A small piston still radiates sound because velocity is slightly in phase with force

Evidence

Acceleration and velocity are always 90 degrees out of phase in a sinusoidal oscillation, so our small piston, it seems, has force 90 degrees out of phase with velocity. How then is any sound energy produced? Even for a small piston, force and velocity aren’t exactly 90 degrees out of phase; there is a small component of velocity that is in phase with force, and thus a small amount of power produced as sound. The reason for this will become apparent as we build up larger surfaces from small ones. — Eric J. Heller, Why You Hear What You Hear- An…, 650

Categorization

• QUESTION: The mechanism by which the in-phase velocity component arises deserves tracing out in the later forced-oscillation discussion.