energy, and they are to be solved subject to appropriate boundary and initial conditions. These equations will be used to formulate and analyse a wide range of problems; our main task will be to simplify these problems to obtain a thorough understanding of source mechanisms together with a quantitative description of the subsequent propagation of the sound including, possibly, its reflection, scattering and diffraction at solid boundaries.

A general introduction to acoustics is presented in this chapter; it forms the basis for the treatment of the fluid-structure interaction problems examined in the rest of the book.