Beats

HOW

1. Strike one of the pipes suspended from the stand. Listen to the sound it makes.
2. Stop the vibration of the pipe by touching it with your hand. Repeat with the other pipe.
3. Both pipes produce sounds of slightly different pitch. Strike both pipes in succession with rapid movements. You will hear a sound of cyclically increasing and decreasing intensity – these are beats.

WHY

Sound waves can merge with each other. The properties of the wave thus created depend on the properties of the constituent waves. When two waves of similar but not equal frequencies combine, beats are produced. Their intensity varies cyclically from a minimum to a maximum value.