It is customary for authors to explain their reasons for writing a book. Mine are twofold. The first is to provide, under one cover, a detailed discussion of the propagation of sound waves in suspensions and of the foundations upon which that discussion is based. The second is due to the historical development of the field and takes longer to explain.

Nearly 100 years ago, there appeared in the research literature an article about the extinction of sound waves by small particles. Since then, very significant advances have been made in the understanding of sound wave propagation in suspensions of various kinds. These advances have occurred as a result of specific needs at a particular time. For example, the first theory for sound wave propagation in bubbly liquids was developed during World War II, as a part of an effort to understand the pressure waves produced by underwater explosions. Similarly, propagation in aerosols received increased attention during the moon-landing program in the 1960s, as a result of certain instability problems encountered in rocket propulsion. Currently, because of industrial and environmental needs, there is a renewed interest in the acoustics of emulsions and hydrosols. One unintended result of this divided interest is that the theories for sound propagation in each type of suspension developed nearly independently from one another. At some level, this has been advantageous because individual suspension types usually require simpler mathematical models for their description.