Pressure gradients drive air flow that cancels out multipole sound

Evidence

On the nodal lines—the dividing lines separating orange from blue—the pressure is not changing as the tines vibrate. Here the air velocity is maximal at the pressure node. The pressure difference, high on one side and low on the other, causes air to rush to and fro as the blue and orange regions exchange each half-period. The greater the pressure gradient—that is, the change of pressure with position—the greater the force and the resulting air velocity. Air accelerates across the nodal line, and the flow from high-pressure to low-pressure regions reduces the pressure difference. This translates into less intense sound. — Eric J. Heller, Why You Hear What You Hear- An…, 623

Categorization

• DEFINE: Explains how a pressure node coexists with maximal air velocity — the mechanism driving multipole cancellation.