Inspired by Mike's exploration: https://www.youtube.com/watch?v=qy1w6rTpC2g
, tried to recreate and play with the setup. Turned into a great demo of the phenomenon at a science symposium, and a physical conformation of the speed of sound. With a measured average interval of 5.92 mm (Pixel counting off level photos). With a 28.5 khz standing wave, the theoretical wavelength would be 11.9 mm. Experientially, 11.84 mm is within half a percent error, good enough for me.
Setup itself is two salvaged pieces of plywood mounted on four M8 rails. Transducer on the bottom, mirror glued to the top. Played around with watchglasses as eliptical reflectors, but to no avail, I think they were too heavy. Driven with a potted 555 sine generator circut, hooked up to the secondary of a transformer. All in all, a really fun project thrown together in two weekends.