Dynamics processing is a core mixing technique, but it’s often misunderstood. We explain how to make a compressor do what you want it to!

Sound is dynamic. It varies in level on a large timescale, such as when a loud finale follows a reflective slow movement. It also varies on a much smaller scale. For example, each time we hit a piano key, the note starts loud and then dies away.

When we record or mix sound, we sometimes want to alter this dynamic behaviour. We might wish to amplify the quiet passages so they can be heard more easily. We might seek to make a performance appear more consistent. We might even want to change the dynamic character of individual notes, for instance by making them sound more or less staccato.

The simplest way to alter dynamics is to adjust the level using a fader. A human listener can follow the large‑scale dynamic variation in a piece of music and make it louder or quieter as appropriate. However, small‑scale dynamic variation — sometimes known as micro‑dynamics — happens too fast for a human operator to respond reliably. Enter the compressor: a device that ‘listens’ to the input signal, analyses its dynamic variation, and adjusts the level automatically in response.

Some compressors have almost no controls at all. Others bristle with an intimidating array of obscurely-named knobs. The aim of this article is to demystify these parameters and help you to use compressors with confidence.

The first thing should always be to ask yourself why you’re using a compressor at all, and what result you’re aiming for.

Insert Or Send?

Conceptually, a compressor is like an automated fader. It’s a transformative rather than an additive process: it modifies, rather than augments, the sound. For this reason, a compressor plug‑in or hardware unit would usually be used as an insert, not an auxiliary send effect. If we want to compress multiple sources at once, we either need to use a separate compressor for each of them, or bus them to a single channel and compress that channel. Both are valid approaches, but they have very different effects and applications.

Watching The Detectors

Most of the key controls on a compressor adjust the circuit or algorithm that decides whether the input signal should be turned down or left alone. This is known as the detector, and it usually receives its own, dedicated copy of the input signal, on a separate path known as the side‑chain.

A basic detector could simply compare the peak level of this side‑chain signal with a chosen fixed value, and trigger gain reduction in the audio path whenever the one exceeds the other. The lower we set this value, the more likely the signal level will be higher at any given moment, and the more often compression will happen. The fixed value in this scenario is known as the threshold, and this is one of the key parameters in nearly all compressor designs.

If the aim is to recreate the way our ears respond to loudness, however, this sort of detection will be a long way wide of the mark. Using peak level as a surrogate for loudness ignores the fact that two signals with the same peak level can sound wildly different to the human ear. It makes no distinction between sudden and sustained sounds, or between low and high‑frequency sounds.

A more sophisticated detector circuit thus measures the rolling average of the side‑chain signal level. This average is known as the ‘root mean square’ or RMS value, and although this still isn’t a perfect substitute for loudness as we perceive it, it’s usually better than the peak value. Some compressors offer the option to switch between RMS and peak detection; usually, RMS will give a smoother and more natural response, but peak will react faster and may be useful on very percussive sources, or when the aim is to ensure that a particular peak level can’t be exceeded.

Another core parameter called ratio tells the compressor by how much a signal that does exceed the threshold should be turned down. A ratio of 2:1 will see the signal turned down by 1dB for every 2dB that the side‑chain signal is above the threshold. Increase that to 4:1 and the signal will be turned down by 3dB for every 4dB above the threshold. Or to put it another way, in order for the output signal to rise by 1dB above the threshold, the input signal must be 2dB above for a 2:1 ratio and 4dB for a 4:1 ratio.

Attacking Instincts

Whether it uses peak or RMS detection, a compressor is of limited value if its reactions are always instantaneous. A device that turns the signal down immediately it exceeds the threshold can’t respond to dynamic variation on a larger scale. It can also cause distortion on low‑frequency sounds, by kicking in on every cycle and thus changing the waveform. Even if we can’t hear distortion, we may well be able to hear other artifacts.

For this reason, nearly all compressors have two additional parameters, known as their ‘time constants’. The attack control allows us to slow down the initial action of the compressor. Let’s say that the detector has determined that 4dB gain reduction is appropriate on a given peak. If we set the attack time to zero, this 4dB level drop will be applied straight away. If we set it to 100 milliseconds, gain reduction will be applied gradually, reaching 4dB only after that time. In human terms, it’s the difference between having a fader instantly jump to the ‑4dB point, and having it moved smoothly to that point.

The release time constant has a similar role at the other end of the process. With an immediate release, the compressor will stop acting the instant the input level drops below the threshold. A longer release time causes it to relax its attenuation more smoothly.

Target Practice

Together, these four key parameters provide a lot of flexibility for shaping the response of the compressor. Lowering the threshold will mean gain reduction takes place more often. It also means the signal is more likely to exceed the threshold by a greater margin, and will thus be turned down further. Increasing the ratio likewise means that signals exceeding the threshold will be turned down more. Longer time constants, meanwhile, tend to make the action of the compressor smoother, and more focused on long‑term dynamic variation rather than micro‑dynamics.

One important thing to note is that detection is a moving target. For example, let’s say we set the attack time to 100ms, and the ratio to 4:1. A peak that exceeds the threshold by 8dB will, in theory, trigger gain reduction that steadily increases to 6dB after 100ms. But if that peak is only 50ms in duration, the signal will fall below the threshold again before the attack phase is over. The detector will quickly ‘change its mind’ and switch the compressor into its release phase, so 6dB gain reduction will never be reached.

Another factor affecting the position of this moving target is the topology of the compressor: whether the side‑chain signal that feeds the detector is tapped before or after any gain reduction takes place. A feed‑forward compressor taps the side‑chain from the raw input signal, but in a feeback architecture, the side‑chain is derived from the processed signal. There are a few compressors that can be switched between the two modes; in general, a feed‑forward compressor has a snappier and more obvious action, but offers less precise control of level, while a feedback design is smoother and more accurate.

Time And Motion

If a compressor has the four key parameters described so far — and most do — that’s enough to make it a very flexible tool. Before we move on to consider some additional controls, let’s consider how these parameters might be used to achieve different results.

On a snare or kick drum track, we could choose a fast attack and a medium release, a high ratio and a fairly high threshold. Then, only the loudest hits would trigger compression, and would do so immediately: useful for levelling out an unevenly played part. By contrast, if we increase the attack time and lower the threshold, we ensure that all hits trigger compression, but only after the initial transient has passed. This has audibly different results. The sound of the drum will be changed, emphasising the attack of each hit and reducing the sustain. We might take this approach if, for example, the drum is ringing more than we want.

Now let’s think about the different ways a compressor placed across the entire mix bus might respond. By setting a very high threshold, the shortest possible attack and the highest possible ratio, we can approximate a special type of compressor called a limiter. In effect, we’re specifying a ‘ceiling’, a level above which no signal is allowed to creep. Any peaks that dare to show themselves above the threshold instantly trigger gain reduction, ensuring that this ceiling level is never breached.

We can achieve a completely different effect by setting a very low threshold, a very low ratio (perhaps as low as 1.1:1) and more moderate time constants. In this scenario, the detector is triggering compression most of the time, and the ‘moving target’ effect described above is continuously in action. The end result is a series of complex but subtle dynamic changes that we hear as a gentle smoothing‑out of the mix.

Filtering Down

Because the side‑chain signal that feeds the detector circuit is not actually in the audio path, processing can be applied to the side‑chain signal without directly affecting the main signal. This possibility is often exploited to apply equalisation or filtering to the side‑chain signal, weighting the response of the detector towards one area of the frequency spectrum. Probably the simplest and the most common application of this idea is to incorporate an optional high‑pass filter into the side‑chain.

The effect of this is to make the detector less sensitive to low frequencies, which often helps to align its action more closely with what we are hearing. Our ears are much more sensitive in the midrange than in the bass; so there are times when we want a compressor to even out the midrange, but it ‘hears’ the low end as being dominant and responds to that instead. That’s especially the case on complex material such as full mixes.

High‑pass filters are found in many compressors. Some also feature additional side‑chain EQ controls, such as the Thrust options on some API compressors. One common type of de‑esser is simply a compressor with an exaggerated treble boost in the side-chain, emphasising the bursts of high‑frequency noise that make up sibilance and triggering compression. Taking this further, some hardware compressors have an insert point that allows you to patch in the EQ of your choice.

Taking Sides

Most DAWs don’t allow one plug‑in to host another as an insert, so you can’t actually patch a software EQ into a software compressor’s side‑chain. However, many plug‑in compressors do have an external side‑chain input or key input which allows a signal of your choice to be fed to the side‑chain and used to trigger compression. If you want to experiment with side‑chain EQ, you can duplicate the source to a second track, equalise it and route it to this input. But it’s also possible to use the side‑chain input for other purposes. In particular, you can route a completely different signal to the side‑chain input, and this is the basis of numerous compression techniques.

This type of side‑chain compression can be used to create an effect that has been a staple of electronic music ever since French house became a thing. It’s achieved by placing a compressor across the master bus, with the side‑chain input fed from the kick drum track. Every time the kick drum hits, the entire mix is compressed, creating a ‘pumping’ motion. Other well‑known applications for side‑chain compression include ducking, whereby instrumental parts or reverbs and delays are ‘pushed’ into the background by a vocal, then allowed to swell again in the gaps.

Up, Up And Away

You’ll sometimes see compression described as a means of making things louder. On the face of it, this seems confusing: as we’ve seen, the fundamental job of a compressor is to turn things down when they get too loud! In fact, there’s no contradiction. What most compression settings do is to reduce the crest factor of the signal: by turning down the loudest peaks, they reduce the ratio between the peak level and the average level. If we then turn the compressed signal back up again so that it peaks at the same level as the uncompressed signal, the average level will now be greater, and hence it will sound louder. All compressors have the means of doing this built in, usually through a control labelled make‑up gain.

Knees Up

As obscurely named controls in music technology go, soft knee and hard knee are right up there. They take their name from a common graphical representation of compression, and relate to the behaviour of signals around the threshold level. In a hard‑knee compressor, the transition from doing nothing to full‑ratio compression takes place as soon as a peak reaches threshold level. In a soft‑knee design, the process is more gradual: compression at a low ratio begins to take place as the signal approaches threshold level, and the ratio steadily increases as it goes further above the threshold. (Dbx coined the alternative term ‘over easy’ to describe soft‑knee compression.)

In most cases, the action of hard‑knee compression is more assertive and less subtle than that of a soft‑knee model; soft‑knee compression is often preferable when we want to retain the illusion that no processing is taking place.

Parallel Lines

Newer compressor designs often now feature a wet/dry mix control. This allows you to implement a technique known as parallel or New York‑style compression, whereby a heavily compressed version of the source is blended with an uncompressed version. This modifies the sound in complex ways, with results that are audibly different from straightforward compression. For more detail, I’d suggest consulting Hugh Robjohns’ in‑depth article on the subject: www.soundonsound.com/techniques/parallel-compression. Alternatively, dive in and experiment! Be aware, though, that there is no universal standard for how the ‘wet’ and ‘dry’ sides of the signal should be balanced, so you may find that a setting which works for one compressor is completely different on another.

A wet/dry control isn’t essential for implementing parallel compression, however. You can achieve the same effect by placing a conventional compressor on an auxiliary channel and sending to it from the dry source channel. The relative levels of the faders on the source and auxiliary channel will then determine the balance of dry and compressed signal.

Compulsive Compression

Like other common processors such as reverb or equalisation, compressors sometimes offer an endless variety of additional controls on top of their core parameters. Sometimes you’ll also find familiar knobs bearing unfamiliar names. Most of this, however, is icing on the cake. Once you understand threshold, ratio, attack and release, and how they interact, you’ll be a long way towards being able to use compression to get the results you want.

So, my advice to anyone starting out is to focus on these core controls first — or rather, second. The first thing should always be to ask yourself why you’re using a compressor at all, and what result you’re aiming for. When we have unlimited plug‑in power at our disposal, it’s easy to get into the habit of applying compression purely for the sake of it. But, like any process, compression is a means to an end, not an end in itself. It’s much easier to choose the right settings when you know what you’re trying to achieve with them!

Check out this list of software and hardware compressors SOS has reviewed