iMovie You can reduce the background noise in video and audio clips without reducing the overall volume and audio quality of the clips. For example, if you recorded a birthday party outdoors and a plane flew overhead, the background noise from the plane could mar an otherwise good audio track. Reducing the background noise can fix the problem.
To reduce background noise in an audio or a video clip:
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In the Project browser, double-click the video clip or audio clip (the colored bar that represents a voiceover, a sound effect, or music you’ve added to a clip, shown below) you want to modify.
Click Audio at the top of the inspector that opens.
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Select the “Reduce background noise by” checkbox in the Enhance section of the inspector, and then drag the slider to set a percentage of noise reduction.
Depending on the quality and type of audio embedded in your clip, there may be some reduction of overall volume and audio quality if you set noise reduction to any value above 75 percent.
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To play back changes to the selected clip, in the Project browser, move the playhead (the red vertical line) to the left of the clip and then press the Space bar; press it again to stop playback.
You can start and stop playback while the inspector is open, which makes it easy to make a change by moving the slider, play it back, and then adjust the slider and play back the changes again until you’re satisfied with the result.
To restore audio to its original state at any time, even after you’ve closed and reopened the project, double-click the clip, click Audio at the top of the inspector, and then click “Revert to Original.”