INTRODUCTION

Acoustics is the science that deals with sound waves such as in a combustion chamber, jet noise, oceanography, meteorology, architectural acoustics, and environmental acoustics. Sound waves may occur in the quiescent air even with extremely small pressure disturbances. This could lead to a noise audible to the human ear. In this case, changes in all flow variables other than the pressure remain constant. On the other hand, the noise level can be extremely high (thunder or explosion), but still with fluctuations of all variables other than the pressure remaining more or less constant. This phenomenon may be referred to as the pressure mode acoustics. When fluids undergo circulations causing significant velocity gradients, vortical waves are generated, which then produce pressure disturbances. The noise coming from this action (vorticity) may be categorized as the vorticity mode acoustics. In many instances in nature or in engineering, we encounter rapid changes in temperature such as in hypersonic flows over a spacecraft creating an entropy boundary layer between the shock layer and velocity boundary layer, subsequently leading to pressure fluctuations. Entropy waves are predominant in this case. We may identify the noise generated by entropy waves as the entropy mode acoustics.

The categorization suggested above was actually originated by Kovasznay [1953]. It is our intention to follow his suggestion in this chapter. However, it appears that the research in the acoustics community in general has been centered around acoustic waveforms (linear and nonlinear–N-waves), sound emission (radiation), and sound absorption (viscous dissipation), under which a large number of subdivided disciplines can be identified.