“Try shifting your attention from one of these two pitches to the other. Can you hear the volume change?

It’s very much like staring at one of those diagrams of a cube that seems to go either into or out of its two-dimensional page (this drawing is called a necker cube). The illustration has no real depth cues so you can imagine it either way. This is a pretty imperfect metaphor for different ways of thinking about sound. But still, these are great illustrations of the fact that we