Sound in a thin plate behaves like ripples on water: it travels, reflects off edges, and changes shape. But sound has a peculiar trait: it can pass through places where waves are forbidden to exist. It's as if ripples on a pond suddenly vanished before a stone barrier and reappeared on the other side, without going around it.
Scientists took a plate of varying thickness — tapering smoothly like a wedge. In such a plate, Lamb waves (vibrations traveling along thin plates) have narrow zones where motion decays. The wave cannot propagate there — by classical rules. But the experiment showed that energy still seeps through this zone. The evanescent wave turns into a traveling wave once past the barrier.
This effect is like a tunnel through a mountain: a train cannot travel through solid rock, but if the tunnel is short, it emerges on the other side. For sound, such a tunnel is a narrow frequency band where the wave is formally forbidden. The tunnel's width depends on the material, and this can be exploited: change the material, change the barrier's permeability.
Calculations matched the experiment with high precision. This means engineers can design structures where sound and vibrations pass through some sections and are blocked in others. For example, damping noise in bridges, airplanes, or instrument casings without adding extra weight.
Wave tunneling is not a physicist's fantasy. This is how light passes through an opaque barrier in a near-field microscope, and how particles pass through energy barriers in nuclear reactions.