First of all, compression has three completely unrelated meanings as it relates to the recording studio. One meaning is the compaction of air molecules followed by rarefaction, which is what takes place in a sound wave. Another meaning is the computer process in which a data file is reduced in size by eliminating redundant, useless, or unnecessary information as determined by various algorithms, such as an MP3 file being created from a WAV file. The third meaning is the reduction in dynamic range of an audio signal using a compressor. It is the third definition we will be discussing here.
A compressor is used to even out sound levels of vocals or instruments. It does this by reducing sounds that exceed a certain level, called the threshold, by a given ratio, called the compression ratio or slope. Vocals typically are reduced with a ratio of 2:1 to 3:1, other instruments 3:1 to 5:1, and a bass by as much as 10:1. The effect is to reduce the overall level of the signal, which is called gain reduction. The signal can then be increased back close to its original level by an amount called makeup gain. This has the effect of raising the quieter passages which effectively decreases the overall dynamic range—the difference in loudness from the quietest to the loudest section in a piece of music.
There are several other adjustments for a compressor. How quickly the compressor starts to act is set by the attack time. How long before the effect begins to decrease is determined by the release time. Another adjustment is the knee, which is at the threshold point where compression begins. With a hard knee there is a sudden reduction in level, but with a soft knee the reduction takes effect more gently.
Parallel compression (sometimes called New York compression) is an technique created by highly compressing one copy of a track and blending it with an uncompressed copy of the same track. This technique preserves the transients of the original signal while increasing the level of softer portions of the signal, which tends to increase the overall level of the track without sounding overly compressed. Bus compression is compressing a bus or group of tracks. If it is applied to the entire mix, it is called mix bus compression.
A limiter prevents an audio signal from exceeding a certain preset value. It is essentially a compressor with an infinite or very high compression ratio.
An expander increases the dynamic range of a signal, which is essentially the opposite of what a compressor does. But it does not act exactly in the opposite manner. You might think that a signal compressed by a 3:1 ratio could be returned to its original condition by expanding it by 3:1 expansion ratio. Not true. Instead of expanding everything above the threshold by that amount, it actually reduces the signal by that ratio below the threshold, a subtle but totally different effect. Otherwise, all the other settings for the compressor are the same on the expander. So when would you want to use an expander? One application would be the case where you have background noise occurring in a live recording. Using an expander may be able to push that noise down to the point where it no longer obvious.
So there you have everything you wanted to know about compressors and expanders and more.
• After Dave Prater shot his wife in a domestic dispute in 1968, Sam Moore of the R&B duo Sam and Dave vowed that he would never speak to Dave again, even though they went on to record and perform together for another 12 years.
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