The concept of acoustic particles is not difficult if you think about it. Any gas, such as our atmosphere, is composed of many, many molecules. An acoustic particle is a small volume, or clump, of gas. The clump is large enough to contain millions of molecules, but is small enough so that one cannot measure any significant difference in pressure, density, etc, within the clump. Because the gas molecules each have some velocity, some of them leave the particle. On average, however, an equal number of molecules will enter the particle for all the molecules that leave and the total number of molecules in the particle will stay the same.