What is a compressor used for? Why do we need compressors? What do all those knobs do and how do I set them? Ask five sound pros and you'll likely get different answers.
Here are my thoughts on basic compression. I'm not going to get into advanced compression used for effects, but I'll try to offer some basic advice on how to tame a signal and create a more controlled, punchier sound.
What is a compressor?
Compressors are devices used to reduce the dynamic range of a sound. The dynamic range is basically the db change from an instrument, or vocalist's, quietest to its loudest signal.
Since most things in real life exhibit very wide dynamic ranges, compression just doesn't sound natural to our ears. On that note, good or natural sounding compression is hard to hear. I say it's hard to hear because when it's wrong, it's noticeable. When it's right, you shouldn't even know it's there. Things will just seem a lot more "in the mix" and in control.
Do I need a compressor?
The best answer to that question is no. I'm a bit of a purist when it comes to sounds. In a perfect world, the instrumentalist or vocalist would be in complete control of his or he dynamic range.
I remember reading a story once about a very accomplished singer. She hated the way compression sounded on her voice, but the recording engineer insisted they use it. Long story short, she sat with the engineer for hours, singing different lines and watching the needles on the soundboard. She learned how to work her mic, and vary her singing techniques to deliver a very even vocal performance.
But in the world we live in and the varied talent of the musicians we work with, compression is often required to make things sit in the mix.
What do all those knobs mean?
All good compressors will have these controls. Basic definitions and tips for the controls are as follows:
What settings should I use?
While there are a few magic numbers and settings, I'm not going to give them to you. Yet. I want you to learn to hear compression. When you hear it, you can fix it.
I'll give you a few guidelines. Remember though, these are only guidelines. Every person sings a little different, every bass player plucks a little (or a lot) different, every drummer hits loud, but different. Use your ears to tame the dynamics of whatever signal it is.
Keeping It Simple
Engage the bypass button on the compressor.
Set your gain correctly on the channel strip of your mixer. This is very important because the compressor will use the channels preamp (gain) as its input.
Now put the compressor back in the line and keep reading.
Note: Please don't do any of this during the service. Experiment during rehearsals.
Bass Guitar Compression Example
Here's what I usually do to achieve a basic setting for just about anything. At first this is going to sound pretty bad, but bear with me; I'm going to show you how to really hear what's going on with compression.
For this example I'll be referring to a bass guitar, but the concept is the same for any other instrument or voice.
Step One:
With the compressor bypassed, go to your gain knob on the soundboard, solo the input and look at the meter on your soundboard. You should have a signal around 0db. Now with the compressor still bypassed look at the output gain meter on your compressor. It should also be around 0 db. If not, consult your users manual there may be a switch on the rear of the compressor to calibrate the circuitry so the meters match up.
Step Two:
Set the ratio to 10:1, the attack to the fastest (all the way counter clockwise) and the release to about 1 second. Now play the instrument, grab the threshold knob and turn it so that you are seeing about –15 on the gain reduction meter. You are now hearing a very squashed and unnatural version of the original bass guitar. And it should be much quieter. Fifteen db quieter to be exact.
Step Three – the most important:
Whenever you compress something, you are taking the loud stuff and making it quieter. Therefore we have to "make up" the gain difference on the input vs. output. Once you've established a certain amount of gain reduction, indicated by the meter with the same name, you should add back (via the compressor's output knob) the same amount. In the above example if we set the bass compressor to read -15 on the gain reduction, then we would set the output gain to +15.
Before we go any further, please note this -15 db of compression is meant as an example and in the real world that is a ton of compression. Most singers or instruments shouldn't need more than -6 or -8, but that is harder to hear, so I'm asking you to learn with -15 or even -20 db of compression so you can really know what compression sounds like. Good compression usually won't be this extreme. We'll get more into that later.
Step Four:
Take the attack control and head clockwise (or slower) with it. You'll start to notice that the beginning of the note starts to sound more defined. This is because the compressor is letting the original "attack" of the bass through because we are delaying when the compressor squashes the sound. A good setting will usually be somewhere between 5 and 30 milliseconds. You'll want to set the attack so that you hear the initial pluck (or transient) come through unaltered, while the remaining part of the note should sound compressed. If it is set too long you'll hear the note pump as the compressor kicks in too late.
Step Five:
Now go for the release control and watch the gain reduction meter. The meter should be letting go of the signal as the note decays. If you hear the instrument decaying and the meter is still hanging down in some serious compression you'll want to make the release faster. Put simply, it should flow with the music.
Don't be afraid to crank those attack and release knobs, feel them out and hear what they do.
Step Six:
Next, back off the ratio to 2:1, listen for a bit, and now crank it to 20:1. Do you hear the difference? The 2:1 setting is only creating a soft effect. The bass will sound pretty natural, but you won't be correcting much dynamics. The 20:1 setting will literally make every note the same volume, but that's no good, as you would be sucking the life out of the music. What you are looking for is that the loud notes are now tame, but the soft ones sound softer.
Every instrument is a little different so play around. What you are going for is manipulating the dynamics of their playing so that the musician/singer's quiet notes are no quieter than the other players and obviously that their loudest notes aren't louder than the others.
Basics
Now that you know what the knobs do, I want to give you some practical starting points, and dispel a nasty myth. Let's hit some of those basics. Here are a few thoughts before we get into the numbers.
When learning compression, start with the "auto" button engaged. If your compressor has one, this button automatically sets the attack and release for you. It is not always completely correct, but it does do a pretty good job of guessing what you want to achieve.
If you have an OverEasy button, turn it on. OverEasy is a trademarked process of controlling the compressor's knee, but simply put, it is a little more musical-sounding type of compression.
When you feel like you understand the threshold, ratio, and output sections, go after learning the attack and release knobs.
Every compressor brand sounds a little different. That's why top studios have many different types of compressors. For our tutorial here, we're more interested in controlling the dynamics of instruments and vocalists.
"I need more compressors" is not always the solution. I run mostly digital consoles these days, so even though I have the ability to put a compressor on every channel, I generally use 6 to 8 compressors in a typical worship band setting and one compressor on each vocalist.
What should I compress?
I typically compress vocals, drums, bass, and acoustic guitar. When I'm short on compressors, I use that priority.
About subgroup compression
Compressing with sub groups is a topic we will save for another day, but in short, be careful using it. A compressor "compresses" the loudest thing it hears. So if you have a sub group of 8 background vocalists and use only 1 compressor on that group, the loudest singer will be triggering the compression for everyone else. It's useful for controlling the overall dynamics of them, but will completely mess up the blend of their vocals in the mix if one person overpowers the others from time to time. A better solution is to have a compressor committed to each individual vocalist, or at the very least, put one on the lead vocal and one on a subgroup of the background vocals
How-to: Hear attack and release settings
Try this experiment. Insert your compressor on a Kick Drum.
Set your Ratio to 8:1
Set the Threshold so you are getting about 10db of compression.
Set Attack all the way fast.
Set Release somewhere in the middle of the dial.
Turn the output gain back up as to match the amount of compression you are seeing. (You can check that you are correct by alternating the bypass switch. You should not notice a difference in volume. Tone, yes, but perceived volume, no.
Now start moving the attack toward the slower setting. You'll notice that the drum starts to get more click or "attack" coming through. Find a nice setting that allows punch but still controls the latter part of the sound.
Turn the Release all the way - fast. Now while listening to the overall tone and decay of the drum, move slowly toward slower settings. The compressor should fully release BEFORE the hit of the next drum, and you should be able to find a setting that adds to the overall tone or thickness of the drum.
Those magic numbers
Now that you've got the basics down, let's move on to the "magic numbers". Remember, though, these are only starting points. At the end of the day, you've got to trust your ears.
Vocals
The idea with vocal compression is to smooth out their dynamic range so they will sit in the mix better. When Sally goes for that high note in the Sandi Patty song, it should not make your ears bleed. As well as when she goes to a quiet section, we should still be able to hear her.
I am a very big fan of more compression with low ratios. Once you learn to hear the higher ratio stuff, try backing your lead vocal down to 1.7:1 but have the gain reduction meter always hitting at least 6 db. Make up the gain on the output and get ready for a very solid lead vocal.
Threshold: Set so there is always a little compression taking place. Maybe –1db on soft sections and –10db on loud sections.
Ratio: 2:1 for lead vocals, and 3 or 4:1 for background vocals.
Attack: Around 25ms gives a nice punch to the beginning of their words while still controlling the rest of it.
Release: Should be somewhere around 200 to 400ms.
Output: Don't forget to bring the output back. If you are seeing –4db on the gain reduction meter boost the output by +4db. That way you make up the gain you lost in the compressor.
Drums
The biggest benefit of compressing drums aside from controlling their dynamics is that we can achieve a punchier sound. Try these settings. One compressor on a sub mix of the drums will often be a good solution. But like I said above, the loudest thing in the subgroup triggers the compressor, so be sure to balance the levels of
the drums so the compressor is hitting evenly on all the drums. That said I always prefer to have an individual compressor on each drum.
Threshold: Set so there is about –4db on normal hits. More compression will return a more "squashed" sound, which is often desired, on a snare.
Ratio: 4:1
Attack: Set as fast as possible. I find 7 to 15ms will let a nice bit of the snap through. Listen to the "click" on the drum when choosing this setting.
Release: 20 to 75ms will fatten up the tone, or meat, of the drum.
Output: Don't forget to make up the gain you lost from compression. If you set it so you see -4db on the compression meter, then turn up the output of the compressor by +4db.
Bass
Since the bass guitar is the feeling and driving force behind most contemporary worship music, I tend to want it pretty solid. I don't like the low frequencies jumping around as if they were coming and going.
Threshold: Set so there is about –6db on the gain reduction meter.
Ratio: 4:1
Attack: 15 to 45ms
Release: 300 to 450ms, depending on the song.
Output: Don't forget to make up the gain.
Acoustic guitar
I like the acoustic to have a nice attack but also to be consistent. Going from finger picking to hard strumming will require a bit of a threshold adjustment usually, but these settings should get you started.
Threshold: Set so there is about –3bd on the gain reduction meter.
Ratio: 2.5:1
Attack: 5 to 15ms
Release: 200 to 300ms. Give or take depending on how fast the song is. You should see the meter fall just a hair faster than the natural decay of the note.
Output: Yup, you got it... make up the gain.
Last but not least...
Remember that nasty myth I mentioned? Here it is: "Compressors can fix bad technique".
Compressors should not be used to fix bad technique. Rather than slapping a 5:1 compressor on a vocalist who doesn't hold the mic correctly, why not spend some time reviewing proper mic technique? (There's a whole Shure Notes on this topic and you can find it right here.)
Remember to be nice. A humble sound person who teaches (and is also teachable) is the best kind to be.
About John Mills A frequent contributor to Shure Notes®, John is an 18-year veteran of the road. He was a frustrated Electrical Engineer who hated college. He left school to pursue a career on the road as a drummer, ended up as a sound engineer and after being blessed to work for many of the top Christian worship leaders, artists and tours, has landed at a job as an audio engineer for a design firm. He says, "I guess Mom was right, she always knew I'd finally got a real job." Check out www.EliteMultimedia.com and www.TechTraining101.com for more about what John is up to.
