The developer calls it musical geometry, and that’s a great description. The idea behind the app is that you have a faucet dropping out balls at a particular tempo. You draw lines for the balls to bounce off of. The faster they are going when they bounce, the higher the pitch of the sound they make.
As you can imagine, if you get enough lines to restrict the balls and direct them in the right direction, you can get quite a sound going. The app is at it’s most musical when you have just a few lines. The sounds it can make in these cases is pretty interesting.
Soundrop is free, and it’s very fun. I’ve found myself more than once wrapped up in Soundrop for more time than I want to admit. Give it a shot, it’s a Universal app that works on both the iPad and iPhone. Though the app really shines on the iPads larger screen.
And did I mention it’s free?