A good way to teach beats is to begin with the maths. Show students that waves with two slightly different frequencies will go in and out of phase, alternately adding and canceling. This can be done on the board, but if computers are available students can use a spreadsheet or graphing program to see this for themselves.
Let students study the graph, then ask them what this would sound like.
Then play the beats and show the equation for beats. It is difficult to test the equation below:
with standard secondary school signal generators as the frequency cannot be set very accurately. The example sound files were produced using technical grade signal generators and the frequencies are accurate to much better than 1 Hz.
You can also use the Combining Sounds software to simulate beats. The sounds will play on a computer speaker, but the beats are produced by adding mathematically the functions for the two waves and then playing the resulting total signal rather than actual addition of sound waves.