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What a compressor actually does
A compressor is an automatic volume control. It watches the level of a signal, and when that level rises above a threshold you set, it turns the signal down by a controlled amount. The amount of turning down is set by the ratio: with a ratio of four to one, every four decibels above the threshold comes out as only one decibel above the threshold. The difference between the input level and the output level is the gain reduction, and it is the number this calculator returns. A singer who jumps from minus twenty to zero decibels, through a compressor with a threshold of minus twenty and a ratio of four to one, comes out at minus fifteen, so the gain reduction is fifteen decibels. That is a large reduction, and in practice an engineer would rarely use so much on a vocal without also adding make-up gain.
Make-up gain is the second half of the story. After compression lowers the peaks, the whole signal is quieter, so engineers add gain back, usually by about the amount of gain reduction, to restore the peak level. The result is not the same as the original: the loud parts are now closer to the quiet parts, because the peaks were reduced and everything was then turned up together. That is the point of compression: it narrows the dynamic range, making quiet details easier to hear and loud moments easier to control. It is also the risk: too much gain reduction flattens a performance, removes punch, and brings up background noise and breath sounds that were previously hidden below the peaks.
How to use the calculator
- Enter the input level in decibels, for example the peak of a loud passage.
- Enter the threshold in decibels, the level above which compression starts.
- Enter the ratio, for example 2 for gentle compression, 4 for firm control, 10 or more for limiting.
- Enter a knee width in decibels, or leave it at zero for a hard knee.
- Read the output level, the gain reduction, and the make-up gain needed to restore the peak.
A worked example
Take a drum bus peaking at zero decibels, a threshold of minus twelve, and a ratio of three to one. The excess above threshold is twelve decibels. Divided by three, it becomes four decibels above threshold at the output, so the output level is minus eight decibels and the gain reduction is eight decibels. Add eight decibels of make-up gain and the peak returns to zero, but the quiet parts of the drum bus are now eight decibels louder than before. Now try a gentler setting: threshold minus twenty, ratio two to one, same zero input. Excess is twenty, output excess is ten, output is minus ten, gain reduction is ten decibels. The numbers show a useful lesson: a lower threshold can produce more gain reduction than a higher ratio, so always look at threshold and ratio together, never at one alone.
Hard knee, soft knee, and what the calculator leaves out
A hard knee starts compressing abruptly at the threshold: below it nothing happens, above it the full ratio applies at once. A soft knee starts compressing gradually before the threshold and reaches the full ratio only after it, which sounds smoother on vocals and acoustic instruments. This calculator models the soft knee with the standard quadratic shape inside the knee band, where gain reduction grows smoothly from zero to the hard-knee value. What it does not model is time: real compressors have attack and release controls that decide how fast compression starts and stops, and a detector that may respond to peaks or to average level. Those controls change how a compressor feels on drums versus vocals, and no static curve can capture them. Use this tool to plan the static curve, then set attack and release by ear on the real material.
Choosing settings that serve the music
Start with the goal, not the numbers. For a vocal that needs to sit steadily in a mix, a threshold near the average loud level and a ratio around three to one is a common starting point. For a bass guitar that needs consistent weight, a lower threshold and a ratio around four to one works well. For a drum bus that needs punch, a higher threshold so only the loudest hits are caught, with a fast attack avoided so the initial transient survives, preserves energy. For a final limiter, a very high ratio and a threshold near the ceiling catches only the peaks. In every case, check the gain reduction meter while listening: if it rarely moves, the threshold is too high; if it never returns to zero, the threshold is too low or the release is too slow. The calculator gives you the target curve; your ears and the meter confirm whether the real compressor follows it.
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