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.
Antoinette Allison of Reynoldsburg, Ohio was staying at the Crowne Plaza Hotel in Syracuse, New York back in April 2011 when she went to the hotel's Library Lounge bar. Syracuse.com reports that Ms. Allison claims that while waiting for food in the Library Lounge, she fell off of a "wooden, high-back bar stool," and landed on her wrist, causing multiple fractures that required surgery. She is now suing the hotel for $1 million.
In her lawsuit, Ms. Allison claims that the bar stool was too high off the ground, and that, "hotel management knew of other problems with the height of the stools." Her lawyer, Mark Ventrone, wrote in a 2012 complaint that, "Said bar stools were more dangerous than patrons would expect and safer designs are on the marketplace."