SoundBrush is a US $1.99 iPad app that lets you create original compositions by swiping your finger across the iPad screen. The description sounded a bit odd, but I had fun playing with SoundBrush and discovering what it could do.
You can have a total of 4 instrumental lines -- piano, strings, flute and synth, and it appears the app selects proper harmonies when you lay your tracks down. I was surprised how good my little compositions sounded, although I wasn't quite ready for a concert.
The app lets you hear the notes as you draw them, and you can super-impose a grid to show you the notes you are drawing. You can delete anything you have written with a double tap, so you can actually edit the music you create. The canvas scrolls, so you can have lengthy compositions. You can also control the playback speed, which also comes in handy when you are editing.
This might be a great way to get a young person involved in music, and the combination of the dynamic colors and music that flows from the drawings you make are bound to be a stimulus to any young mind. I'm no kid, and I really enjoyed experimenting. I would have loved to have the app creators include some sample compositions, because starting cold, you don't know what to expect. I also think having some percussion options would be nice. Using the app doesn't require any musical knowledge, but I expect the more you know, the better your compositions will be.
Soundbrush requires iOS 5 and an iPad, and it supports the retina display on the new iPad.