Audio Compressors, arguably one of the hardest things to understand and let alone hear when it comes to working with audio. Compressors have a wide set of applications and a wide variety of controls. Today we’ll break it all down so that you can walk away from this lesson understanding compression like a pro!
In my journey I’ve found that compressors take a specific perspective to understand how they work. Often I hear people using words to describe what a compressor does, but they don't actually help and more often than not, leaves one having more questions than before.
Let’s Dive in!
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The Purpose
Originally created to control audio levels automatically, a compressors’ main job is to control dynamics.
For example, if a vocalist was whispering in a microphone you would instinctively turn the volume up in order to hear what they were saying. Then randomly they started yelling. Jumping backwards with a rush of adrenaline you reach for the volume to turn it down. The drastic change in volume was unexpected and took you by surprise.
If a compressor was in the mix, this intense fluctuation of volume wouldn’t be an issue because the compressor would prevent such a wide range of dynamics from taking place. If you’re lost on what dynamics are, check out this lesson on dynamic range.
Compressors begin to work at a specifically set threshold. In the example above, that threshold would be set above the whispering but below the yelling, thus keeping the yelling portion to a similar volume level as the whispering portion, but without you having to adjust the volume manually.
This is a very brief description of what a compressor can do, and is by all means not the only way to use a compressor. Before we get into different practices, let's cover some different types of compressors.
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Types of Compressors
A VCA Compressor (Voltage Controlled Amplifier) uses a VCA to adjust the gain of the incoming signal. While a VCA in and of itself is not within our lesson scope. At its core, it's a gain knob that is controlled by a Voltage signal.
This type of compressor is very fast, precise, and clean. Its transparent and perfect for drums, vocals, and mastering.
These types of compressors will typically have the full set of compressions controls:
Attack, Release, Threshold, Ratio, and (sometimes) Knee
Famous Examples:
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SSL G-Series Bus Compressor
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DBX 160
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API 2500
A FET Compressor (Field-Effect Transistor) uses transistors to emulate the sound of tube compression, but with a much faster response.
Due to the emulation of tube compression, these compressors DO add color, aggression, and harmonics to your sound. They tend to bring out more transients and brighten up a sound due to the addition of harmonics. They are great for drums, vocals, and guitars. Providing a little character along with your compression.
While the 1176 (most renowned FET) does not have a threshold, it uses the input gain to push the audio into compressions and the output gain to return the audio to the desired level. Most FET compressors are modeled this way.
Famous Examples:
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UA 1176 (one of the most famous compressors ever)
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Purple Audio MC77 (modern 1176 clone)
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Warm Audio WA76
An Optical Compressor (Opto) uses an LDR (Light-Dependent Resistor) and a light source to control gain. The brighter the light, the more compression is applied.
Opto compressors tend to be smoother, more organic, and transparent. Slower attack and release times and tends to be a more gentle compressor type.
Very common on Vocals, strings, and bass. A well rounded and built for beef compressor. More often than not, you’ll find clones of the LA-2A, a legendary compressor that only has Gain and peak reduction knobs.
Famous Examples:
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Teletronix LA-2A (legendary vocal compressor)
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Tube-Tech CL 1B (modern high-end opto compressor)
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UA LA-3A (faster optical compressor for guitars & vocals)
A Variable-Mu Compressor (Tube Compressor) uses vacuum tubes to control gain reduction. The more gain you put into the compressor, the more compression you’ll get out of it.
Often defined as rich, warm, and smooth, due to the coloration of the vacuum tubes. Variable-Mu compressors are great for smooth/ vintage vocals. They are amazing for mastering and adding glue and warmth. Overall brilliant for orchestral and bass dynamics.
These compressors come with a variety of controls ranging from just gain and threshold to the standard full set of controls.
Famous Examples:
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Fairchild 670 (holy grail of tube compressors)
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Manley Vari-Mu (modern version of Fairchild)
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Gates STA-Level (classic for vocals & bass)
As you can see, compressors are really based on the electrical components they are built with internally. The circuitry within them is what makes them special. This is what really defines the analog vs digital debate amongst audio engineers. A debate we won't get into here.
Let's get into the typical controls of a Compressor. While I won't cover the unique compressors like the LA-2A or 1176 and how they differ from standard compression controls. I will cover the standards and the perspectives for each control so that you have a better understanding of what each one does. This will also shed a little light on those unique compressors that do things a little differently.
Threshold
The question that provides an answer is “at what dB level do i want my compressor to start compressing”.
In this example we’ll look at a kick drum, a sudden on/off sound that will help us visualize all the controls we need to understand.
For simplicity, our kick drum reaches the 0dB mark just to keep numbers easy for our demo
As you can see in the image above, the threshold is currently set to 0dB, meaning there is no compression happening to our sound, because our sound does not pass 0dB, it does not cross the Threshold.
Let's now set the Threshold by asking the question:
“at what dB level do i want my compressor to start compressing” for our example let's set it to -14dB. Though, in practice this level will vary based on the actual level of your audio.
As you can see in the image above, with our Threshold set to -14dB. The kick drum’s dB level now surpasses the Threshold, thus activating the compressor. Our kick drum exceeds the threshold by +14dB because originally our kick drum was set to 0dB.
Now that we have set our Threshold, and we know the kick drum exceeds that Threshold by +14dB. We can now take a look at the Ratio. This is where some perspective thinking comes in.
Ratio
When we think about compression, the first thing that comes to mind is typically squeezing, crushing, or cramping a specific amount of something into somewhere. I want you to forget that idea. Instead I want you to adopt a new perspective of what compression is.
A compressor limits the amount of audio that passes through it. It doesn't squeeze or clamp down on the audio, its a buffer that keeps audio from passing through it and this is done in Ratios.
2:1 for every 2dB that passes the threshold, only 1dB makes it out
4:1 for every 4dB that passes the threshold, only 1dB will make it out
100:1 for every 100dB that passes the threshold, only 1dB will make it out
What does this mean for our example? Lets break it down.
In the above image you can see that the Ratio is set to 1.00:1 or 1:1. This means that for every 1dB that passes out threshold 1dB will make it out. In other words no compression is happening because every dB that passes our threshold makes it through.
In the above image we can now see that the ratio is set to 2:1. This means that for every 2dB that passes our threshold, the compressor will only let 1dB out.
Our original signal reached 0dB and we set our threshold to -14dB. This means that our kick drum surpasses the threshold by +14dB.
Since we set our ratio to 2:1, this means that for every 2dB that passes the threshold, only 1dB is let through. This means that we can take +14dB over threshold and divide it by 2, we’ll get +7dB that are actually let through.
This means that if our threshold is set to -14dB and our original signal was 0dB, 14dB over the threshold. Our compressed signal will now be at -7dB at its peak. Because only 1dB for every 2dB that passed our threshold was passed through.
Lets go over that again:
In the image above you can see we have a Ratio of 4:1 set. Meaning that for every 4dB that passes our threshold, only 1dB is passed through.
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Our original signal peaks at 0dB
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Our threshold is set to -14dB
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Our Ratio is 4:1
How many dB over the threshold does our kick drum signal exceed? It exceeds +14dB over our threshold.
Our ratio is 4:1, so for every 4dB that passes the threshold, only 1dB passes.
14db ÷ 4 = 3.5dB
This means that out of the +14dB that our kick drum passed over the threshold.
Only 3.5dB were actually let through.
This means that our output signal will only be 3.5dB past our threshold of -14dB.
-14dB + 3.5dB = -10.5dB
Our kick drum now peaks at -10.5dB instead of 0dB.
One more for good measure:
The above image displays a ratio of 100:1
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Our original signal peaks at 0dB
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Our threshold is set to -14dB
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Our Ratio is 100:1
How many dB over the threshold does our kick drum signal exceed? It exceeds +14dB over our threshold.
Our ratio is 100:1, so for every 100dB that passes the threshold, only 1dB passes.
14db ÷ 100 = 0.14dB
This means that out of the +14dB that our kick drum passed over the threshold.
Only 0.14dB were actually let through.
This means that our output signal will only be 0.14dB past our threshold of -14dB.
-14dB + 0.14dB = -13.86dB
Our kick drum now peaks at -13.86dB instead of 0dB essentially limiting the signal (foreshadow of net weeks lesson on limiting)
I know, I know, you didn't sign up for a math class. But here you are doing ratios addition and division. I’m hoping these examples have helped you understand how threshold and ratio work together.
Really quickly I want to cover the Knee setting as it has some relation to threshold (at what point) and ratio (how much) settings.
The Knee will dictate the leniency of how and when the compressor reacts to the signal passing the threshold and how the ratio is applied.
A Hard knee will be instant and tends to be good for fast compression for things like drums for a snappy response.
A Soft knee will gradually apply the compression the more the signal exceeds the threshold. The more the signal exceeds the threshold, the more compression is added. Soft knees are more for smooth and rounded sounds like orchestral and vocals.
Hard knee:
Waits for the signal to hit the absolute threshold before applying quick compression as soon as the threshold is met.
Medium Knee:
Gradually starts applying compression as the signal approaches the threshold
Soft Knee:
Begins applying compression far before the threshold is met.
Attack & Release
Now it's time for the next two primary compression settings:
Attack (at what point) and Release (for how long).
A lot of people will talk about Attack as “how soon” will the compressor act. I don't like to think of it that way. It gives the wrong impression in my opinion.
I think of Attack as “how long after”.
How long after the threshold is crossed does it take for the compressor to turn on?
A lot of people will talk about Release time as “how long will compression stay”. To me this kind of makes sense.
I think of Release as “how quickly will it stop”.
How quickly will the compressor turn off/ reset?
Both Attack and Release are measured in Milliseconds. They are both time based functions. They are both much more easily understood when watching the audio through an Oscilloscope. An Oscilloscope is a tool that allows you to view the waveform instead of the spectrum. Another very useful tool to have in your toolbox.
A little more math so you can understand how I'm getting the following numbers.
60,000 ÷ BPM = millisecond of 1 beat (¼ note) as there are 4 beats per measure (bar), and 4 measures (bars) per phrase
In our example we’re at 115 BPM so 60,000ms ÷ 115BPM = 521.74 ms per beat (¼ note)
And our kick drum is a 1/16th note so
¼ 521.74 ÷ 2 = ⅛ 260.87ms
⅛ 260.87ms ÷ 2 = 1/16th note at 130.43 ms
This being said, out kick drum is 130.43ms long.
Here is our Kick drum with No compression at all. The 130.43ms mark is dead center of our Oscilloscope.
Let's use an ∞:1 ratio, meaning absolutely no dB are passing our threshold. Let's set the threshold to -60dB (the lowest we can), whilst using a 0.005ms attack time. This will absolutely crush our kick drum as you can see below.
In the above images you can see that we’ve instantly smashed the kick drum. And looking at the arrows we have just about a straight line all the way to the. We haven’t let any dB through. With the red compression line moving all the way down.
Our Attacked time of 0.005ms indicates that 0.005ms after our kick drum passed the -60dB threshold, our compressor kicked into gear. This is almost instantaneous. This is also where my perspective of “How long after the threshold is crossed does it take for the compressor to turn on?” comes from.
Because we can't be instant, we can only react from here on out. Adding time to out attack only delays our reaction time.
Take a look at what happens when we increase our Attack time to 65.22ms which is half of 130.43ms.
You can now see that by increasing our attack time, we allow more kick drum to happen before the compressor reacts.At this point, we’ve allowed 65.22ms of our kick drum to pass through the compressor before it started to actually compress. Looking at the arrows, we can see the line isn’t straight down anymore. Our red compression line indicates that we actually let about 25dB through before hitting peak compression.
So what happens when we set the Attack time to the full length of our kick drum ?
You can see in the above images that the maximum amount of compression does not take place until the kick drum is just about over. We are still applying an insane amount of compression. But if you take a look at the top arrow, notice the dark grey starting to travel downwards is almost aligned with the bottom arrow, where the red compression line begins to travel upwards. This is indicating that by the time the kick drum is finished playing we’ve now reached full compression. Full compression wasn't reached until the end of the kick, gradually taking its time. Letting a whole 30dB through before reaching peak compression.
What if we double the length? The longest Attack time this compressor will achieve is 250ms as seen below.
As you can see by the bottom arrow. Full compression almost didn’t even happen. Letting a whopping 40dB through before reaching is peak compression.
The longer our Attack time, the longer it takes for our Compressor to react, allowing more audio to pass through it before it begins to compress.
This is how we can shape transients or make drums more punchy! Allowing the initial punch through and then compressing the remainder of the
Here is our Original uncompressed Kick Drum:
Here is our kick drum with a 7ms attack, a smooth knee, a threshold of -19dB and a ratio of 2:1 and gain staged to match the original dB level before we applied compression.
This is the art of transient shaping with compression, just one of the many things a compressor can do.
Lastly, let's talk about Release time. Release time is how long it will take for the compressor to reset after the audio drops below the threshold.
Having too long of a release time may prevent your compressor from ever fully resetting. When working with punchy sounds like drums, having too long of a release time will prevent the punch of the proceeding drum hits to occur.
Below is a short 10ms Release time. This means that 10ms after the sound falls below the threshold, the compressor will stop applying compression.
You can see how the compression curve is fairly steep.
Looking at the 2,500ms Release time below, this prevents our kick drum from ever being heard, because once the compressor is triggered, it will take 2,500ms to reset. In other words it would take a full measure (bar) to reset!
You can see that with the extremely long Release time, the kick drum peaks around -42dB because the compressor never has time to “Release” before the next kick happens. thus, preventing the kick drum from reaching its original 0dB peak.
A healthy Release time time can help shape your audio signal whilst still allowing the following transients to come through.
Wow, we’ve covered a lot of ground with this intro to compressors! By now you should have a much better understanding of compressors. They will always be a little black box of magic, smoke , and mirrors. But by understanding these key principles of how they work will take you very far!
Its very easy to over use them, but one trick that will always help you make a good judgment is
GAIN STAGING
Make sure you take note of your audio’s dB level before you compress it. Apply your compression, and then increase the gain to match the dB level that it was before you compressed it. This will be the ultimate deciding factor to help you make a decision on whether your application of compression makes sense within the context of your mix.
Compression compresses your audio, it reduces the dynamics and at the beginning, makes it sound a bit quieter. But it's so very important to gain stage and turn it back up to its original dB level by doing this you’ll realize that compressors actually make your audio “beefy-er” and “louder” because it reduces the distance between the quietest and loudest parts, making your audio a little more even all around.
Understanding all of this will really help you out in the next lesson when we cover Limiters. Which are basically compressors that don't let anything through.