Constricting a tube’s open end transforms it into a Helmholtz resonator.

Evidence

Starting with a half-open tube, a constriction applied at the open end turns the system into a Helmholtz resonator. There is no neck, but air near the opening has mass and acts like a short neck. The lowest mode of the tube smoothly evolves into the Helmholtz mode as the constriction is applied. The volume inside the chamber acts as a spring, with the whole chamber rising and falling in pressure almost uniformly. The wavelength of the sound is much larger than the resonator itself, which is why the pressure is nearly uniform within the inner chamber at any moment. — Eric J. Heller, Why You Hear What You Hear- An…, 1649

Categorization

• DEFINE: Explains what makes a Helmholtz resonator—massive neck air plus spring-like cavity volume—and why its pressure is uniform.
• XREF: The Helmholtz resonator model connects directly to why cupped hands around the mouth or a bottle’s neck tune how we hear and project sound.