Evanescent waves confine sound to surfaces when step spacing is sub-wavelength

Evidence

There is a second type of chirp, sounding like raindrops in a bucket of water. (That effect is due to vibrating bubbles trapped underwater.) To hear the effect at Chichén Itzá, you must have your ear close to a bottom (or top) stair, as someone else steps up the stairs near the top (or bottom). Each step taken by a climber evokes a raindrop sound to someone listening with an ear near the inner corner of a step many steps away. The climber hears nothing strange. … The necessity of listening very near the stairs means that an evanescent wave is involved. An evanescent wave decays exponentially without undulating in at least one direction, but it may undulate in the other two. The exponential decay away from the surface effectively confines it there. Sound waves launched by subsonic surface traveling deformations are evanescent: they are loud near the surface but decay as you get away from it (see figure 7.38). The staircase wave is trapped for the same reason: short-circuiting. Alternate stairs are closer together than half a wavelength of the sound frequency of the chirp. The wave cancels itself ever more completely, the farther away from the surface it gets. — Eric J. Heller, Why You Hear What You Hear- An…, 737

Categorization

• DEFINE: Clear definition of evanescent waves: exponential decay without undulation in one direction, confining sound to a surface.
• XREF: Connects to the earlier Kukulkan chirped echo (first remarkable effect) and to the general physics of surface-confined waves.