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Compressor & Limiter use on your mix - audio dynamics handling for starters

posted on #1
Posts: 2039
Joined: 30.12.10
In this Tutorial, I'll cover some very basic things about how to improve your mixes outcome by using compression.

To suit the "beginner" frame, lets make sure everybody knows what the following things are, and explain them in short for those who do not:

The Dynamic range - defines the possible spectrum between silence and maximum-loudest signal. If we exceed the maximum range of our recording setup, the sound becomes distorted- this is called "clipping" or "peak" and often indicated by red lights, as it should not happen.
Every instrument, microphone, amp, A/D-converter, monitor and ear has its own dynamic range (think of the maximum "loudness" of a drumset compared to an acoustic guitar, or in-ear heaphones compared to a PA speaker), we will focus on our recording & mixing softwares dynamic range here.

Mind: Dynamics have to do with "how loud is it" - when mixing for dynamic improvements, keep your fingers away from your monitors volume control!
Of course you can let any recrding be real loud by cranking up your monitors, what we are trying to achieve here is to let your music sound a bit louder without touching the monitor volume. That is what a compressor will do.

Master Fader / mastering - the "master" channel is the channel on your mixer (or on your DAWs mixer) which controlls the mixes volume - it will be the most-right fader. All single tracks signals are combined in this channel, and your mixdowns volume depends on its settings. Keep it set to 0dB at all times!
While the instrument-specific sound-editing (EQs, effects...) happens in the single tracks channels, there are some things which are commonly applied to the mixed signal at the end of the chain. These improvements on the mixed signal are called "mastering", which is a part of every professional production.

Headroom - think of the maximum volume as the ceiling, and the current volume as your head. Headroom is "how much more volume can be added before we bang against the ceiling" :)

Normalizing - "Normalizing" is a function offered by many DAWs, it works like this: The normalizer seeks the loudest peak in a recording and analyses the headroom that is left "above" that loudest peak, lets say 1dB.
Then, it raises the volume of the whole recording by 0.99db.
Result: Everything is a bit louder, while the loudest peak is still not clipping at 0dB. Downside: "Everything" includes all noise & hum on your recording which also gets louder.
Normalizing? Not advised!

DAW - "digital audio workstation" - all kinds of multi-track recording software

Ideal mastering results / use of dynamic range
Well, this is not answerable, really - depending on the kind of music you are mixing, you will have different approaches. A few hints:
- Do not waste headroom.
When recording, make sure to use the whole dynamic range, or at least 80%. If your recordings loudest peak is at -25dB (or at or above 0dB!), go and record again. Something between -3dB and -6dB is great, mind we are talking about the one loudest peak, which may only be a few milliseconds long!

When mastering, do not waste headroom either! Its absolutely OK if your loudest peak reaches -0.001dB. If it does not reach -3dB, I'd raise the single tracks levels until it does.

- Do not "compress flat" - as we will see in a bit, too much compression will sound bad and make remixing your track harder!

Peaks & average volume
Up untill this point we have been avoiding the dreaded clippings by focusing on the loudest peak. Depending on the recorded instruments dynamic range, the average volume may be pretty close to the loudest peak.
With many instruments, this is not so.
Example: You recorded an acoustic guitar with delicate pickings and some percussive slaps at the very end. Those slaps will go up to -1dB, while the picking part only sports -20dB on your recording.
You have no clipping, but the pickings really get lost in the mix... and this is when you need compression!

What the compressor does
In a nutshell, the compressor will keep the peaks below clipping, while raising the less-loud signal to a suitable volume.

Compressor settings
Your cmpressor will propably feature some presets, it is not too hard to understand how it works, tho, so here is what you can adjust:
1. "Threshold": Tells the compressor on which peaks it should start to act
0dB: nothing happens, -6dB: all peaks above -6dB get a "treatment" etc etc

2. "ratio": defines how the range defined by "threshold" shall be treated -
1:1: nothing happens,
1:2: assuming our threshold is set to -6dB, and the peak is -1dB, there are 5dB difference. This gets divided by the ratio (2), and the compressor will reduce the peak by 2.5dB. Result: the peak is at -3.5dB after compression.

3. "make up gain": after the peaks have been "flattened", we can use the won headroom to increase the overall level. This is similar to normalization, and some compressors have an automatic make up gain. If yours does not, raise the make up gain until your loudest peak is close to 0dB.

Can I hear compression?
Not if it is done well! Compressed music has less dynamic range than uncompressed audio, but this should not become obvious to the listener.
Most people will be happy about the "loudness" of your track, which makes it easy to listen to in noisy surroundings and at low volume.
If you spot a track and feel you really need to crank up them speakers, the mixer propably forgot to master to a decent volume!
One positive effect of mastering properly is that you do not have to change volume on your cars stereo with each track.
Especially radio is compressed in a way to make any track equally loud.

Now, for the sake of getting you started, here are some hints on the settings:
Threshold - set it low enough to catch some peaks, but way above the average signals volume. You will propably have some kind of monitoring on when the compressor is acting, make sure it does at some point, but not full throttle all the time.
Ratio - the smaller, the better. Generally, ratios from 1:1 to 1:3 are OK, everything harsher should be used with care because it will get audible.
I would use a 1:6 ratio on a single bassdrum or for a dub-bass track, but never on a voice or in a mastering mix.
Try it, you'll hear why.
Choosing a "1:unlimited" ratio will simply cut off all peaks above the threshold. That is what a limiter does, too - it is OK to catch some millisecond peaks as created by drums, but is too clearly audible to use with longer-lasting peaks.

I hope that at this point you are sitting there with a compressor loaded into your master channel and looking at it while playing your track. If you can switch it on and off, compare the difference it makes - your track should be a tad louder and the average volume display a bit higher when using the compressor.

Finally, lets cover a wikiloops specific scenario: the remix
You are going to create a remix, so you will have a wikiloops backing track imported to your recording software, and your own recording on a seperate track.
Lets assume you played the acoustic Guitar as in our first example, while using a well-mastered, loud backing track.

When mixing the backing with your guitar, you notice the backing is so "powerfull", that your guitar seems a little lost.
You tried above mastering-compression, but the result sounds really bad.
Why? because you are compressing the already compressed peaks of the backing another time, which will sound bad and not make the backing any quieter!
So, what to do?
Remove/disable the mastering compressorand solve this on the single tracks side.
First, lower the backings volume with its single-track fader untill you hear your guitar as you'd like to.
Go by ear, even if you end up with only 50% volume on the backing but a nice mix, that is OK. (Actually, I'd recommend setting a compressor on the guitars channel if it is that quiet)

No matter if you choose to work on the guitars dynamics in its own channel, your last controlling check will be back at the master volume.

Since you lowered the backing tracks volume, the mix will propably have won some headroom... keep the relation between the backings and the guitars volume and move both up untill your mixes levels are getting close to 0dB. Never push a single track fader higher than 0dB, btw.
Guess what, now its time to enable the mastering compressor again and let it raise the mixes volume.

I hope you enjoyed the read, happy mixing!
Edited by Dick on 12-07-2017 22:35
"Sorry - had to do it!" - Les Claypool

yes, you are looking at the administrators signature.
posted on #2
Posts: 461
Joined: 07.01.13
Cool :D you started it !!! yay ;)
clusters Clusters CLUSTERS !!!!!!
posted on #3
Posts: 8
Joined: 03.05.14
Thanks for sharing u'r knowledge :)
posted on #4
Posts: 60
Joined: 25.08.14
A good vid for basic understanding and use of compression:
posted on #5
Posts: 5
Joined: 02.04.16
Thanks Dick -- I'm glad I found this thread. The right way to use a compressor has been a bit of a mystery to me so far, and my contributions seem to be quieter than others. I'll try to put your tips to use in my next tracks.
posted on #6
Posts: 312
Joined: 27.02.15
Excellent thread, Dick. Compressors/Expanders still confuse me - I really struggle to use them on my snare where I want the grace notes brought up but the main peaks brought down. The same applies to my overheads which are the key to really getting 'air' into my snare sound. I seem to end up with everything flattened or it having no effect at all. I'll try some of your suggestions :)
Edited by mpointon on 12-09-2017 12:06
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