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Propagation of sound waves and pulses through a weak inhomogeneous medium with a van der Waals interface is studied in the non-dissipative approximation by using a perturbative technique. It turns out that in some cases the resonance frequency of sound standing waves in a cavity will change from one resonant mode to another according to the detailed shape of the interface. Also, important differences between the standing waves and the pulse techniques are found. These results may provide a promising way to probe weak inhomogeneous interfaces.