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Visualising Waves


A laser beam is pointed at a mirror mounted on a speaker cone. The reflected beam moves as sound is produced by the speaker, producing Lissajous figures.


This demonstration may take some time setting up and care must be taken when using the equipment so that the laser beam does not point towards anyone's eyes.

Equipment typically required (per demo):

  • Laser (Class II)
  • Small mirror
  • Loudspeaker with accessible cone
  • Signal generator and leads to connect to loudspeaker
  • Stands and clamps as required
  • Double sided tape

The Aim of this demonstration is to show how sounds from the signal generator are converted into movement by the moving coil of the loudspeaker. The outcome of this demo is similar to that of the loudspeaker and balls equipment setup.

Instead of placing balls onto the cone of the loudspeaker, in this setup a mirror is attached to the cone of the loudspeaker using double-sided tape. A laser is then placed near-by so that the beam hits the mirror at an obtuse angle and is reflected onto a wall or screen. It is essential that the beam cannot reflect off of glass within the lab and come into contact with viewer's eyes, so accurate positioning is important to ensure safety.

A signal generator is attached to the loudspeaker making sure that the output of the generator does not exceed the maximum power of the loudspeaker. When tones are generated, the cone will move in and out, creating Lissajous figures.

Clamps and stands may be required to aid accurate alignment of the beam. A more robust setup could be created which consists of a loudspeaker and laser mounted within a box. The advantage of this is that set-up time will be greatly reduced.

Both volume and tone of the signal will displace the beam in different ways and this may be investigated in the lesson. Comparisons could be applied between this demo and the output of an oscilloscope.



Relevant safety precautions must be taken when using a laser - check that the laser is labelled 'Class II' and that students cannot stare into the beam or reflected beams.

The contents of this page are for information only. Please refer to CLEAPSS, SSERC or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.