GarageBand '11: Set the audio resolution
By default, GarageBand records and exports audio at CD quality (44.1 kHz sample rate, 16-bit depth). You can have GarageBand record Real Instrument regions, and export projects, at a higher audio resolution (24-bit depth).
Recording at a higher audio resolution increases the size of your projects. Exporting at a higher audio resolution increases the size of the exported file.
To set the audio resolution for recording and export:
Choose GarageBand > Preferences, then click Advanced.
In the Advanced pane, choose Good, Better, or Best from the Audio Resolution pop-up menu.
A brief explanation appears below the pop-up menu when you choose each item, to help you choose the best setting.