It is shown that the master equation for a superfluid Helmholtz resonator is co 2 = u n 2 A/LV, where oo is the angular-resonant frequency, u n is alternately the velocity of first, second, and fourth sound, A is the cross-sectional area of the neck of length L, and V is the volume of the resonator. The fourth-sound resonance occurs when the resonator is packed with a porous medium which locks the normal fluid component. Experimental results on such a resonator are in agreement with this result.
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