The traditional purpose for a limiter is to catch stray peaks jumping out, preventing distortion or clipping when raising the mix to a more suitable loudness level. This is why you'll sometimes hear them being referred to as peak limiters. In more recent times, the limiter's purpose has evolved somewhat. Rather than catching the odd stray peak, they are more often used as a solid ceiling to force the mix up against, gaining high levels of RMS. The whole subject of loudness and RMS energy ties in very well with the limiter.

Limiters are mainly found to be the last process in the master chain, as once you have used a limiter to full effect, the audio's condition is such that any further processing will not blend as well as when applying the same process earlier in the chain. In fact, further processing after a limiter can either harm the mix or un-do some of the earlier processing - you will soon understand why. 

A limiter works in a very similar way to a compressor only much simpler. They typical have three parameters, threshold (dB), release (ms) and output/ceiling (dB). Just like the compressor, the limiter will attenuate the signal level at the point of it passing the threshold, only the amount of attenuation isn't dependent on the ratio as with a compressor. It simply limits the signal level from reaching any higher than the threshold, hence the name limiter. For this reason, there can be no adjustment of attack, the attack time is quite simply instantaneous. The release can be adjusted and, apart from the threshold, is the only way to change the way the limiter behaves and sounds.

Setting the limiter's release is similar to that of the compressor in the sense that too long will give a lagging behind feeling and a significant drop in perceived volume. However, fairly short release times are more acceptable as this is what the limiter is designed to do. So to get the limiter working in a transparent way, we don't necessarily have to have the limiter moving with the rhythm as with the compressor. The instant attack and a fast enough release (so that we don't feel it lagging behind) will provide a suitably transparent way to obtain more RMS energy. As with the compressor, there will be a sweet spot for the limiter's release – too quick will sound edgy, too slow will sound squashed or suffocated.

Warning - the instant attack will impact on the transient hits so go easy.

Limiters provide more RMS in the same way as compression - the reduced peak levels create more headroom above the peaks allowing there to be an overall gain without the risk of hitting the top and causing clipping. In most cases, the limiter performs the gain-increase automatically - however low you pull down the threshold, the limiter will increase the output gain by the same amount, so the output will always be the at the top of the ceiling.

Similar to most compression techniques, we usually want the limiting to be transparent, although as a limiter increases the RMS level of the mix, you can appreciate that the limiter can give a certain amount of fattening too, which is sometimes desired.

There's a fine line between pushing your limiter a little harder to achieve the fattening effect and over-cooking.

A limiter acts like a brick wall at the top stopping any stray peaks from triggering the reds, and with the right release setting, can do this in a transparent way. So much so that many peaks can be forced over the threshold to obtain the desired increase in RMS level, which in turn gives us the perception of increased loudness and fatness. You can generally achieve more RMS with a limiter than you can with a compressor.

As with the compressor, it is important to understand that as a peak is reduced, for that very instance in time, the whole signal's level is reduced, not just the tips of single waves. When a limiter is pushed too far, RMS heavy elements, like a kick drum, can sound as though they are literally punching a hole through the mix, due to such a severe drop in level at that very instant. This relates back to the compression technique where the compressor's reaction to the kick drum can be of help when translating its weight and solidity to different systems. In the case of the limiter reacting in the same way, the effect is not quite so pleasant because of the enormous attenuation and aggressive attack, and therefore should not be used for this purpose (as long as you have your compressor setup correctly, there should be no need to use a limiter for this effect, or risk of it happening unintentionally).

David Eley - TGM Audio

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