Photo by Frankie Lopez on Unsplash
I remember it vividly: my high school buddies and I had just mic'd up our drummer's kick drum and it was time to record. I inserted my simple Alesis micro limiter in the kick channel and tried to dial in the punchy, sharp sound I had in my head. Though I was expecting miracles, the limiter wasn't able to fix a poorly tuned, poorly mic'd kick drum.
I made many mistakes that session, but my biggest problem was not having a clue about dynamics processing. It seems that a complete understanding of compression and limiting--and how to apply them--is a challenge for many people. If you're in that camp, this article is for you.
Three Functions, Three Controls
Compression has three vital functions: dynamic range reduction, peak limiting and envelope adjustment. Each works by reducing the gain (volume) of the input signal. We'll cover each of these functions in turn. A few clarifications: “Limiting” is just a term for a specific application of compression. Noise gating (or expansion) is a form of dynamics processing, but we won't cover that here.
Though most compressors offer many knobs and buttons, there are three main controls you need to understand to really apply dynamics processing effectively. The first is the threshold, which sets the signal level at which the processing begins. Until the signal comes up to the threshold level, the dynamics processor leaves it alone. The ratio control tells the processor how much gain reduction to apply after the signal passes the threshold. The attack time control sets how fast the processor responds once the threshold is passed.
Think of the compressor as an elastic band across a doorway. Until you walk into the band (at the threshold), your progress is unhindered. Once you hit the elastic band, you have to work extra hard to move forward. The thickness of the band (the ratio), determines how hard you have to push. If the elastic band could wait before pulling you back, that would be an analogy for the attack time. Dynamics processing works like this elastic band: it makes the signal push harder to achieve an increase in volume.
Of the three parameters, two have the greatest effect on the amount of gain reduction you achieve. Lowering the threshold causes more of your input signal to push into the compression. Increasing the ratio makes the signal work even harder once it passes the threshold.
A fourth control (release time), tells the compressor how fast to return to unity gain as the input signal decreases in level. This control will affect the sound in certain applications, but the auto release setting usually does a fine job. Finally, a makeup gain (or output level) control allows you to compensate for signal level lost through the compressor.
Compressor Controls at a Glance
Threshold - The signal level at which the compressor starts to work
Ratio - How hard the compressor works
Attack Time - How quickly the compressor gets to work
Release Time - How quickly the compressor stops working
Makeup Gain - Returns the processed signal to the same perceived volume
Sit in the Mix
Most instruments and voices have tremendous dynamic range, meaning they can go from very quiet to very loud (spanning a 40 dB range, or more). In the context of a mix, this means an instrument could be inaudible one moment and painfully loud the next. Talented musicians can control their volume (within a 10 dB range, for example), but many still play with greater dynamics than the mix requires. With modern music, having a part or an instrument “sit in the mix” usually means it varies in level just a few dB over time.
One option to keep a part sitting in the mix is to constantly ride the fader, but who can do that across the whole mix? A better solution is to let a compressor reduce the dynamic range of the input. This usually involves a low ratio, a low threshold and a fast attack time. Here are some good starting settings for dynamic range reduction.
Talented musicians can control their volume (within a 10 dB range, for example), but many still play with greater dynamics than the mix requires.
Dynamic Range Reduction
Ratio - 1.5:1 - 4:1
Attack Time - 1 - 20 ms
Threshold - adjust as needed
Gain Reduction - 3 - 8 dB (max)
Peak Control
A compressor can also strictly enforce a maximum signal level, something very important in this age of digital everything and hard clipping. In this role, a compressor is often called a limiter, because it quickly (and severely) limits the signal level the instant it reaches the threshold.
Peak limiting involves a high threshold so the signal is affected only as it approaches the danger zone. A high ratio makes signal increase past the threshold minimal. A fast attack time keeps transients from sneaking through. Here's a suggested starting place for limiter settings:
Peak Limiting
Ratio - 10:1 - infinity:1
Attack Time - as fast as possible (0.1 ms - 1 ms)
Threshold - 2 dB below max level
Gain Reduction - 10 - 15 dB (max)
Set incorrectly, a limiter is the fastest way to ransack your sound. If a limiter is engaging more than on rare occasion or reducing gain by more than about 10 dB, back off the input level or raise the threshold.
Because compression and limiting achieve very different ends, they are often used together in series. Compression evens out the overall volume, followed by a limiter to enforce a “ceiling” level. If a channel has both, try engaging them. You may also have success applying compression at the channel level and limiting on a bus or subgroup.
The Envelope, Please
The final and most creative application of compression involves using it to change the dynamic shape (or envelope) of the sound. Sounds ramp up from silence with an “attack” phase, continue into a “sustain” phase, then drop back to silence with a “decay” phase. The balance between the attack and sustain levels gives a sound much of its character, and a compressor lets you change this balance.
Percussive sounds generally have a very strong attack component followed by a quieter sustain. This includes the output from drums, other percussion, acoustic guitar, piano, bass and more. In a mix, you may find that the initial attack of sound from these instruments is loud enough to spark red lights while the sustain portion (which really defines the tone) is too quiet.
To decrease the attack portion of a sound relative to its sustain (body), use a fast attack time and moderate ratio. Though auto release time will usually sound fine, settings are suggested below in case it doesn't. If the sound ends up choked or lifeless, try raising the threshold or slowing (increasing) the attack time.
Decrease Attack, Add Body
Ratio - 3:1 - 5:1
Attack Time - 1 - 5 ms
Release Time - 20 - 50 ms (or auto)
Threshold - adjust as needed
Gain Reduction3 - 6 dB (max)
Sounds that need a punchier, snappier attack can also benefit from compression. In this case, the goal is to boost the volume of the initial transient relative to the sustained sound. A slow attack time lets the transient sneak through before the compressor “grabs” and reduces the body of the sound. This is the go-to method to make snare and kick sound crisp and fat. Below is a good starting point for adding attack and snap.
Increase Attack
Ratio - 2:1 - 5:1
Attack Time - 10 - 40 ms
Release Time - 100 - 250 ms (or auto)
Threshold - adjust as needed
Gain Reduction - 6 - 12 dB (max)
Season to Taste
The recipes above represent a starting point. Every device and plug-in is different, as are the tastes of every engineer. The path to success, however, is largely the same: dial in your initial settings, bring up the make-up gain, toggle bypass to match the compressed and original signal. Listen for the compression effect. Adjust the controls to suit. Keep an eye on the gain reduction, but trust yours ears.
Finally, about those other compressor parameters: let your ears be your guide. Knee, peak/RMS, opto, mix--all will have an effect (sometimes subtle) on the sound. Tweak them to hear what sounds best.
