Compression is often one of the most confusing effects to understand.
This isn’t just for beginners, by the way.
Even many professional guitarists don’t really know how to work their compressor pedals and just use their ear until they find a sound they like.
And this understandable.
With controls like Threshold, Ratio, Make-Up Gain, and Knee (I mean come on, what in the world does Knee control?), it’s no wonder that anyone who isn’t an audio engineer has to wonder: What does a compressor pedal do?
In this article, I’m going to de-mystify compressor pedals so that you know what they do and how they work.
The Short Answer
Without getting too technical, a compressor pedal evens out the dynamic range of an audio signal. It makes the loudest parts quieter and the quietest parts louder.
The Long Answer
It is because compressor pedals amplify valleys and attenuate peaks in your audio that you will sometimes hear compressors called a “Leveling Amplifier”.
Compressors are one of the oldest bits of audio technology. They were first used on telephones for voice audio because some people talk louder than others. With a compressor, telephones made quiet talkers and loud talkers all sound at about the same volume.
Compression was also heavily used in radio broadcasting for similar reasons. This would be helpful if there was an audience clapping because the audience wouldn’t overwhelm the level of the broadcaster.
This effect has evolved a great deal over the years, moving from rack mount systems, to guitar pedals in the 1970s, and these days compressors are used as outboard gear or even as digital plug-ins on your computer.
Whatever kind of compressor you use, they all work to even out the dynamic range so that you can hear all parts of the audio evenly.
Parameters and Controls
For us guitarists, the most common form that we see compressors in is in effects pedals. No matter what compressor pedal you use or what controls are on them, they all aim to do the same thing.
Let’s look at some common controls and parameters you will see on compressor pedals.
- Threshold: where the compressor begins to work. Setting high means compressor will only work on peaks. Lower means compressor will work sooner. Far enough will be always on.
- Ratio: Set of numbers (2:1, 4:1, etc.). How much compression is applied. Lower the number, the less compression being applied. Infinite is known as Limiting.
- Attack: how fast the compressor clamps down on signal. Good way to target the attack on guitars or drums
- Release: how fast the compression lets go. Sustain
- Knee: How compression is applied. Soft knee gently applies compression the further signal goes past.
- Makeup-Gain/Volume: Allows you to get back to unity gain before adding compression.
- Blend: mix between compressed signal and dry signal
Types of Compressor Pedals
There are six different kinds of compressors (optical, VCA, FET, Valve, Multi-Band, and Parallel that are commonly used in music, but here are the three most common ones that you will see in pedal form:
This is the most common type of compressor and stands for “Voltage Controlled Amplifier”. They don’t color or distort your sound, which makes them really versatile and conjunction with other pedals. Unless otherwise specified, most compression pedals are likely to be VCA.
These compressors use an LED and a photocell to monitor the input level. The brighter the lightbulb is, the more light the photocell reads, which causes it to reduce the gain more. This is a slower technology, but lends itself to sounding really natural, similar to how tube amps have a slight bloom or lag to them that many guitarists prefer. They have a smooth attack and have to be pushed to the limit to be a noticeable effect.
Parallel compressors use the dry and wet (compressed) signal in parallel and are then blended together. This use of blending is what makes parallel compressors more and more popular in today’s market. These are a great choice if you are looking for a compression pedal purely to add sustain to your guitar solos.
Why Use a Compressor?
Aren’t dynamics in guitar playing a good thing? Why would you want to take away dynamics?
Compressors can be a polarizing effect for those that rely heavily on dynamics for their expression. Many often suggest that if you can hear a compressor, then you’re using it too much.
One thing that compressors do well is even out the natural dynamic differences in guitar. When you strum a guitar, the attack is naturally much louder than the rest of the held out note. To try this out, play a funky rhythm part on a clean, electric guitar. It almost hurts your ears!
Compressors are great at evening out the attack on clean rhythm guitar.
This is what makes compressors like the MXR DynaComp a hit in Nashville, Tennessee, where country music reins supreme. Many players use Telecasters, which are bright. The MXR DynaComp in particular has a dark sound to it and is used to squash fast chicken pickin parts. This is why country guitar sounds smooth and squishy, while remaining twangy.
Another thing that compressor pedals do is add sustain to your playing. This is another feature that is great for clean, electric guitar. If you want to play a clean guitar part for a solo, you can use a compressor to help hold your notes longer.
Recommended Compressor Pedals
- MXR Dyna Comp – Best Under $100
- Jackson Audio Bloom – Best High-End Option
- Aguilar TLC Compressor – Best for Bass
Hear It All Evenly with a Compressor Pedal
I hope this article has been helpful and answers the often asked question of “What does a compressor pedal do?”
Compressors allow you to hear all parts of an audio signal evenly and are a great pedal to boost quiet playing or tame loud playing. They are also great for adding sustain.
To learn more about our favorite pedal compressors and which ones we recommend, be sure to read our full write-up.
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Davis Wilton Bader is a professional guitarist/writer based out of St. Louis, MO. He plays in the bands Lumet and The Outskirts.