The Audio Compressor: Breaking Down The Parameters

An Avalon AD2044 audio compressor and a Neve 33609/J audio compressor in a rack mount

When it comes to mixing and mastering, the ability to use an audio compressor effectively is like the ability to walk. It’s just something you are going to have to do all the time. Many of us have a good grasp of what an audio compressor is for and why we should use one, even if we don’t fully understand everything that is going on. For those of you who are still a bit lost or have no idea at all, this article will act as compression parameters 101. Let’s jump straight in!

Audio Compressor Step 1: Threshold

Probably one of the most important parameters of any audio compressor. The threshold is used to determine the point at which compression is engaged. For example, let’s say we have our threshold set to -12dB. This means that any signal that exceeds -12dB will be affected by our compressor. Equally, any signal that is under -12dB will be deemed too quiet and thus not affected.

As a rule, setting your threshold first is always a good place to start. Regardless of the other parameters, setting your threshold to a point that provides the beginnings of the sound you’re looking for is a great starting point. Having your ratio and attack & release settings tuned to perfection but your threshold nowhere near the signal level is practically useless.

Audio compressor threshold with various ratios

Step 2: Setting your Ratio

Now you’ve got your threshold in roughly the right place, you can start to adjust the ratio of your audio compressor. Put simply, the ratio is the multiple at which signal is reduced by once it passes the threshold. With our threshold set to -12dB and with a ratio of 3:1, a signal that comes in at 0dB will be compressed to -8dB. Still following me? If not, here’s the equation.

Output dB=((Input dB-Threshold)/Ratio)+Threshold dB

So in our previous equation, we are calculating output dB by taking our 0dB input and subtracting our -12dB threshold. This provides us with the number 12. We then divide 12 by 3 which yields 4 and finally add our threshold dB back in. This shows us that a signal that started at 0dB will now be output at -8dB.

The more we increase our ratio, the more aggressive our compression will become. If you’re looking to keep things more gentle and musical, you’ll probably want to keep your ratio down. If you’re looking to get aggressive and punchy, try cranking it up. Some audio compressors won’t let you go above a certain figure (due to their circuit type design) whereas others can go as far as ∞:1. This would be otherwise known as a limiter as with an infinite to 1 ratio, all signals above the threshold would be squashed to that level.

Step 3: Attack & Release

Next, we need to address the attack and release times. This isn’t always something you will have to do mind you. Audio compressors like the classic DBX160 have fixed AR times, implemented to achieve a specific sound. However, more often than not you will have some form of AR to play around with. You can control the speed at which your compression sets in and how quickly it stops by adjusting these two knobs. Let’s look at a couple of typical examples for varying attack and release times.

Fast Attack & Release

Big sounding kick drums offer up huge transients and a lot of punch. As such, we can often end up with a flabby sound that needs to be tightened up in order to cut through a mix. In this case, a fast attack time is going to trigger our audio compressor almost as soon as the signal passes through it. The result will be a strong and loud initial punch with a quick drop in dynamic range for the remainder of the hit. This is how we get it to really cut through and tighten up.

With our fast attack, we would also look to have a relatively fast release. As kick drum sounds tend to last milliseconds, we don’t want our compressor to still be acting by the time it reaches the next hit. This would result in an inconsistent sound and weak kick drum hits. Equally, we don’t want it to act too quickly or we will end up with a pulsing sound as the kick drum gets quieter and louder again in a short space of time.

Slower Attack & Release

So what if you’re dealing with slower and more musical sounds? Vocals tend to be more flowing and need shaping far less than transient heavy signals. In this case, we would be looking to tame a few louder areas and increase a few quieter areas to provide a more consistent performance. Medium to slow attack times will allow the compressor to flow more with the vocalist without creating sudden dynamic changes. Equally, medium to slow release times offer a more elegant transition in dynamics, especially with long notes. Your best bet here is to experiment and find a form of AR shaping that suits your needs.

Step 4: Now For Some Make-Up Gain

As you may have guessed, the make-up gain is used to add gain back into your now compressed signal. Simply use this knob to dial in some gain and bring your signal back up in the mix. The key thing to remember here is that you won’t necessarily need to increase the gain the same amount as you have reduced it. By compressing a signal, you have reduced the dynamic range of the said signal. This means that the louder parts are quieter and perceptually, the quieter parts appear louder. As you increase the make-up gain, not only do you turn the compressed sections back up but you also increase the volume of the unaffected signal. As such, if you increase the gain by the same amount you compressed, your uncompressed signal is going to be louder than it was before compression.

An audio unit gain knob

By using make-up gain properly, it can also provide a great way to A/B the effect in context. As the uncompressed and compressed signal will be almost the same average level, you can flick the audio compressor on and off repeatedly to hear what is it is actually doing. This way, you can decide if you have achieved the desired effect and then move on.

Other Parameters

Knee

Another parameter you might see on your compressor plugins or hardware is the knee. Much like our attack & release, this parameter doesn’t always appear as certain compressors are designed to either be hard or soft knee. If your compressor does have this parameter, it’s something you can look to adjust when you’re setting the ratio.

The knee can be thought of in two ways. Hard knee means that as the signal passes the threshold, it is instantly affected by the ratio we have set. Soft knee effectively applies an element of smoothing to this transition. It allows the gradual introduction of compression to signals that pass the threshold. Hard knees are good for when you want aggressive and sudden compression. Soft knees typically suit more musical signals. However, a soft knee can be advantageous even with heavy transient compression if you are yielding too much of a clicky sound.

Sidechain & Sidechain Listen

Many compressors have a sidechain section built in these days as it’s become such a useful production tool. This section allows us to set up a trigger for our compression other than the incoming signal. As such, we can use compression to make space in a signal for another. Many sidechain features are also paired with a ‘sidechain listen’ section that comprises two filters. These high-pass and low-pass filters let us zone in on a specific frequency within a signal. An example of this would be focusing in on the kick drum in a drum loop. We could use its rhythm to create space in another sound. For tips on using sidechain compression, check out my article on sidechain techniques.

TL;DR

Audio compressors may seem daunting but getting to grips with the controls makes them nothing more than a fluffy little kitten. Implement the parameters in the correct order and pay close attention to what each of them does. Remember: threshold; ratio; attack & release; make-up gain. Follow this guide and think about each step logically and you’re bound to end up with a much better mix.


Feature image provided under creative commons attribution 2.0 by Justin De La Ornellas.

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