Wb-9 Circular Standing Waves (The Ring of Fire)


To demonstrate circular standing waves in a pipe using sound waves and gas jets to indicate the pressure profile. May be useful when discussing the Bohr theory of the atom and the quantization of electron orbits.


  • Horn Loudspeaker
  • Signal generator
  • Power amplifier
  • Gas cylinder with special pressure reduction valve and hose
  • Fire extinguisher
  • Heat proof mats

Ring of Fire 1 Ring of Fire 2

Circular Standing Waves Apparatus.


A circular pipe has a series of small holes drilled equidistant along its length and is connected to an LPG supply. A horn loudspeaker is driven by a signal generator and P.A amplifier combination. The gas supply must be used with the special hose (shown in the diagram picture). This hose adjusts the pressure and flow rate to give optimal flame height consistent with clarity and safety. As the frequency is varied, standing waves are produced in the pipe when the wavelength matches the boundary conditions, as indicated by the height of the flames.

This demonstration has the following safety issues.

The demonstration must be used with the indicated hose attachment.

The demonstration must be supervised by the lecture demonstration technician.

A fire extinguisher must accompany the apparatus.

The demonstration is for intermittent use only (10 minutes on with a 10 minute cooling off period)

Heat proof mats will be supplied with the apparatus.

The unit must not be placed beneath Room Fire Heat sensors

An exclusion zone will be included to protect staff, students and lecturers.