A compressor turns down the loud parts, so logically the result should come out quieter than it went in. Every time I put one on a vocal the opposite happens and it sounds louder, closer and more present in the mix.
Where is the extra loudness actually coming from, if the thing is only ever reducing gain?
Accepted Answer
From the makeup gain, plus a change in what loudness means to your ear.
The compressor reduces the peaks. That leaves headroom, and you then raise the whole signal to use it. Peaks end up where they were, and everything quiet is much higher than before. Average level rises while peak level does not.
Our sense of loudness tracks average level far more than peaks, so it sounds louder even though the meter maximum is unchanged.
The second effect is that detail previously buried becomes audible: breaths, consonants, the tail of a note. That reads as presence rather than volume, which is why a compressed vocal sits forward in a mix.
One thing worth watching, because it is the usual beginner failure and it is not a volume problem.
Attack time set too fast removes the transient, which is the part that tells you what the instrument is. Fast attack on a snare gives a flat thud rather than a hit.
Often the better move is a slower attack that lets the transient through and catches the body behind it. If a compressed track sounds louder but duller, the attack is the first thing to check, not the ratio.
It is also why loudness wars happened. Heavy compression raises average level so a track sounds louder next to others, and at the extreme everything ends up equally loud and equally lifeless.
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