Longitudinal wave motion
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In this type of wave motion, the individual particles of the medium vibrate parallel to the direction in which the wave travels.
Example: The waves along the length of a spring when one end of it is suddenly compressed or pulled out and then released.
Sound waves in air
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Let a long coiled spiral spring be suspended horizontally from a wooden frame using a number of threads. Ordinarily, the turns of the spring will be equidistant. On giving a sharp tap to one end of the spring, a few turns will come closer together. This state of compression moves forward along the length of the spring. Similarly, if the end of the spring is sharply pulled out and released, the distance between the turns becomes greater than usual. This state of extension also travels along the length of the spring. If the end of the spring is alternately pushed in and pulled out, a series of compressions and elongations are found to travel along the length of the spring. The vibrations of the particles are horizontal and the wave propagation is also along the horizontal. Thus, it is a longitudinal wave.
Let a stiff metallic strip (like a hacksaw blade) be fixed vertically on a rigid support. When its other end is pulled to one side and released, it begins to vibrate. The extreme positions of the vibrating strip are shown by dotted lines in the figure. As it suddenly moves to the right, it pushes the layers of air in front of it and causes a compression of air. This compression is then propagated onwards through the air. The strip while coming back, overshoots its equilibrium position and moves to the left causing a compression of the air layers on the left side while it creates a rarefied region on the right side. Both these conditions, namely compression and rarefaction move onwards in the medium as the strip vibrates. The vibrations of the particle are parallel to the wave propagation. Thus, it is a longitudinal wave. Such a wave does not require a shearing stress and hence, can pass through any medium possessing elasticity of volume (i.e., in solids, liquids or gases).
Sound waves in liquids
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In a liquid, the motion of the particles may be neither purely transverse nor purely longitudinal, but a combination of the two. The path followed by the particles is either a circle or an ellipse. The paths of several selected particles, their positions and the shape of the wave are shown in the figure. The particles move in circular paths if the wavelength is equal to or smaller than the depth of water. As the water becomes deeper, the path becomes elliptical and at the lower levels, the wave motion is entirely longitudinal.
Comparison between transverse and longitudinal waves
| Transverse waves | Longitudinal waves |
| The particles of the medium vibrate at right angles to the direction of wave motion | The particles of the medium vibrate parallel to the direction of wave motion |
| The wave is propagated in the form of crests and troughs | The wave is propagated in the form of compressions and rarefactions |
| This type of wave motion is possible in solids and on liquid surfaces | This type of wave motion is possible in any medium (solid, liquid or gas) |
| These waves Can undergo polarization | These waves do not undergo polarization |
