Neckless Helmholtz resonators use near-opening air as their oscillating mass to reduce friction

Evidence

If a corked bottle contains air at higher pressure than outside, and the cork is suddenly popped, an oscillating decay ensues, with a frequency given by formula 13.1, and a quality Q determined by the power radiated as sound together with the friction in the neck of the resonator. There is no neck on the resonators shown in figure 13.7, so formula 13.1 does not apply directly. However, there is a small amount of air residing near the opening that acts as the mass. (One reason Helmholtz’s resonators have no necks or very short ones is to reduce the friction that otherwise would have occurred in a tubular neck, thus increasing their Q.) — Eric J. Heller, Why You Hear What You Hear- An…, 1230

Categorization

• DEFINE: Clarifies what the effective ‘mass’ is in a neckless Helmholtz resonator and why necks are shortened.