Compression is a tool that affects the dynamic range of a signal, be it a singular instrument or a group of instruments. Compression reduces the dynamic range so that the difference between the quietest and loudest sounds are less than it was before. Compression acts like an imaginary engineer with their fingers on a fader. When the sound is quieter, the engineer pushes the fader up so that the signal becomes louder. When the signal is loud, the engineer pulls the fader down so that the signal becomes quieter.
While there are a few different types of compressors, in this post I’d like to discuss the different parameters or controls of a basic compressor. Not all compressors you find will have all of these parameters, but they will include at least some of them.
The parameters are:
Before I get started with the different parameters, I’d like to first explain the concept of gain reduction. The whole goal of compression (unless you’re just feeding your signal through a compressor to color your sound with the tone of its tubes or something) is to get gain reduction. It is the inevitable result of compression. How much compression you are doing is determined by measuring how much of that signal has been reduced. On all compressors, you’ll find a “GR” (gain reduction) meter. This helps to show you how many dB of gain reduction is happening and thus, how much compression is happening. Going back to the imaginary engineer metaphor, gain reduction refers to how far the engineer pulls the fader down.
The threshold is the level at which the compressor starts compressing. When your level gets past a certain point, the compressor will kick in and begin to reduce the level. The threshold allows you to change how loud the signal gets before it starts being reduced by the compression.
When you see an option for “knee” on a compressor, the language used is usually choosing between a hard or a soft knee. It describes how gradual or drastic the transition is between the compressed and uncompressed sound is. A soft knee gives you a smoother transition between no reduction and reduction. A hard knee gives you a more dramatic transition.
The ratio knob on a compressor determines how much gain reduction will be applied once the signal passes the threshold. If your ratio is 1:1, the signal coming in will be the same going out. As you begin to increase the ratio, more and more compression is applied. If, for example, you set your compressor to be 4:1, the signal coming in has to be 4db above the threshold in order for the output to increase by 1db. Limiters, which are infinity:1, control the signal but not allowing any signal to increase by 1db. Therefore, the output signal will never be louder than the threshold.
Attack and Release
The attack time refers to the amount of time it takes for the signal to be compressed after it reaches the threshold. Have a very fast attack time often times will greatly affect transients (short-duration sound at the beginning of a waveform). It can result in a darker-sounding output. Having a slower attack time will not affect transients and will result in a brighter-sounding output.
The release time refers to the amount of time it takes for the compressed signal to get back to normal.
Because compression’s goal is to literally reduce the gain of certain signals relative the quieter ones, in order to get the level back to where it was, the make-up knob is there to level match. One method many engineers use is to switch between bypassing the compressor and activating the compressor to make sure the overall levels are similar. If the compressed signal is too quiet, they nudge up the make-up gain until it is strong enough to compare to the uncompressed signal.