Sound Waves in Air
A single-frequency sound wave traveling through air will cause a sinusoidal pressure variation in the air. The air motion which accompanies the passage of the sound wave will be back and forth in the direction of the propagation of the sound, a characteristic of longitudinal waves.
Physics professor Clint Sprott of the University of Wisconsin shows one way to visualize these longitudinal pressure waves in his "Wonders of Physics" demonstration show. A loudspeaker is driven by a tone generator to produce single frequency sounds in a pipe which is filled with natural gas (methane). A series of holes is drilled in the pipe to release a small amount of gas. Igniting the gas produces flames for which the height increases with the pressure in the pipe. The pattern of the flames shows the pressure variation and can be used to roughly measure the wavelength of the pressure wave in the pipe.
Shown below is more detail on the attachment of the loudspeaker to the pipe. The loudspeaker is driven by the amplified output of a tunable oscillator.
A series of small holes were drilled at regular intervals in the pipe. They appeared to be about 8 mm apart.