Copy of The sound of two sine waves
This shows, and lets you listen to, what happens when you combine two sine waves. You can control the overall volume, as well as the relative volume of each sine wave, and the pitch (in Hz).
If the sound isn't playing online (it seems to work in Chrome), you need to download the worksheet and open it in a local copy of GeoGebra.
Experiments: When happens when one pitch is twice the other? 50% more than the other? What about when the two pitches are very close but not identical (what you can hear is sometimes called 'beating', and is used by musicians to help tune their instruments).