By Doug Olson
Many of our recording school students have a passing familiarity with dynamics processors such as compressors and gates. These are mainstays of the recording engineer’s toolkit. The idea most people have is that a compressor’s job is to take the loud parts of a recording and turn them down, while leaving the quiet parts alone. This is basically true – a compressor can be used to make a track’s level more even. Gates, on the other hand, serve to turn down the unwanted quiet parts, such as cymbal sound between snare drum hits, while leaving the loud parts alone. Again, this is a good basic description of what a gate – technically a “downward expander” – does.
But there is a lot more to it than that. To tell the truth, the subject of dynamics processing could probably fill an entire book, so for today I’d just like to focus in on one of it’s less well-known aspects: upward expansion.
To illustrate the concept, I’ll use a Pro Tools plug-in that I think is an amazing piece of recording technology – McDSP’s ML4000. It’s a multiband compressor, limiter, expander and gate.
I’m going to play around with a short recording of a drum kit. Here it is, unprocessed:
Unprocessed Drums Unprocessed Drums
Now, let’s use the ML4000 to compress it:
I’m only using the compressor here, and I’ve adjusted the thresholds for each band to give about 6 dBs of gain reduction. To give the drums a little extra “pop, ” I’ve set the attack time to 15 milliseconds. This is slow enough to let the drum transients through, but fast enough to still grab the tail of each drum hit. The release is 100 milliseconds, which is fast enough to allow the compressor to recover between hits. The compressor is letting transients poke through, then grabbing the loud part of the tail of each hit and turning it down, then letting the volume recover before the next hit. This creates a bit of a bounce, and “glues” the drums together a bit. Here’s what it sounds like:
Now, many people are familiar with gates, which are a special case of the more general idea of dynamic range expansion. Expansion could be described as the opposite of compression – rather than reducing the difference between the loud and soft parts of a sound, expansion increases it. A gate takes this concept to the extreme by eliminating part of the sound, such as unwanted high hat bleed. There’s no need to stop there, though. Upward expansion is a way to use an expander to do something more like what a compressor does. But where a compressor detects the loud parts and turns them down, an upward expander detects the quiet parts and turns them up.
Here are the ML4000 settings I used to create an upward expansion effect:
In this case the expander turns up the level as the sound falls below the threshold. The settings I used in this case are a little less subtle than they were in the last example. Check out how much the sound in between the drum hits is brought out:
The ML4000 lets you combine both compression and expansion (and gating and limiting too, if you want).
It sounds like this:
You might feel like this is an interesting sound, but the high hat cymbal is getting too intense. One of the cool things about using a multiband processor like the ML4000 is that you can adjust the settings for each frequency band independently, so as to make its effect more or less pronounced across the frequency spectrum. Here I’ve tweaked the settings to keep the compressed drum sound but make the high hat a little less pronounced:
I really love the level of control that is possible with the ML4000. It is complicated, and takes some practice to learn, but I find it to be one of the most powerful tools available to the sound engineer. So powerful, in fact, that you have to be careful – you can do a lot of damage with it if you don’t know what you’re doing. But hey – that’s what audio engineering school is for!