They are using a multitude of digital effects on his vocals. Some of that is light stereo flanging. Some of it is automatic pitch correction. The light stereo flanging with a little bit of delay creates a beautiful stereo spacious effect utilizing a bit of the Doppler effect. That's because the flanging is low frequency modulated with a sine wave of very low frequency. And two different delay settings are utilized for each channel in the stereo image also creating the haas effect which presents a feeling of spaciousness in the stereo image. So it's not just one thing.

Not sure what to suggest for you in Garage Band since I don't do Macintosh? Most everything you need is probably already in that program and it's just the manipulation of those effects in combination with the other affects. It's what audio engineering is all about. And that generally require some trial and error. These effects are very precisely accomplished through great recording technique, knowledge and experience. There is no single effect that can produce all of that in most programs. So it's a layer of effects. You know they spent more than a few minutes on that particular mix since so much money is on the line for successful artists and/or performers who aren't necessarily artists. Artists is a term used much too loosely in our business. And I can assure you that that effect was not necessarily dictated by Justin Biber himself but by his producer and engineer.

All vocal doubling through digital processing are merely different time delays in both channels from a single mono source track. And this is from a single mono source track. In earlier days like with the Beatles, they literally tracked the vocal twice from two different takes. Today it's nothing more than audio software computer games which produces these fabulous effects and sounds. One of the ways in hybrid analog/digital that we accomplished a similar effect were with devices such as Lexicon PCM 70's, Yamaha SPX 90/SPX 90 II. With on that unit I/we would select a preset called " Pitch Change C ". It would take the mono input source slightly taking the pitch up in one channel and slightly taking the pitch down in the other channel outputs. While also mixing in the mono source in the center. I've used that a lot over the years with my SPX 90 II. Along with a light and short ambient plate reverb effect also added from an additional digital reverb unit such as the Lexicon PCM 70/60's and others that came later. And a lot of that can be accomplished in many different audio software programs. So it may also be possible to do within Garage Band? I'm just not really clear on the sophistication of that particular program?

I love those digital effects
Mx. Remy Ann David