First off, a compressor is sometimes a limiter, but a limiter is never a compressor. A dynamic range compressor is essentially a tool that takes any given signal and - when used correctly - can lower the volume peaks of a track but can also be used to make the softer parts loud, ultimately giving the entire track a more balanced volume output. Imagine if you have a song that starts off with some low volume vocals, such as a whisper or spoken word, and then sometime after that the song begins to reach its climax, complete with big guitars, loud drums and screaming vocals. Without a compressor, those quiet vocals might be completely muted by the time you adjust the volume of your louder segments. Conversely, if you were to raise the whispers loud enough to be heard, the initial louder parts will be ridiculously loud. Essentially, a compressor compresses the dynamic (volume) range of the track. A limiter on the other hand limits the amount of a signal passing through. Both use a user dialed in volume output cap (known as the threshold) but instead of taking the volume overage and compressing it, a limiter just completely removes it. One more time just to get this crucial point across; the main difference between the two is that a compressor squeezes down excess sound while a limiter completely removes it.
Also, compressor can be used as a sort of limiter as long as its ratio setting is sufficiently high. Let elaborate on that a bit. A compressor works through a few settings. You have the threshold which - usually expressed in decibels - acts like the cap. Any volume that goes above this threshold will get compressed. The amount of compression applied to the signal that goes above the threshold is based on the ratio. Pretty much, if you set this ratio pretty high, let’s say anything above 10:1, the compressor essentially becomes a limiter since is squeezes the signal so much that as far as our ears are concerned, the compressed signal is not there. Technically, it’s still there though, so it’s not exactly like a limiter. But even when a compressor acts like a limiter, they still sound a little different from each other.
Knowing When To Use Them
Depending on your situation, there might be certain moments when using a limiter might be the better choice, even if you plan on using the compressor as a limiter. It might be a bit tough to completely understand this without being able to actually hear what I am about to explain, but you can easily replicate this example on your own music editing program. Anyways, since we are still fresh from yesterday’s drum mixing article, let’s say you have a snare drum that peaks a bit too loud so you add some compression with a threshold of -10db and a ratio of 10:1, meaning that any sound above -10db will be flattened to 10% of its original strength. If you pay attention, you should be able to hear the actual compression squeezing down on the excess volume which comes off as a sort of slight sucking sound. Depending on how much excess sound is being compressed (along with the attack and release settings), this sound might be more or less pronounced, but either way, its not a very pleasant sound.
Now let’s say you're in that same situation - snare’s too loud - but you decide to use a limiter instead. The threshold will still be set at -10db so in the end we will end up with more or less the same end result in volume cut. Anyways, using a limiter instead of a compressor will let you avoid this annoying sucking sound. Not only that, you can actually up the gain on the snare for some tonal colorations. I know what you’re thinking: By raising the gain, aren’t we raising the volume, which is the opposite of what we’re trying to do? Kind of, but stay with me here. By raising the gain and then adding a limiter, you can achieve a very gritty rock snare sound but still achieve your desired output level. The limiter is still cutting everything above our -10db threshold, but we get to keep the added tonal changes of upping the gain. As a side note though, if you do happen to try this out on a snare or anything else for your own project, be aware that by raising the gain/volume of the entire track - even if you use a limiter to cut off the excess - the background noise will also be raised. This is where gates such as noise gates and high pass filters come in which are essentially limiters but work to cut off signals below a set threshold, such as where white noise resides, known as the noise floor. Just keep that in mind and don’t say I didn’t warn you.
So to recap, using either a limiter or compressor in this situation can still give you the same amount of output reduction but the limiter will ultimately sound cleaner since it completely removes the unwanted signal above the threshold, rather than just squishing it down. This was just one example of when a limiter might be the better choice but in the end, it comes down to how you want to shape your own project. Still though, its always good to know what options you have to play with. Anyways, both limiters and compressors are great tools that each serve a purpose in our quest for that perfect mix.