When an audio signal is inverted the sound wave is essentially flipped upside-down. The wave's positive peaks become negative and its negative peaks become positive. Many audio recording and editing applications, such as Audacity and Goldwave, come with a built-in sound wave "Invert" effects tool. The current GarageBand release, GarageBand 11, does not feature an audio signal inversion tool. To invert a track in GarageBand, you must install a third-party Audio Unit plug-in that can perform this task.
Download an AU plug-in that features a phase-invert tool such as Sonalksis freeG channel strip AU plug-in. Save the plug-in to your desktop.
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Click the "Finder" icon in the dock to open a new "Finder" window. Browse to "Macintosh HD," "Library," "Audio," "Plug-ins" then "Components." Drag the AU plug-in from the desktop to the "Components" folder.
Launch GarageBand. If the application is already open, close it and re-launch it. Open the project you want to edit.
Double-click the header of the track that contains the sound wave you want to invert. The Track Info pane will open on the right.
Go to the "Edit" tab in the Track Info pane. Click on "Click here to add an effect." Scroll down to the "Audio Units Effects" section. Select the AU plug-in you installed in Steps 1 and 2.
Click the AU plug-in's designated phase inversion button to invert the track. With the freeG plug-in, for example, the button is labeled "Flip." Click "Apply."