lesson
Updated 6 days ago
Imagine a vocalist who whispers intimately on the verse at −32 dBFS, then belts the chorus at −4 dBFS. In a dense mix, the verse is completely inaudible, but turning up the fader blows your speakers on the chorus.
A compressor acts like an automated engineer with lightning-fast reflexes, turning down signals that exceed a chosen level to narrow this dynamic range (the decibel difference between the quietest and loudest parts of an audio signal).
In the 1930s, Western Electric engineers developed automatic gain control circuits to prevent powerful AM radio transmitters from overloading during unexpected loud broadcasts. Today, those same core parameters give you surgical control over musical dynamics.
How does a compressor know exactly when to start turning down the volume, and how aggressively to react?
Threshold and Ratio
The threshold is the decibel level (−dBFS or dBu) at which compression begins. Any signal below the threshold passes through unaffected (a 1:1 input-to-output ratio).
The compression ratio determines how much the signal exceeding the threshold is attenuated. A ratio of 4:1 means that for every 4 dB the input rises above the threshold, the output is only allowed to rise by 1 dB.
📊Interactive transfer function graph showing Input Level (dBFS) on x-axis (-40 to 0) vs Output Level (dBFS) on y-axis (-40 to 0). Draw a 1:1 linear dashed reference line. Set a Threshold marker at -20 dBFS. Draw solid compression curves above -20 dBFS for 2:1 (slope 0.5), 4:1 (slope 0.25), 8:1 (slope 0.125), and 20:1/Brickwall Limiting (nearly flat line). Include clear interactive toggle tabs for each ratio so clicking 4:1 highlights that line in bold blue (#22b7ff) and displays an annotation box: 'Input: -8 dBFS (12 dB over threshold) -> Output: -17 dBFS (Gain Reduction: -9 dB)'. Clean light background (#f8fafc), dark text (#1e2945).
To calculate the output level mathematically when an input signal (Lin) exceeds the threshold (T):
Lout=T+RatioLin−T
Gain reduction (GR) is the amount of level lost: GR=Lout−Lin. For an input of −8 dBFS with a threshold of −20 dBFS and a 4:1 ratio, the signal is 12 dB over threshold; the compressed excess is 12/4=3 dB, yielding an output of −20+3=−17 dBFS and a gain reduction of −9 dB.
Ratios between 1.5:1 and 2:1 provide subtle, transparent dynamic glue for mix buses, 4:1 to 8:1 provides authoritative peak control on vocals and bass, while 20:1 and above acts as a limiter to prevent any peaks from exceeding a ceiling.
What happens right at the threshold point—does the compressor slam down instantly, or ease in gradually?