Kundt's tube is one of several classic acoustics instruments used originally to measure, and now mostly to demonstrate, phenomena related to standing waves. Unlike Ruben's tube (which uses open flame and requires a source of combustible fuel) or a Chladni plate (which uses loose fine powder) to create similar effects, Kundt's tube is both completely safe and completely clean.
The example shown here was built by San Luis Obispo resident and nuclear engineer Mike Sullivan. It consists of a clear tube with a speaker at each end containing hundreds of fluorescent plastic beads. The speakers produce two different pitches -- one (to left) fixed at A220, and one (to right) that can be varied over a range of about a half-step up or down from A220 by turning the single knob.
Sealed Tube Speaker System Shows off Standing Waves sans Fire, Mess