Sound intensity can be modeled by tracking energy flux through expanding spheres

Evidence

Suppose people have been streaming out of a subway station in a park for some time at a constant rate, heading in various directions. We could count the number of people emerging from the subway per unit time by watching how many passed through a small circle surrounding the subway exit, or we could take a larger circle much farther away. Either way, we should get the same answer if people have been coming out at a constant rate for a long time. Of course, the density of people passing through the small circle will be much higher than in the larger one. In fact, the density will drop off as the inverse of the radius of the circle if the people head out uniformly in all directions. — Eric J. Heller, Why You Hear What You Hear- An…, 141