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Air, an app by Opal Limited, is available for US$1.99 at the App Store.
The iPhone has an odd sort of potential as a musical instrument. From the moment the App Store opened, there have been applications that attempt to approximate real instruments, even giving you the ability to make multitrack recordings. I've yet to hear a real magnum opus composed on one, though I'm sure someone somewhere has tried. I was about to say that I've also never seen an all iPhone/iPod touch band, but I thought to check out YouTube before committing that to words, and sure enough ...
But a touchscreen device like an iPhone isn't limited to valves, frets or a traditional piano-style keyboard arrangements. In fact, anyone who's learned on a real instrument might find a touchscreen's lack of tactile feedback off-putting and difficult to deal 
with. So instead of trying to cram a drum set or a piano into an iPhone, some developers take a different approach and try to come up with new ideas for music creation that better fit a small touchscreen interface.
That's the route Peter Chilvers and Brian Eno took in creating Bloom several months ago. Now Chilvers has partnered Sandra O'Neill to create an app called "Air," with concepts credited to Eno as well.
Tap to Play
Like Bloom, Air is not so much an instrument that lets you play any song in any style as long as you know the right technique. Instead, it's sort of a sketched-out aural canvas to which you apply the paint.There are two modes: Listen and Conduct. Conduct allows you to actively control the sounds by tapping various sectors of the screen. It lets you choose between controlling voices (continuous "ahs," no lyrics), piano notes or "random," wherein some tap zones are piano, some are voices, and some sound like far-off crashes of percussion.
Select a mode, and you're taken to a screen with 12 triangular touch zones. Tap one, and a note will play -- lots of sustain and reverb -- and colors shift around kaleidescope-style. Everything's in the same key, so it's hard to come up with anything that sounds really unpleasant.
Just Listen
Listen mode turns Air into a non-stop ambient music generator. You can select whether you'd like it to play piano, voices or both, and you can adjust the spacing in between the notes it generates. You can also select up to 8 "players" -- that is, if there are other iPhones/iPod touches in the vicinity and you want to make them part of the same mix, you can run Air on all the devices at once and they'll spread out the tones between them, letting the music come through multiple sets of speakers.
There's also a sleep timer. Yes, all these fluffy ambient tones do make it easy to pass out.
Whether you're using the active Conduct mode or the passive Listen mode, the result sounds a whole lot like "1/2" on Eno's "Music for Airports," thus the name of the app.
Bottom Line
As an ambient music generator, Air does the job. Hook it up to a set of speakers and it really will make it sound like you're in some kind of dreamy sci-fi/fantasy movie, or stuck in an airport.
However, as music creation apps go, this one just doesn't capture attention the way Bloom does. Air is limited to 12 sectors, and the touch interface doesn't feel as responsive as Bloom's. Neither of these apps is really designed to play in sync with other musicians like in the YouTube clip above, but in terms of just messing around solo, Bloom is more fun.
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