Compression is a powerful tool, not only for controlling the dynamic range of signals so our mixes stay more consistent, but also for shaping the parts of the sounds that we’re given. Some of these topics are a step beyond the basics, so if you need a refresher on the fundamentals of what compression is doing, check out my previous articles here (part 1) and here (part 2) on ProSoundWeb.

Compression is a way to turn down the signal automatically after it reaches a certain level, so we have control of its dynamic range. But if we utilize the compressor’s time-based controls – the attack and release times – we can actually change the envelope of the sound, or the balance between the different micro-dynamic characteristics of a sound source, so we can fit it in our mix better.

The Envelope, Please…

We can describe the way sound sources start, sustain and stop in the time and level domain with four sections: attack, decay, sustain, and release. It’s called the sound’s “envelope.” Understanding it is easier with an example.

Let’s look at a piano. When a key is pressed, the hammer strikes a string. This percussive sound is the attack, or how quickly the initial sound rises. The decay is next, but we’ll circle back to that. The sustain is how long the note rings out while being “held.”

If we keep the piano key pressed down, the damper stays raised above the strings, allowing them to continue vibrating. The transition between the initial attack and the sustain is called the decay. On a piano, the decay can be described as pretty quick. After the sustain, we have the release, or the way the sound stops.

If we gently lift our finger off the piano key, the damper comes down slowly, causing the string to stop vibrating slowly. If we lifted our finger suddenly, the damper would abruptly stop the string from vibrating, for the most part.

The quick attack and decay are called the “transient” of the sound. Some sounds that are mostly transients are percussive, like hand claps. There’s almost no sustain or release for a clap. Drums have a high transient component, but they can ring and sustain if tuned in a particular way. The piano has a transient component in the attack, as does a strummed acoustic guitar.

If we look at a gently bowed violin, the amplitude increases relatively slowly, so we’d say that the sound has a slow attack. Since its level doesn’t drop after the attack, we’d say it has no decay, and only sustain.

Confusing The Terms

Now let’s go back to a compressor’s controls and address one point of confusion along the way. The release is one term that’s used both in the envelope and on the compressor’s controls. The release time on the compressor is how quickly or slowly the compressor’s gain reduction responds to decreased input level. It either says, “When I’m supposed to have less gain reduction, I’m going there quickly,” or “I’m going there slowly.”

The initial part of the sound is called the “attack,” and one of our compressor’s time controls is called “attack.” But they’re not the same thing. The attack time on the compressor is how quickly or slowly the gain reduction responds to increases in the input signal. The compressor’s attack is describing how it responds, not how it is in the first place. To help keep things clearer, I try to use “transient” in place of “attack” when describing the fast start of a sound envelope related to compression. Hopefully this helps you too.

Peak Versus RMS

Another control on the compressor that often goes unnoticed or misunderstood is the Peak versus RMS setting, so let’s explore that for a minute. Sound waves and audio signals swing back and forth from their resting state at 0, so taking an average of those signal levels would yield an average of… 0. So, some really smart math people came up with a way to take a type of average of an oscillating signal and created Root Means Squared or RMS. Short story: RMS is average for audio signals.

Peak measurements take into account the very highest and lowest signal level (because they’re oscillating, remember?). A signal with high peak energy but low RMS energy would be a transient, like drums or percussion. A signal with low peak energy compared to the RMS energy would be something like a string or synth pad.

For a compressor, Peak or RMS mode has to do with how the detector circuit measures the input level to apply the ratio. It will subsequently affect how the attack and release time behave.

On a piano, the hammer hits produce higher peak energy than RMS energy. If we put a compressor in Peak mode, its detector will pay more attention to those hammer hits, given that our attack time is fast enough. If we’re in RMS mode on the compressor, the detection will focus more on the average levels and sustain of the piano to apply the gain reduction. If we use RMS mode on a signal that has primarily peak levels, like drums, it may not give us quite the control we want with the attack and release times.

Changing The Envelope

Armed with compressor and adjustable attack and release times, we can change the balance of our sound source’s envelope. We’ll stick with the piano for the example.

Imagine our piano is in a very dense mix with layers of distorted, sustaining electric guitars and big drums. We’re having trouble getting the piano to cut through the mix without all the sustaining overtones competing with the electric guitars and the vocals. So, what can we do? Use a compressor with a slow attack time to accent the transient part of the piano sound so it can “pop out” in the mix briefly to catch our ear without having the sustaining overtones vying for sonic space.

If you want to try this on your own, set your compressor’s ratio to 3:1 or 4:1, and pre-set the attack time on the compressor to 20 milliseconds (ms). If possible, choose “peak” mode rather than RMS. The appropriate release time will vary with the speed of the notes played, meaning the more time between hits on the piano, the longer the release time will need to be. Start with 350 ms and go from there.

Now, lower the threshold until you’re getting about 5 to 8 dB of gain reduction and listen to how the balance of the transients and sustain changes. Take the attack time to 10 ms and listen to the change, and then all the way down as fast as it will go. You should be able to hear how the initial hammer hit changes with the attack time. You may need to adjust the threshold to keep the compressor from sounding strange while adjusting the attack time. Now listening in context, you should hear the hammer hits pop out of the dense mix, while the sustain of the piano is lower in the overall balance.

Let’s take the same piano sound and imagine a different context. Now we have a sparser arrangement with more acoustic instruments, such as mandolins, acoustic guitars and even a banjo. The song is a ballad, and the musical arrangement has more whole-note hits, where there’s a lot of space between notes. The other stringed instruments have little sustain, but the piano could fill up that space in the mix if we use the compressor the right way.

We begin the same way as before, but this time set the attack time to around 6 ms and switch the mode to RMS (if you can). Fine-tune the release time and the threshold to push the decaying signal back up as the gain reduction goes down.

Start with the same 350 ms release time, but then adjust it faster to see how that goes. You’ll likely find that the initial hit of the piano pushes the compression gain reduction down, and then it quickly bounces back up. This will push up the sustain relative to the initial transient, but it won’t push up the tail-end of the piano note.

Now slow down the release time until the gain reduction just about comes to 0 dB by the time the next hit comes. If you take it too slow, you’ll have a similar result to having the release time too fast – the sustain of the piano doesn’t get boosted at the end.

I’ll leave you with this parting thought. Great mixes are a combination of contrasting types of sounds – loud and quiet, bright and dark, as well as instruments that are percussive and others that sustain. Using compression this way can help bring excitement and contrast to the dynamics of your mixes.

There are a lot of other ways to apply this technique with the compressor, so get creative to shape the tones you’re given to produce exciting textures and mixes.