If you have a working knowledge of expansion (covered later, don’t worry), you’d be forgiven for scratching your head. Surely this must be expansion, right? After all, you’re pushing a signal up, not pulling it down.
To that point, I’d reply that you must take into account the governing context of the signal as a whole: In upward compression, the overall dynamic range of the material is still being reduced, not expanded. Therein lies the difference. Yes, the lower level is now louder, but the higher level remains the same, and the net effect? The dynamic range has been compacted—just like in regular old compression.
I like to use an upward compressor on any room-based track that isn’t quite roomy enough. Let’s take the room mics of a drum set: in the studio, perhaps the drum booth was rather small, resulting in a closed-in sound. Using an upward compressor in this scenario can allow you to bring out the ambiance—the space between the transients—and get a roomier feel in your track.
You might be saying, why not use a transient-shaper for this? You sure could—but they aren’t very transparent as a rule; they can often add distortion. Sometimes I love that distortion. But sometimes it’s uncalled for, and that’s when an upward compressor is your friend.
You can also use an upward compressor to create a room-mic sound when no room mics were actually used in the session. In fact, recently I did this on a mix, turning a spare tom mic into ambiance.
Let me explain: I was mixing a live track for the indie band Leland Sundries, and I wanted more room tone than was provided in the recording. I didn’t like what artificial reverbs were giving me, as this was a tightly miked, live-mix scenario. Luckily for me, the rack tom on this tune was never played in the song—not even once! Instead, the mic had picked up a weirdly balanced picture of the whole kit: kick, snare, hat, cymbals, and floor tom were all represented in excellent proportions.
But it wasn’t roomy sounding. Squashing the track with a downward compressor wouldn’t have worked, because that would’ve emphasized the transients, and I was going for the space between the transients. With an upward compressor, I was able to bring out the ring of the kick and snare, turn up the splash of the cymbals, and emphasize other room-based reverberations.
If you want to try this for yourself, it’s possible with many popular software compressors. The compressor in Neutron 2 can be toggled into upward mode when you use negative ratios.
Other plug-in manufacturers supply compressors that work in an upward fashion too; if you see a range value in your compressor, and the range control can be oriented in both a positive and negative directions around the zero point, then chances are the processor can provide this variant on compression.
Just make sure, when experimenting, to pay extra-careful attention to your threshold, attack, and release parameters, and make sure to use very low ratios (otherwise it’s hard to control). To successfully pull off upward compression requires an exacting touch, because the way transients react to this process is somewhat counterintuitive—i.e., you’ll probably want a faster attack than you’d think, and the release will behave differently as well. I tend to use fast attack values and fast to medium fast release settings when utilizing upward compression.
In drum room-mic scenarios, multiband upward compression can also be your friend, allowing you to avoid affecting the cymbals too much, and letting you dig into the snare and toms. Your mileage may vary though—in fact, you might find yourself with no upward compressor in your arsenal at all...
...Or so you would think, if you didn’t read this tutorial!
It turns out that you can use parallel compression to approximate much of what upward compression can give you. It works as follows:
Send an instrument or stereo bus to a compressor on aux track. Set the threshold very low, so that the compressor is constantly working, giving you gain-reduction on soft passages and loud passages alike. Counter to how you might use a compressor on the track itself, you’d employ very fast attack times, and, if you’re like me—that is, generally preferring faster releases for musical pump—you might want to employ slower releases than you’d normally use; the key here is to juice the release settings so they won’t introduce distortion.
These settings, auditioned in solo, might not sound very musical, but when you blend the parallel compression back in with the unaffected signal, you’ll start to feel how it works: because the compressor is kicked into high gear on loud moments, you won’t really notice the parallel track when the music is really hot; however, when the music dips down in level, the parallel track will be more audible, and bolster the overall level of the material. The result? Lower levels have been brought up, almost as they are in upward compression.
2. Upward Expansion
Upward expansion is akin to regular old compression in a key way: it affects a louder signal as it crosses the threshold. However, instead of clamping down on the signal, the expander pushes the sound up in level. Here is where the process earns its name—it expands the overall dynamic range of the signal.