Chapter 3: Propagation and Transduction of Acoustic Waves in a Dissipative Fluid

In the previous chapters, we discussed the excitation and propagation in an inviscid ideal fluid which has no energy loss of any kind associated with the propagation. In practice, the fluid in which the acoustic wave propagates is not ideal; we have to consider the irreversible processes such as viscous, thermal conduction and relaxation, etc. accompanying with the propagation of the sound. During the propagation of acoustic waves in such a viscid medium, the “ordered” acoustic energy is converted to “disordered” thermal energy when the acoustic energy is absorbed by the medium, consequently its amplitude decays with increasing travel distance. In addition to the aforementioned irreversible acoustic energy absorption, the acoustic wave amplitude also attenuates due to the spreading of acoustic wavefront (spherical waves) or scattering from small particles (floating dusts, air bubbles in water, water-droplets of fog, etc.), the wave propagation direction consequently is partially altered. Thus, in the original direction, the acoustic wave energy is attenuated.