Compression can also be used to take an entire mix and reduce it to a maximum specific volume. This is called “limiting” because the maximum level is limited to a set threshold. There are software and hardware limiters that are designed specifically for for this purpose, but the software will use autmated settings. Automatically set limiting values make sense because limiting is easy to set up. But in this example, we will use the same compression software from the last example and set the limit values manually to illustrate what is happening.
In this case, we have used extreme settings to force the whole track into a set threshold. The results are very messy and distorted, because the overall track has completely lost its dynamic range so that all the tracks are squashed into the same place. Extreme limiting can be a good effect to use if you want to insert song track but give it more color and grittiness. Here are the settings used above:
The threshold value in a limiting scenario is often set to just below 0 (zero) in order to limit any peaks that rise above the reference level of 0db. The result of this effect ought to be imperceptible if done right. Only when limiting was needed and not applied will you hear the effects, usually resulting in analogue distortion or digital artifacts depending on the mastering source. However, in order to demonstrate the effects of hard limiting on already loud material, I used a relatively extreme level of -20db, just to show how the compression is effecting the sound. In this case, everything above -20db is being effected (which should be most of the sound).
The ratio setting for limiting should be at maximum. This high setting determines that all sounds above the threshold (in this case -20db) should go no further. Obviously, the Waves Renaissance Compressor I am using does not go up to a ratio of infinity, so 50db will have to do.
The attack for peak limiting should usually be reduced to the lowest level because the point of “hard” limiting is to let nothing through the set threshold. Applying any amount of attack means you are allowing some of the signal to escape before the compression is applied, and this just would not make sense in most limiting scenarios.
Because we are limiting the master track output, the gain is going to effect the overall output of the whole mix. Therefore, I recommend watching yout output levels on all visual feedback as you set the final output gain on a limiter. A rule of thumb is as close to 0db as you can get without going over (this shoul dbe obvious given it is the point of setting the limiting compressor in the first place), but many mixing engineers will swear that the most important factor is how it sounds. I would just caution that going over 0db can result in really awful distortion when coming from digital applications, so be mindful of that.
As mentioned above, limiting or “hard limiting” in compression is relatively unsexy work, but can be especially helpful when it comes to mastering. In order to ensure that track are not going to distort or be too loud when played on other equipment, it can be a good idea to run them through hard limiting. However, as shown in the video above, improperly using limiting can result in undesireable effects and can totally ruin a good mix.