May 19th, 2020

Demystifying Compression: An Exposé on the Secret Smurf Army Inside Your DAW

Many aspiring Diamond Ears are asking, 'What exactly is a compressor and what do all those controls mean?' As a sound educator, I get this question often and the folks over at SoundGym invited me to share the answer.

The first thing you need to understand is that sound people are pretty lazy. Not lazy in the sense that we don't like to work. Rather, we will work incredibly hard to create ways to avoid having to work hard at things we don't like.

Case in point: we created an entirely new system of measurement (dB) that is derived using logarithms and exponentials in order to avoid having to do simple addition and subtraction on numbers that are greater than three digits. Yes, ours is a very special compulsion. I once spent two months writing a computer program that could remote control a reverb processor so that I could avoid a single 30-second walk up a flight of stairs.

A compressor is the brainchild of sound people from a previous generation who worked really hard to create a tool so they wouldn't have to work so hard at riding the faders on their mixing console.

A Compressor is Simply an Automatic Volume Control

In order to understand a compressor, you need to realize that it's not as complicated as you think. A compressor is nothing more than automatic volume control.

I like to think that there's a little army of sound Smurfs inside the box that do all that work for me that I don't want to do. I even found an actual sound engineer Smurf figurine at a souvenir shop in Pigeon Forge, Tennessee several years ago. I've included a picture of him sitting on my desk with his headphones and boom mic ready to be put to work.

For a compressor, the Smurf is sitting inside the box listening to the sound through his headphones with his tiny hand on a tiny fader. His prime directive? If it gets too loud, turn it down. Naturally, the Smurf has a few questions:

How Loud is Too Loud?

The term 'too loud' is awfully subjective. You need to tell the Smurf in no uncertain terms, what you consider to be too loud. This information is communicated to the Smurf through the threshold control. On a standard compressor the threshold is simply a line that you draw somewhere on the audiometer.

Any sound that is louder than that threshold line is too loud. Any sound that is quieter than that threshold line is acceptable. In the included screenshot of a compressor plugin, the threshold is set at -20dB. I'm telling the Smurf that anything louder than -20dB is too loud.

When it gets too loud, how much should I turn it down?

Now that the Smurf knows when something gets too loud, you need to tell him how much to turn it down. The answer, of course, is 'it depends'. You need to give the Smurf a simple rule to follow that allows him to turn it down relative to how loud it gets.

This information is communicated using the ratio control. The ratio is expressed in a pair of numbers. For example, a ratio could be 2 to 1, or 4 to 1, or 8 to 1, or even ∞ to 1. The idea here is that the 1 represents how much the sound went above the threshold. The Smurf then takes the larger number and uses it to divide that value. Here are some examples:

1:2 Compression Ratio:

The sound gets turned down to half the amount that it exceeded the threshold. So, if the sound went 10dB above the threshold, the Smurf will turn it down -5dB.

1:4 Compression Ratio:

The sound gets turned down to a quarter the amount that it exceeded the threshold. So, if the sound went 10dB above the threshold, the Smurf will turn it down -7.5dB.

1:8 Compression Ratio:

The sound gets turned down to an eighth the amount that it exceeded the threshold. So, if the sound went 16dB above the threshold, the Smurf will turn it down -14dB.

One thing to notice is that in all three of these examples, the sound still goes louder than the threshold, just not as much. If you never want the sound to go above the threshold, you'll need to set the ratio to = 1.

This means the Smurf will divide the amount that exceeded the threshold by infinity. In other words, he'll turn it down by whatever amount he needs to in order for it to be no louder than the threshold. This is what we call a limiter. A limiter is nothing more than a compressor with a ratio of 1.

Another thing to notice is that if you put in a ratio of 1 to 1, that effectively bypasses the compressor because the Smurf will not turn it down at all.

How fast do you want me to turn it down?

Once the Smurf has recognized that something is too loud, and has figured out how much to turn it down, how quickly should that fade go?

The Smurf can do it as fast as you want. This is communicated with the attack control. An attack of 0mS means the Smurf completes the fade instantly. An attack of 10mS means the Smurf will fade it down slowly over 10mS. Deciding the attack value depends on the sound that you're trying to compress.

With a transient sound like percussion, a long attack might mean that the sound never gets turned down fully before the hit is over. With a smoother sound like a vocal, a short attack might sound like you robbed the performance of all its excitement.

When it gets quiet again, how fast do you want me to turn it back up?

Eventually the incoming sound will drop below the threshold again and if the Smurf has the fader down, you're going to want him to turn it back up so you don't lose the quiet stuff. You need to tell him how fast to do that. This is communicated through the release control.

A release time of 0mS would mean that he would instantly pop the fader back up for the quiet stuff. A release time of 100mS means that he would slowly restore the fader level over a period of 100mS. If your release is too short, you'll hear the tails of the loud stuff pop in unnaturally. If your release is too long, the Smurf might not get the fader up there before something gets loud again.

Deciding how long to set your attack and release depends entirely on the kind of sound you're compressing. There's no rule about that. Just listen and adjust both until they sound natural. Generally speaking, start with shorter attacks and longer releases and then tweak from there.

Cleaning things up

After the Smurf has done all that work turning loud things down, the whole track is probably going to sound quieter now. Most compressors have a makeup gain control to compensate for this overall reduction.

This is something you have to do yourself. The Smurf is too busy turning things down. So just use that makeup gain to take your now less-dynamic sound and turn it up to a level that sits well in your mix.

Deploying your army

Now that you know how to teach the Smurf to ride the fader for you, you can deploy a whole army of Smurfs to ride all the faders! Every time you put a compressor on a channel, the Smurf will take over that tedious work for you.

The Smurf's skills are not limited to just riding faders though. You can employ them to do all kinds of undesirable tasks for you. Maybe you'd like a Smurf to automatically control a mute button for you? Just put him inside a gate plugin and he'll pop that mute button on and off for the rest of his life. Maybe you'd like a Smurf to EQ your sound for you as the frequency response changes? Just put a whole family of them inside a multi-band compressor or a dynamic EQ and each of them will take over a volume control for a specific frequency and ride that particular fader forever.

There's no end to the amount of sound work you can avoid doing by getting a Smurf to do it for you. The Audio Ear Training Games on SoundGym is a great way to learn how to teach your Smurf to take over the tedious work of riding the fader.

Jason Romney is a Professor and Director of the Sound Design program in the School of Design and Production at the University of North Carolina School of the Arts. He has designed sound at theatres all over the USA. He is a member of the Theatrical Sound Designers and Composers Association and a Vice-Commissioner for Sound with the United States Institute for Theatre Technology. His book, Digital Sound and Music, co-authored with Jennifer Burg and Eric Schwartz can be found online at digitalsoundandmusic.com or in print from Franklin Beedle & Associates. Last week, after over a year of practicing, he finally achieved Diamond Ears status on SoundGym!

 


Comments:


profile
Alex Voloshyn
Jan 18, 2023
In 'If you never want the sound to go above the threshold, you'll need to set the ratio to = 1' typo I guess - '...set ratio to ∞ to 1'.
@Cuantas Vacas I'm also concerned about Smurfs human rights here, 'and he'll pop that mute button on and off for the rest of his life' so sad..
profile
Cuantas Vacas
Jan 18, 2023
This is apology of plain slavery. Gargamel would be proud!
profile
Wolfgang Eschenfelder
Jan 18, 2023
This is one of the best and simplest explanations of a compressor. Well done.

Login to comment on this post