Apple Final Cut Pro 5 - Manual (Page 914)

Chapter 7 Mixing Audio in the Timeline and Viewer 127
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Example: Using Keyframes to Adjust Audio Levels
You need at least two keyframes to make any dynamic change from one volume level
to another in a clip.
In the example above, the section of the clip to the left of the keyframes is at –30 dB,
and the rest of the clip to the right of the keyframes is at 0 dB. This is the simplest type
of level change you can make.
A more sophisticated change in levels— for example, introducing a slight boost in the
level of a few notes in a music track—requires three keyframes:
In this example, the volume level of the clip starts at –3 dB and then rises along a
curve, peaking at +6 dB on the note that’s playing at that point. The volume level then
lowers along another curve, ending back at –3 dB.
Three keyframes allow you to boost or attenuate (lower) a section of audio along a curve,
but to make less gradual changes to longer sections of audio, you’ll need to use four.
In this example, the volume level, instead of rising or lowering constantly, changes from
–3 dB to –26 dB during the first two keyframes, and then remains constant. The final two
keyframes boost the level back to –3 dB, where it remains for the duration of the clip.