The ratio on a compressor controls how aggressively the compressor reduces volume once a signal crosses the threshold. A ratio of 4:1 means that for every 4 dB a signal goes above the threshold, only 1 dB comes through on the output. Higher ratios squeeze the dynamic range harder; lower ratios are gentler and more transparent.
How Ratio Works in Decibels
Ratio only affects the portion of a signal that exceeds the threshold. Everything below the threshold passes through untouched. Once the signal crosses that line, the ratio determines how much it gets turned down.
At a 2:1 ratio, a signal that’s 8 dB above the threshold comes out at just 4 dB above. Send three signals through at 2 dB, 4 dB, and 8 dB over the threshold, and they’ll come out at 1 dB, 2 dB, and 4 dB over. The louder the signal, the more gain reduction it receives. At 4:1, a signal that’s 6 dB over the threshold comes out at only 1.5 dB over, meaning the compressor is applying 4.5 dB of gain reduction. The math is straightforward: divide the amount above the threshold by the first number in the ratio to get the output level above the threshold.
Low Ratios vs. High Ratios
Ratios between 1.5:1 and 3:1 are considered gentle compression. They narrow the dynamic range without dramatically changing how the audio sounds. A low ratio paired with a low threshold tends to produce transparent, even compression across the whole signal. It’s useful for raising the overall perceived loudness of a track without making the compression obvious. Vocals often sound best in this range (2:1 to 3:1) because you get enough control to keep them sitting steadily in a mix while preserving natural expression.
Ratios from 4:1 and above compress more aggressively. These settings are more audible and start to change the character of the sound rather than just taming its volume. Drums and bass commonly use ratios around 4:1 to tighten up the performance and make each hit more defined. The higher you push the ratio, the more the compressor flattens the loudest moments of the signal, which can add density but also risks making things sound squashed if you overdo it.
When Compression Becomes Limiting
Push the ratio high enough and a compressor essentially becomes a limiter. There’s no universal cutoff, but most engineers consider ratios above 10:1 to be limiting territory. Some define it as 20:1 or even infinity:1, where the output can never exceed the threshold at all. The practical difference is that a limiter acts as a hard ceiling. It’s the same process as compression, just taken to an extreme where peaks are caught and stopped almost entirely rather than gently reduced.
How Ratio Affects Punch and Transients
Ratio has a direct impact on how punchy a sound feels. Higher ratios tend to produce more punch because they increase the contrast between the initial hit of a sound (the transient) and the sustained body that follows. At 2:1, the difference between the loudest peak and the rest of the signal stays relatively small, so the sound feels smooth but not particularly impactful. At 4:1 or above, that gap widens, and the transient pops out more dramatically.
There’s a catch, though. Ratio interacts closely with attack time. A high ratio paired with a very fast attack clamps down on transients before they even get through, which makes the sound dull and lifeless instead of punchy. The punch comes from letting the initial peak slip past the compressor before the gain reduction kicks in. So if you’re after impact on drums or percussion, a higher ratio with a slower attack (somewhere around 10 to 30 ms) lets the snap of the hit come through while still controlling the sustain.
How Knee Settings Change the Ratio’s Behavior
The knee setting on a compressor determines how the ratio gets applied near the threshold. With a hard knee, the compressor does nothing until the signal crosses the threshold, then applies the full ratio immediately. A signal at 1 dB below the threshold is uncompressed; at 1 dB above, it gets the full ratio treatment. This creates a sharp, noticeable transition that can sound aggressive or precise depending on the context.
A soft knee eases into the ratio gradually. Instead of waiting for the signal to cross the threshold, the compressor starts applying light compression as the signal approaches it and doesn’t reach the full ratio until some point past the threshold. The threshold sits at the center of this transition zone, and the width of the zone (measured in dB) determines how soft the knee is. A wider zone means a gentler, more transparent onset of compression. Soft knee settings are particularly useful when you want higher ratios without the compression sounding abrupt or heavy-handed.
Ratio and Makeup Gain
Compression reduces the level of your loudest moments, so the overall signal comes out quieter than it went in. The more gain reduction you apply (which increases as you raise the ratio or lower the threshold), the more you’ll typically need to compensate with makeup gain. Makeup gain is simply volume added after the compressor does its work, bringing the output back up to roughly the same peak level as the input.
A common rule of thumb: match the makeup gain to the average gain reduction. If the compressor is reducing peaks by about 4 dB, add 4 dB of makeup gain. This keeps the perceived loudness consistent when you toggle the compressor on and off, which is essential for making honest comparisons. Without it, the compressed signal will sound quieter, and your brain will interpret “quieter” as “worse” regardless of whether the compression actually improved things. A practical starting point for most sources is a ratio between 2:1 and 6:1 with the threshold set to produce 3 to 6 dB of gain reduction, then bringing the output up by the same amount.
Common Ratio Ranges by Instrument
- Vocals: 2:1 to 3:1. Keeps natural dynamics intact while preventing lines from disappearing in the mix.
- Drums: 4:1 or higher. Tightens hits and adds punch, especially on snare and kick.
- Bass: Around 4:1. Evens out the volume differences between plucked and sustained notes so the low end stays consistent.
- Mix bus or mastering: 1.5:1 to 2:1. Gentle “glue” compression that brings elements together without flattening the overall dynamics.
These are starting points. The right ratio depends on the performance, the arrangement, and how much dynamic range the source material has. A vocalist who stays close to the mic and sings evenly needs far less compression than one with dramatic volume swings. Trust your ears over any preset, and always A/B with matched levels to hear what the ratio is actually doing.