This interesting device is a 3-inch-diameter (~76 mm), 2-foot-long (~61 cm) tube, which has a line of 31 0.055-inch-diameter (~1.3 mm) holes equally spaced from one end to the other. (The inside length of the tube measures 23-1/4 inches, or 59.0 cm.) A loudspeaker is screwed onto one end (at left in the photograph above), and a gas tap is attached at the opposite end. A function generator (at left) drives the loudspeaker. When you flow natural gas through the tube and ignite it above the holes, you will see a line of flames. With no sound, or with the function generator set at a frequency for which the length of the tube, which is closed at both ends, does not correspond to an integral number of half wavelengths, the flames are all the same height. When you tune the function generator either to the fundamental frequency of the tube, or to a multiple of that frequency, this sets up a standing wave inside the tube, giving rise to displacement nodes at the two ends, with a series of displacement antinodes and nodes in between. The numbers of these antinodes and nodes, of course, depend on the particular harmonic to which you set the function generator. The photograph above shows the second harmonic, with one displacement node in the middle and two displacement antinodes on either side. The regions where the flames are the lowest correspond to displacement nodes, and those where the flames are highest, to displacement antinodes.

In a sound wave, the displacement is least where the pressure is greatest, and it is greatest where the pressure is lowest. So displacement nodes correspond to pressure antinodes, and displacement antinodes correspond to pressure nodes. Because the pressure is greatest at the displacement nodes, and least at the displacement antinodes, the speed of the exiting gas is greatest at the displacement nodes and least at the displacement antinodes. The faster the gas exits the tube, the lower the static pressure around the jet, and the greater the amount of air, and thus oxygen, that is entrained in the jet, which leads to more efficient combustion of the gas. As a result, the flames at the displacement nodes are agitated, blue and short, and those at the displacement antinodes are calm, yellow and long. (The yellow is from incandescence of soot particles formed as a result of incomplete combustion of the gas.)

The fundamental frequency for this device is approximately 330 Hz. This varies slightly as the tube warms up, since the frequency depends on the speed of sound (v = fλ, so f = v/λ), and the speed of sound is √(γRT/M), where γ is the heat capacity ratio, C_p/C_v, R is the universal gas constant, T is the temperature in Kelvin, and M is the molar mass of the gas. (See demonstration 44.03 -- Speed of sound in air vs. helium.) Thus, as the tube warms up, the resonant frequency rises somewhat. As the above indicates, you can use this demonstration to measure the speed of sound in the tube. If you wish to check the measured speed against theory, methane, CH₄, has a molecular weight of 16.03 g/mol and γ = 1.32 (according to data on the Engineering Toolbox web site). R, of course, equals 8.314 J/mol·K (or 8.314 × 10⁷ erg/mol·K).

You can easily show patterns for the fundamental and harmonics up to the sixth harmonic.

To see an interesting paper that explores the gas dynamics of this device, click here.