~A six-foot long, four-inch diameter pipe is closed at one end and has a
polyethylene diaphragm and a speaker across the other. The pipe is fed with
natural gas which escapes from a row of small holes along the top edge of the
pipe. When it is lit, a row of small flames forms. If the pipe is now driven at
one of its natural frequencies, a standing wave is set up in the gas which
varies the gas velocity/density and consequently varies the amount of
gas being fed to the flames and is directly proportional to therelative position
of the standing wave in the pipe to the exit holes of the gas. Thus,
the flames no longer burn with the same length of flame, but with a
sinusoidally modulated envelope.~