Sound 8 for iPhone

"Sound 8 is an interactive 8 track sound looper.

In this application, each sound is represented by a circle of a different color. You can assign a default sound or any of your own imported sounds* to each circle.

A sound's pitch and volume is altered through the device's accelerometer or using touch input and gestures. Swipe gestures control the accelerometers speed and allow for randomization of pitch and volume. In addition, each sound's panning can individually be set.
It is also possible to stop acceleration completely, or to automatically position sounds in "Automove" mode.

To see an example of how to import your own sound files please visit the sound8 support page.

*supported file format is 16-bit mono WAV or AIFF.
maximum sound file size (per circle) is 5.3MB, or ~1minute @44.1kHz 16-bit mono wav."

Sound 8 - Oliver Grimm - iTunes
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I just checked this one out. It's a surprisingly cool little app. A nice feature is an included mini tutorial in the app to get you started. What really makes it interesting is the ability to download your own samples off the web. If you look at the bottom right screenshot you will see an address bar. The Support site linked above gives an example of how to download off of SoundCloud directly. Do a search on the right or on C for SoundCloud and see what comes up. That should be interesting. One thing I would like to see is the ability for the circles to move about indefinitely, bouncing off of the walls once you get them going [Update: this has been added!]. That way you could have it randomly do it's thing while you play other gear. Right now they settle to one spot once they level out with gravity until you move them again by either dragging them or using the accelerometer. The loops do continue to play endlessly with whatever effect they settle with though. Been listening to the red one while writing up this post. :) Regardless, this is a very interesting app.