Apple Soundtrack Pro 2 - Manual (Page 471)

Appendix B Audio Fundamentals 471
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.
Stereo Audio
The human ear hears sounds in stereo, and the brain uses the subtle differences in
sounds entering the left and right ears to locate sounds in the environment. To re-
create this sonic experience, stereo recordings require two audio channels throughout
the recording and playback process. The microphones must be properly positioned to
accurately capture a stereo image, and speakers must also be spaced properly to re-
create a stereo image accurately.
If any part of the audio reproduction pathway eliminates one of the audio channels,
the stereo image will most likely be compromised. For example, if your playback system
has a CD player (two audio channels) connected to only one speaker, you will not hear
the intended stereo image.