Surface acceleration, not velocity or motion itself, produces the sounds we hear.

Evidence

The sound we hear coming from surfaces has its origin a short time earlier in the movement of the surface. A pool ball rolls silently across the table, save for the sound of the felt. If it strikes another ball, there is a characteristic loud clack. Accelerations and decelerations of the surface of the ball are responsible for the sound. The acceleration is short lived, so the velocities attained are not large: velocity = acceleration × time, but cue balls are in contact for perhaps a few milliseconds. The velocity is about 1 to 10 m/s, so the acceleration is on the order of 100 to 10,000 m/s2, up to 1000 times the acceleration due to gravity! — Eric J. Heller, Why You Hear What You Hear- An…, 643

Consider a seemingly prosaic phenomenon: tap a fingernail on a wooden tabletop. You hear it—but why? You move your finger only a few centimeters per second, and the table is initially stationary. On contact, no part of the table surface will move any faster than your finger was moving. The finger wasn’t producing any sound, so why should the surface? The answer is acceleration: the desktop surface, initially at rest, suddenly deforms under pressure of the finger, even if by only a few microns or even less. (The finger also decelerates. Deceleration is just acceleration in the reverse direction. The finger is more pliable than the desk, and the acceleration of the surface of your finger is less dramatic). The tabletop abruptly moves down at the point of impact—a very strong (if brief) acceleration. The acceleration generates a small zone of rarefaction, followed by a compression as the surface rebounds. Sound has thus already been launched, but the battle is not over. The deformation of the surface propagates along the desktop, displacing more air and generating more sound. — Eric J. Heller, Why You Hear What You Hear- An…, 661

Categorization

• DEFINE: Clean statement of the central principle that sound from surfaces comes from acceleration, not constant motion.
• XREF: Connects to the book’s own earlier treatment of vibrating sources, and to the general theme of acceleration-driven emission.
• SEED: A crisp everyday-mechanics explanation usable in a piece about how normal motions produce sound — the acceleration-not-velocity principle is counterintuitive and highly shareable.
• CANDY: The fingernail-on-table mystery with its acceleration answer is a memorable, elevator-level fact about sound production.
• corroborated: 2 independent passages support this claim — Eric J. Heller, Why You Hear What You Hear- An…, 643; Eric J. Heller, Why You Hear What You Hear- An…, 661