Fancy Footwork

When the kids and I are doing donuts in the parking lot at the dollar store–that is, driving the minivan in such tight circles that the wheels begin to screech and squeal–we are making minivan gangly banging sounds. Such behavior leads to especially complex rubber‑meets‑road hits and slides, sounds we revel in as we’re doing it. But the patrons at the dollar store hear an additional feature. The patrons hear Doppler shifts, something that the kids and I do not hear because we are stationary relative to the minivan. For the dollar store patrons, the pitch of the envelope of minivan gangly bangings rises and falls as we approach and recede from them in our donuts. In fact, it is because my minivan is veering so sharply that its ganglies begin banging in a more complex fashion. Compared to minivans not doing donuts, minivans doing donuts change pitch faster and have more complex “gaits.” Greater pitch changes therefore tend to be accompanied by more complex gait patterns.

This pitch‑rhythm connection is also found among human movers. When we turn, we are likely to have a more complex gait and gangly pattern than when we are simply moving straight ahead. For example, when you turn left, you must lean left, lest you fall over on your right side; and your legs can no longer simply swing straight past each other, but must propel the body leftward via a push or pivot. And many turns involve more complex footwork, such as sidestepping, trotting, twists, and other maneuvers we acrobatic apes regularly carry out. For example, when a basketball player crosses the court, his or her path is roughly straight, and the resultant gait sounds are a simple beat. Once a player has crossed the court, however, his or her movements tend to be curvy, not straight, as players on offense try to free themselves up for a pass, or players on defense loom in for a steal or shadow the offense to prevent a pass, in each case setting off a richer pattern of gangly sounds.

Does music behave in this way? When melodic pitches change–a signal that the depicted mover is turning, as we discussed in Chapter 4–does the rhythm tend to get more complex? As a test for this, I sampled 713 two‑beat intervals having at least two notes each from the Dictionary of Musical Themes , and for each recorded whether the pitch was varying or unvarying, and whether the rhythm was simple (one note on each beat, or “just the footsteps”) or complex (more than “just the footsteps”). (Data were sampled from 2/4 and 4/4 time signature pieces, and from every tenth theme up to “D400” in the Dictionary.) When pitch changed over the two‑beat intervals, the probability was 0.66 that the beat was complex, whereas when pitch did not change the probability was only 0.35 that the beat was complex. Consistent with the prediction from real‑world turners, then, these data suggest that when music changes pitch–the Doppler signature of a mover changing direction–its rhythm tends to become more complex. That is, as with people, when music “turns,” the ganglies start flying.

We see, then, that melody interacts with rhythm in the way Doppler interacts with gait. Now let’s ask whether loudness also interacts with rhythm, as expected from the ecology of human movers. We take that up in the next Encore section.