Sound & signal
Ratio
Ratio sets how hard a compressor reduces anything above the threshold. At 4:1, four decibels of input above the threshold become one of output. At 20:1 or beyond it stops being compression and becomes limiting.
In practice
Low ratios shape, high ratios limit. Past about 10:1 you have built a limiter, whatever the box is called.
Two to four to one is the useful range for making a record sit better against the ones around it, and it should be almost inaudible. If you can clearly hear the compressor working, it is either the ratio or the attack, and on a DJ mixer the honest answer is usually to turn it off and fix the problem with gain instead. The one place a high ratio genuinely belongs in a booth is a microphone channel, where a speaker who moves around needs a ceiling more than they need dynamics.
2:1–4:1
The useful range
10:1+
Where it becomes a limiter
The mic
Where high ratios belong
Where these come from: Standard dynamics practice, not a catalog figure. For what we can measure, see the control beside it.
Why it matters
It is the number that turns one processor into another. There is no mechanical difference between a compressor and a limiter — a limiter is a compressor with the ratio wound past the point where anything meaningful gets through, and knowing that removes most of the mystery from both.
For DJs the practical range is narrow. Gentle ratios shape a thin record into something that sits with its neighbours; anything aggressive is doing the venue’s job for it, and doing it worse, because the venue’s limiter is calibrated to its own speakers and yours is not.
Frequently asked
What ratio should I use?+
Two to four to one for shaping, and it should be close to inaudible. If you can hear it working, either back it off or fix the underlying level problem with gain.
At what ratio does a compressor become a limiter?+
Conventionally around ten to one and unambiguously past twenty. There is no hardware difference — it is the same processor with the control wound up.
Should I compress the master on a DJ mixer?+
Rarely. The venue is limiting downstream against its own speakers, so your stage removes dynamics without adding level anyone hears.
Related terms
Threshold
Threshold is the level above which a compressor or limiter starts working. Below it nothing happens; above it the processor reduces gain by whatever the ratio says.
Attack and release
Attack sets how quickly a compressor responds once the signal crosses the threshold; release sets how quickly it stops once the signal falls back. Together they decide whether you hear the processor or the music.
Multiband compression
Multiband compression splits the signal into frequency bands and compresses each separately, so a loud bassline does not pull the whole track down with it. Club system processors use it; DJ mixers generally do not.
Limiter vs compressor
Both reduce dynamic range. A compressor turns down loud signals gradually by a ratio; a limiter stops them passing a ceiling at all. A limiter is a compressor with an extreme ratio and a fast attack.
Explore: LUFS meter · Compressor · Limiter vs compressor · All DJ terms