The knowledge: compression techniques

Compressor controls explained

Like manual methods, a compressor also manages dynamics by reducing level, but it does it automatically, depending on the settings you choose. To help understand how this works, let’s keep the car suspension analogy in mind. The suspension is a compressor, of sorts, controlling the ‘dynamics’ of the road’s surface – the bumps, peaks and troughs.

Smooth it over

The overall smoothness of the ride is determined by the stiffness of the springs in the suspension. If you lean on the car, how easily does it sink down to absorb the weight? In a compressor, this overall ‘softness’ is determined by the ratio.

A lower ratio gives you a smoother ride; most family cars have their suspension set up this way, but there’s more movement in the car and less control for aggressive driving. Higher ratios are analogous to a sports suspension in a car, where the smoothness of the ride is less important than control.

Next up is the speed with which the suspension reacts to bumps in the road. In a compressor, the attack and release times determine how quickly a compressor reacts to the input signal (attack) and how quickly it relaxes afterwards (release). The attack time needs to suit the material. Imagine a car hitting a speed bump too fast – the suspension can’t react fast enough to smooth out the thump. The same thing happens with audio if the attack time of a compressor is too slow.

The final crucial setting is the threshold. The threshold determines when a compressor starts working, depending on the input signal.

Make-up gain

One setting on a compressor that doesn’t fit the car analogy is make-up gain. Compressors control the dynamics of the input signal and usually reduce its dynamic range. Loosely speaking, they turn the louder stuff down.

This means that when you first patch an analogue compressor in, it will probably make the overall signal sound quieter. To avoid having to keep pushing the fader up to compensate, most compressors allow you to add make-up gain. This is just a way of lifting the signal back up to balance the reduction in the dynamic range caused by the compressor.

You may not have seen this control if you use a compressor plugin, though – many digital compressors now include automatic make-up gain as standard – so the effect you hear as you dial in more compression is simply that the quieter signals get louder, with the peaks staying roughly the same. This can make using them quicker and easier, but it can also lead to them being over-used.

Our ears tend to assume that anything louder sounds better. With automatic make-up gain, we keep piling on more compression, thinking it sounds better, and being distracted from the negative aspects: reduced dynamic range, pumping and even unnecessary distortion.

Gain reduction

One way to avoid this is to keep an eye on the compressor’s metering: the overall gain reduction. How much gain reduction is needed is dependent on what you’re recording; vocals and bass may need heavy compression while keyboard sounds and strings hardly need any.

As a rule of thumb, though, pay close attention if you start to see more than 8-10dB gain reduction. If you wind the threshold control down further and don’t hear any real changes, you should probably ease it off and try some different settings.

Knee

The knee of a compressor refers to when and how the ratio starts to change when the compressor starts to take effect. A ‘hard knee’ means the compression becomes immediately active as soon as the input signal hits the threshold, whereas a ‘soft knee’ means the compression becomes audible more gradually. A soft knee also means gentle compression starts happening further below the threshold.

If a car’s suspension bushes start to wear out, you’ll feel something like a ‘hard knee’ effect. Normally they smooth out the smaller bumps that aren’t big enough to involve the main springs. As they become worn, however, they don’t do this as effectively and the action of the suspension becomes more abrupt.

“How much gain reduction is needed depends on what you’re recording”