Apple Final Cut Pro 7 - Manual (Page 836)

0 dB on an analog meter refers to the ideal recording level, but there is some allowance
for stronger signals before distortion occurs. This safety margin is known as headroom,
meaning that the signal can occasionally go higher than the ideal recording level without
distorting. Having headroom is critical when recording, especially when the audio level
is very dynamic and unpredictable. Even though you can adjust the recording level while
you record, you can’t always anticipate quick, loud sounds. The extra headroom above
0 dB on the meter is there in case the audio abruptly becomes loud.
Dynamic Range and Compression
Dynamic range is the difference between the quietest and loudest sound in your mix. A
mix that contains quiet whispers and loud screams has a large dynamic range. A recording
of a constant drone such as an air conditioner or steady freeway traffic has very little
amplitude variation, so it has a small dynamic range.
You can actually see the dynamic range of an audio clip by looking at its waveform. For
example, two waveforms are shown below. The top one is a section from a well-known
piece of classical music. The bottom one is from a piece of electronic music. From the
widely varied shape of the waveform, you can tell that the classical piece has the greater
dynamic range.
Waveform from a well-known
classical piece
Waveform from an excerpt
of electronic music
Notice that the loud and soft parts of the classical piece vary more frequently, as compared
to the fairly consistent levels of the electronic music. The long, drawn-out part of the
waveform at the left end of the top piece is not silence—it’s actually a long, low section
of the music.
Dynamic sound has drastic volume changes. Sound can be made less dynamic by reducing,
or compressing, the loudest parts of the signal to be closer to the quiet parts. Compression
is a useful technique because it makes the sounds in your mix more equal. For example,
a train pulling into the station, a man talking, and the quiet sounds of a cricket-filled
evening are, in absolute terms, very different volumes. Because televisions and film
theaters must compete with ambient noise in the real world, it is important that the quiet
sounds are not lost.
836 Chapter 52 Audio Fundamentals