CHAPTER VIII - SOUND

Abstract

Publisher Summary This chapter discusses sound waves. A body oscillating in a fluid causes a periodic compression and rarefaction of the fluid near it, thereby producing sound waves. The energy carried away by these waves is supplied from the kinetic energy of the body. Monochromatic waves are important as any wave can be represented as a sum of superposed monochromatic plane waves with various wave vectors and frequencies. This decomposition of a wave into monochromatic waves is simply an expansion as a Fourier series or integral. The terms of this expansion are called the Fourier components of the wave. The energy flux density in a plane sound wave equals the energy density multiplied by the velocity of sound. The propagation of a sound-wave packet is accompanied by the transfer of fluid and is a second-order effect. Turbulent velocity fluctuations also are a cause of sound excitation in the surrounding fluid. If there is something in the path of propagation of a sound wave, then the sound is scattered. Beside the incident wave, there appear other scattered waves, which are propagated in all directions from the scattering body. The existence of viscosity and thermal conductivity results in the dissipation of energy in sound waves, and the sound is consequently absorbed, that is, its intensity progressively diminishes.