Understanding dynamic range is essential when recording, mixing and mastering your tracks. This post is a clear explanation and helpful overview of the topic from Mark Garrison's book Encyclopedia of Home Recording.
Dynamic range is a measurement of the difference between the quietest possible sound (silence) and the loudest possible sound that can be handled by a given medium. The potential dynamic range of music in an acoustic environment is as high as 120 decibels (dB). The dynamic range of a CD is closer to 80 dB, FM radio is only about 50 dB, and AM radio a mere 30 dB. With these severe limitations on dynamic range, compression becomes necessary in order to allow the changing dynamics in music to be heard.
Another reason for limiting the dynamic range of a recording is consideration of the listening environment. Many people listen to music in environments with a lot of background noise, such as in their car or at work. In these situations quieter passages will be masked by the background noise. Careful compression can be used to reduce this problem.
Multi-band compression breaks the signal into different frequency ranges and compresses each range separately. This is a bit of a middle-ground between compression and equalization. The signal is broken up into frequency bands, just like with an equalizer, but rather than boosting or attenuating the volume of these bands like an equalizer, a multi-band compressor applies compression to the bands separately.
Multi-band compressors are most commonly used in the mastering process, when instruments can no longer be affected separately. Because the frequency bands are compressed separately, an overly dynamic instrument in one range will not affect the amount of compression in other ranges.
Compression and Mastering
During the mastering process, compression is usually added to the completed mix. This is done with the dynamic limitations of different mediums in mind (as mentioned above), but is also used to increase the perceived volume of the mix. In modern music there is a bit of an unspoken competition to make louder mixes, the reasoning being that if your song is just a bit louder than any other on the radio, it will be noticed more.
The advertising industry uses this same technique, which is why television commercials are often far louder that the programs themselves.
When To Compress
It has been said that the sound of modern music is the sound of compression. By the time we hear a sound in a modern recording it has generally been compressed multiple times.
During the tracking process, a little bit of compression can act as a safety net, preventing the signal from clipping. This can save a good take that may otherwise have been unusable due to distortion.
Depending on the style of music, instruments may be compressed again (sometimes quite heavily) during the mixing process in order to have them all sit nicely in the mix. This is also a first chance to get the overall volume of the mix higher.
Once the track is mixed, it will usually be compressed again during the mastering process to further increase its perceived volume. This needs to be done carefully as it is very easy at this point to lose the natural dynamics of the song.
If the track is then broadcast over the radio, it is compressed again (rather heavily) in order to deal with the limited dynamic range of radio broadcasting.
Significant amounts of compression have become not only accepted but expected in modern recordings.