Helmholtz resonance requires wavelengths far exceeding the vessel size so internal pressure stays constant.

Evidence

At the frequency of the Helmholtz resonance for a given vessel, the wavelength is invariably greater than the size of the vessel, and sometimes very much greater. The Helmholtz mode has essentially constant pressure inside the volume V; if the wavelength were close to the linear dimension of the resonator, it wouldn’t be possible for the pressure to be constant inside. — Eric J. Heller, Why You Hear What You Hear- An…, 1228

Categorization

• DEFINE: Clarifies the defining condition of the Helmholtz mode: pressure constant inside because wavelength ≫ vessel dimension.