Unlike an expander, which increases dynamic range, a compressor reduces dynamic range. In recording, running a signal through both a compressor and an expander can be very effective.
Why would you want to reduce and enlarge the dynamic range at the same time? Actually, you wouldn’t.
They don’t both come into play at the same time; an expander does its thing when signals are at their quietest (or nonexistent), and a compressor does its thing in the louder part of the dynamic range.
So if you’re recording a cymbal crash through both a compressor and an expander, the expander will be working before the sound begins; the expander’s gate opens up immediately when the cymbal is struck, and then the compressor takes over.
The compressor works perhaps for a few seconds while the cymbal decays (with the expander doing nothing, since the signal is over the expander’s threshold).
Then the signal enters a kind of no-man’s-land, between the compressor’s and the expander’s active ranges, where neither circuit does anything to the signal.
Finally, when the expander senses that the crash is decaying below its threshold, its gate begins to close again (Figure 1).
This process accomplishes two things: the expander cleans up the noise before and after the crash, and the compressor tames the initial peak and thereby allows the whole signal to be brought up in volume, allowing it to have more punch and presence in the mix.
If you were recording or sampling a series of cymbal crashes, one after another, the compressor would be even more beneficial: It would tend to even out the crashes in volume, which would make the quieter crashes less likely to get buried in the mix and the louder ones less likely to overwhelm the mix.
As a bonus, compression makes a sound less likely to overload stages downstream in the signal chain—which is particularly important if you’re recording digitally.
Figure 1: A crash cymbal (left), and the same crash cymbal through a compressor/expander (right). The compressor and the expander come into play at different points of the cymbal’s decay (click to enlarge)
Here’s a look at a compressor’s typical parameters and how to use them:
Threshold: To understand how compression works, it helps to imagine expansion upside-down. When a signal rises past the compressor’s threshold, the compression circuit begins to kick in, and when a signal falls below this threshold, the compressor stops working.
So compression happens only when the signal is above the threshold—just as expansion happens only when the signal is below the expander’s threshold.
Given a gradually rising signal, compression can kick in suddenly, which is called hard-knee compression, or the circuit can come into play gradually as the signal rises, which is called soft-knee compression.
Some compressors allow you to specify which kind it performs; soft-knee compression tends to sound more transparent and natural.