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
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.
Demo room information