Types of mechanical waves

Mechanical waves can be classified by the direction in which atoms or molecules are temporarily displaced from their mean positions opens in a new window relative to the flow of energy.

Mechanical waves can be one of three types:

  • longitudinal
  • transverse
  • torsional.

In this unit, we shall only consider the first two types. It is important to note that mechanical waves temporarily disturb a medium. The medium is not transported but the energy is.

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Longitudinal waves

A longitudinal wave is a disturbance in which the particles of the medium move in the same plane as the flow of energy (ie parallel or anti-parallel to the flow of energy).

motion of a single particle is backwards and forwards in line with the flow of energy

Particles (atoms or molecules) that make up a medium are not isolated. Neighbouring particles exert electrostatic forces on each other so a shift in a particle's position results in forces pushing or pulling the particle back to its mean position. A returning particle will overshoot its mean position, due to its inertia, before once again experiencing the push of neighbouring particles. This to-and-fro motion, or oscillation, is parallel and anti-parallel to the direction of energy flow.

Longitudinal waves are characterised by an alternating series of regions of closely spaced particles (compressions) and widely spaced particles (rarefactions).

The oscillating motion of the particles is shown in this animation. opens in a new window

Longitudinal waves can exist in solids, liquids and gases. A sound wave in air (eg thunder) is an example of a longitudinal wave.

View an actual longitudinal wave. opens in a new window

Transverse waves

A transverse wave is a disturbance in which the particles of the medium move in a plane perpendicular to the flow of energy.

motion of a single particle is at perpendicular to the flow of energy

Transverse waves are characterised by a sinewave like appearance – even though the particles of the medium are always moving perpendicular to the direction flow as shown in the animation below.

View a transverse wave animation. opens in a new window

Mechanical transverse waves exist in solids and on the surface of liquids, but not in gases. The oscillation of a stretched string is an example of a transverse wave.

View an actual transverse wave. opens in a new window

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Ocean waves

Not all waves are one type only. An example is wind-generated ocean waves.

View this animation of the true nature of these waves. opens in a new window

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You may wish to consider the following as a group.

  • Is a 'Mexican wave' at a sporting arena a mechanical wave? Discuss.
  • How would you classify a small ocean wave – transverse or longitudinal? Explain.
  • Can sound waves be transverse waves? Explain.
  • Explain how a torsional wave disturbs a medium.