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.
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.
Waveform from a well-known
classical piece
Waveform from an excerpt of
electronic music
26
Part I
Audio Mixing
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.
The goal is to make the quiet sounds (in this case, the crickets) louder so they can
compete with the ambient noise in the listening environment. One approach to making
the crickets louder is to simply raise the level of the entire soundtrack, but when you
increase the level of the quiet sounds, the loud sounds (such as the train) get too loud
and distort. Instead of raising the entire volume of your mix, you can compress the loud
sounds so they are closer to the quiet sounds. Once the loud sounds are quieter (and the
quiet sounds remain the same level), you can raise the overall level of the mix, bringing
up the quiet sounds without distorting the loud sounds.
When used sparingly, compression can help you bring up the overall level of your mix
to compete with noise in the listening environment. However, if you compress a signal
too far, it sounds very unnatural. For example, reducing the sound of an airplane jet
engine to the sound of a quiet forest at night and then raising the volume to maximum
would cause the noise in the forest to be amplified immensely.
Different media and genres use different levels of compression. Radio and television
commercials use compression to achieve a consistent wall of sound. If the radio or
television becomes too quiet, the audience may change the channel—a risk advertisers
and broadcasters don’t want to take. Films in theaters have a slightly wider dynamic
range because the ambient noise level of the theater is lower, so quiet sounds can
remain quiet.