Air motion is usually caused by more massive objects set in motion.

Evidence

To understand impedance in air, we begin by considering solid elastic bodies, such as pucks on an air hockey table or coins on a slick surface. We need to understand such things in any case, because usually before air is set in motion, something more massive, like a string or a block of wood, is set in motion. Each puck or coin is a lumped object—we ignore the details of atomic or molecular structure inside, but keep essentials such as size, density, and elasticity, just as we will for air cells when we return to them. The essentials are used to build a theory of what happens when adjacent lumps interact. — Eric J. Heller, Why You Hear What You Hear- An…, 59

Categorization

• DEFINE: Introduces the lumped-object modeling abstraction that carries through the chapter’s theory of sound propagation.