Go here to play with drum polyrhythms and sine waves.
Slowly converging and diverging drum patterns - equally soothing and disconcerting. Ideal background music for grocery shopping, juggling 4D objects, tribal rites, and stress-testing your CPU fans (it may give them a hard time).
The slider changes the speed and pitch of one of the two identical sounds. The most easy-listening settings are 1) all the way left 2) right in the middle 3) all the way right.
Credits: Sine example - Daniel Shiffman