What happens when you add similar sine waves together? Any musician can tell you: as you tune an instrument closer and closer to the correct tone, you start to hear "beats", when the tone sounds louder and softer and louder and softer and ... You get the idea.
What does it look like? Take a look.
Your job is to reproduce this sound, and draw it, using the following link: http://www.falstad.com/fourier/e-logsound.html
and write a summary of what Beats are using the following article: http://en.wikipedia.org/wiki/Beat_%28acoustics%29