After guiding you through the basics of compression a few issues back, it’s time to look at its mastering brother in arms, the limiter, and all of its subtleties

One subject that seems to cause more confusion than any other among beginners and experienced engineers alike is the real difference between compression (in its many forms and plugins) and limiting. Here, we’ll build on your knowledge of compression to explain what these subtly different tools are, how they work and how they can help you.

Limiters, levelling amps and expanders

The difference between a compressor and a limiter (sometimes known as a peak limiter) is essentially very small. Both operate in a similar way, reducing the volume of a signal when its level exceeds a given threshold. The usual rule of thumb is that if the ratio is 10:1 or higher, the unit is considered a limiter. However, there are a huge number of exceptions to this rule and you’ll find many compressors that offer higher ratios and limiters with lower ratios. A ‘brickwall’ limiter uses a ratio of ∞:1 in order to ensure that the signal never exceeds the threshold.

Occasionally you may also come across a levelling amplifier. This is really just an oldfashioned term for a compressor. It was most famously used on the legendary Teletronix LA-2A of the 1960s and crops up occasionally on new equipment. Thanks to the vintage link, it’s more closely associated with retro-sounding compressors that colour the sound of any signal they compress. However, this is only a very vague rule of thumb. In reality, the differences between compressors, limiters and levelling amplifiers are so small that they should really be considered different variations on the same basic idea. The good news is that this means the controls tend to work in pretty much the same way. If you know how the threshold, attack and release settings work on a compressor you’ll have no trouble with a limiter or levelling amp.