Demonstration consists of a pair of tuning forks mounted on wooden boxes which are open on one end. One of the tuning forks has a pair of adjustable weights which attach to the forks.

This demonstration can alternatively be run using a pair of organ pipes (where one of them is adjustable to be very close to the other). In principle any pair of oscillators that can be adjusted to be close to each other will work for this demonstration.

Demonstration of the constructive and destructive interference ('beating') observed when two nearly identical frequencies are played together. Relevant Wikipedia page

• Pair of tuning forks
• (Optional: Vernier Labpro, Microphone, and computer with LoggerPro)

Or alternatively:

• Pair of organ pipes

• Place the pair of tuning forks with resonance boxes on a table so that the open ends of the boxes face the audience. Now, strike both of the forks with a rubber mallet. If the adjustable fork is damped so that it's frequency is slightly different than the other, beats will be heard.
• If you are using a LabPro, attach it to the microphone sensor and a computer with LoggerPro open. Set data collection for a few seconds and display the graph of the beating frequencies

• If you do this with a pair of organ pipes, you will need an assistant. Get your assistant to sound the non-adjustable pipe continuously, and begin sounding the adjustable pipe yourself. Adjust the pipes so that they are sounding nearly the same note, and beats should be heard.

The frequency of the 'beats' tone that you hear is equal to the difference between the two other frequencies being played. If the frequencies are sufficiently close, you may not hear any beats at all, and if they're too far apart the beats will have a high enough frequency to be heard as a combination tone.

The basic principle is an extension of superposition.