A Helmholtz resonator’s phase shift turns acoustic short-circuiting into constructive output

Evidence

The quarter-wave tube used in a Savart cup setup can be replaced by a Helmholtz resonator. Figure 13.6 reveals the resonator to be 90 degrees out of phase with the incident amplitude on resonance. This phase shift, together with another phase shift owing to the finite distance to the out-of-phase part of the near field, allows constructive addition rather than destructive cancellation of the formerly short-circuiting fields of a Savart cup, tuning fork, or block of wood as in a marimba. — Eric J. Heller, Why You Hear What You Hear- An…, 1901