Transverse Waves

For transverse waves the displacement of the medium is perpendicular to the direction of propagation of the wave. A ripple on a pond and a wave on a string are easily visualized transverse waves.

Transverse waves cannot propagate in a gas or a liquid because there is no mechanism for driving motion perpendicular to the propagation of the wave.

Longitudinal Waves

In longitudinal waves the displacement of the medium is parallel to the propagation of the wave. A wave in a "slinky" is a good visualization. Sound waves in air are longitudinal waves.

Geologic exampleCork dust example

Traveling wave concepts

Sound propagation concepts
HyperPhysics***** Sound R Nave
Go Back

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.

Low frequency

High frequency

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.

Ear's sensitivity to such waves

Periodic motion concepts
HyperPhysics***** Sound R Nave
Go Back