What Are Compressor Pedals (Guitar/Bass) & How Do They Work?


Compression is one of the most important and popular processes in audio so it stands to reason that a compressor pedal in a board/rig would be fitting.

What are compressor pedals and how do they work? Compressors pedals reduce the dynamic range of the input guitar/bass signal. A compressor pedal will reduce the amplitude of the input signal by a set ratio if the signal surpasses a set threshold. Compression improves sustain; brings quiet and loud notes closer, and slightly distorts the signal.

In this article, we’ll discuss compressor pedals in much more detail, covering the inner workings of compressors and how they affect guitar and bass guitar signals. I’ll share a few compressor pedals throughout the article and offer some tips on how to get the most out of your compressor pedal if you choose to use one.


Table Of Contents


What Is Compression?

“What is compression” is a question anyone will inevitably ask as they get into the world of audio. This type of audio processing is somewhat difficult to conecptualize at first but is rather easy to understand once it clicks.

To begin our definition of audio compression, let’s consider its alternate name: dynamic range compression.

Dynamic range compression is the effect we’re after. Digital audio can be data compressed to reduce file size (that’s not what we’re talking about here).

So then, dynamic range compression, as the name suggests, compresses the dynamic range. That means that an audio compressor acts to compress or reduce the dynamic range of the signal its acting upon.

The dynamic range of a signal is simply the range between the signal’s lowest amplitude (typically the noise floor of the signal) and the highest amplitude. Dynamic range is typically measured in decibels (dB).

If you’re confused at all about decibels, please check out my article on the subject: What Are Decibels? The Ultimate dB Guide For Audio & Sound.

Compressors reduce the dynamic range by attenuating the loudest parts of the signal.

Simply put, compressors suppress loud notes so that quiet notes sound comparatively louder.

That’s the “what”, now let’s look at the “how”.

How Do Dynamic Range Compressors Work?

So we know that compressors act to reduce the dynamic range of a signal by attenuating the loudest parts of the signal. Let’s start there.

Audio processing units have long been capable of reading the amplitude of both analog (electrical) and digital (binary) audio signals. Analog signal amplitudes are typically measured by peak or RMS voltage (often in decibels dBu or dBV). Digital signal amplitudes are often measured in dBFS.

Compressors, from analog hardware units to digital software, can effectively read how strong an audio signal is at the input.

A compressor will have a threshold setting. This threshold refer to the input signal amplitude at which the compressor will start working. In order for the compressor to work, this threshold must be sett below the maximum amplitude of the signal. Otherwise, the compressor would never get triggered into action.

Okay, so what happens when the input signal surpasses the set threshold? How does the compressor attenuate the signal?

If the signal was to get completely cut off at the threshold, we’d have a limiter (which is really just a very hard compressor).

A limited signal looks something like this:

With typical compressors, this is where the ratio parameter comes into play. The ratio of a compressor represents the amount of attenuation that will take place as the signal amplitude surpasses the compressor’s threshold.

The compression ratio means the following:

[input signal dB above the threshold] : [output signal dB above the threshold]

So a ratio of 4:1 means that for every 4 dB the input signal surpasses the compressor’s threshold, the output signal will only be 1 dB above the threshold.

Those are the two main parameters we need to know to understand the basics of compression. Here are a few illustrations to help us visualize compression:

Input Amplitude Vs. Output Amplitude Of A Compressor With Various Ratios
Simplified Illustration Of A Compressed Sine Wave With 2:1 & 4:1 Ratio

The amount the input signal is attenuated is referred to as “gain reduction” and is generally measured in decibels as well.

Beyond the basics, there are also attack and release parameters on compressors:

  • Attack: the amount of time it takes for the compressor to react once the threshold is surpassed.
  • Release: the amount of time it takes for the compressor to disengage once the input signal drops back below the threshold.

Having longer attacks can make the guitar/bass (or other audio) more percussive by allowing the transients to pass before the attnuation clamps down on the signal.

Shorter attacks reduce transient information and make for a thicker signal.

Fast release times can help the sound remain more natural so long as there isn’t too much gain reduction. Faster released also increase the perceived loudness of track by allowing the sub-threshold signal to remain close to, if not at, its original level.

Slow release times will help smooth out dynamic performances. They’re often used to push an element further away in a mix. Generally speaking, we wouldn’t want overly long release times on our guitar or bass compressor pedals.

Compressors will typically have make-up gain to bring the peak signal level of the output back up to what it was at the input. With the make-up gain set to equal out the peaks, we can really hear the compression of the signal. On guitar and bass, it typically sound like a increased sustain; a thickening of the signal, and greater perceived loudness.

That’s the “how”, now let’s look at the “why?”

Why Is Dynamic Range Compression Used?

Compression, in the context of a mix, is used for various reasons:

  • It helps “glue” the tracks together into a cohesive sound.
  • It can be side-chained to duck certain elements out of the way of others.
  • It can improve overall headroom by reducing the peaks of certain tracks.
  • It can improve the character of certain elements in the mix.

This list could go on.

For individual instruments like guitar and bass, which are the instruments that typically run through pedals, the effect is cherished for a few other reasons:

  • Compression can improve the sustain of the guitar/bass.
  • Compression will make the transients of notes across all strings more equal in amplitude.
  • Compression can yield a nice overdrive/saturation effect, which sounds great on guitar and bass.
  • Compression pedals have make-up gain and so they can also act as boost pedals.
  • Compression will make the instrument sound more present, overall.
  • Compression can be utilized at any stage of the signal chain.

So there are plenty of reasons to add compression to our audio signals. Having a great compressor pedal (or multiple pedals) can make a noticeable and positive difference in the sound of our guitar or bass.

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What Is A Compressor Pedal?

A compressor pedal is a compressor unit in a stompbox format designed to compress guitar, bass and other instrument signals.

They can be analog or digital.

These pedals typically have most of, if not all, the controls of a typical compressor. They’ll generally have an on/off (on/bypass) switch and should be built to withstand the wear and tear of being stomped on and taken on tour.

Other than that, compressor pedals act just like other compressors.

The Origin Effects Cali76 Compact Deluxe (link to check the price on Amazon), for example, is based on the legendary Universal Audio 1176 studio compressor. It’s an analog compressor stompbox pedal with dry/wet mix; input and output level; ratio; attack, and release controls.

Origin Effects Cali76 Compact Deluxe

The Boss BC-1X Bass Comp (link to check the price on Amazon) is a digital bass compression pedal. It has output level, release, ratio and threshold controls along with an led indicator to show the amount of gain reduction when the pedal is in use.

Boss BC-1X Bass Comp

Boss is featured in My New Microphone’s Top 11 Best Guitar/Bass Effects Pedal Brands To Know & Use.

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Is A Compressor Pedal Necessary?

Compressor pedals are by no means necessary. However, as we’ve discussed, they can do wonders for our tone when used correctly.

Just a few benefits of compressor pedals include:

  • Increase in perceived loudness.
  • Added sustain to the signal.
  • A more even and equal transient profile for the instrument (particularly with bass).
  • An increase in transients (with a slower attack).
  • Helps to more easily fit the guitar or bass into the mix.

So no, compressor pedals are not necessary, though they are a worthy investment for many players.

Related article: Are Guitar Effects Pedals Necessary Or Worth It?

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Typical Controls On Compressor Pedals

We’ve touched on the typical parameters of compressors already. Compressor pedals will typically have controls to alter most, if not all, these parameters in one way or another.

The most common controls for compressor pedals to have are as follows:

Output Level

The output level control (sometimes referred to as make-up gain) controls the output signal level of the compressor pedal.

This is often used to bring the peak level of the compressed output signal back up to what it was at the input pre-compression.

However, increasing the output level can also allow the compressor to double as a boost pedal, assuming the gain is clean enough to not affect the tone too much!

Input Level

The input level control adjusts the signal level going into the compressor circuit.

Increasing the input level will make the compressor more active without making any adjustments to the compressor itself.

Mix

Some compressor pedals allow users to mix the compressed/wet signal and the direct/dry signal together at the output.

This control may be labelled as blend.

Threshold

The threshold control adjusts the set amplitude point at which the input signal will trigger the compressor into working.

Threshold is sometimes labelled as “sensitivity”.

Ratio

The ratio, as we’ve discussed, relates the input signal level above the threshold (in decibels) to the output signal above the threshold (in decibels).

The ratio control will alter this ratio. The higher the ratio, the more dynamic compression will take place above the threshold.

Attack

The attack control will adjust the time it takes for the compressor to kick in once the threshold is surpassed at the input.

Release

The release control will adjust the time it takes for the compressor to disengage once the input signal drops back down below the set threshold.

Sustain

Sustain controls tend to have something to do with the amount of compression and the release of the compressor.

Increasing a sustain control will squash the signal more in order for the tail end of a note to sound more comparable to the transient part of the note, thereby increasing the perceived sustain.

Slower release times can also increase perceived sustain by keeping the signal under compression as the input amplitude dies out.

Tone

Tone controls, when the are added, have little to do with the compressor but are worth mentioning anyway.

Adjusting tone controls typically has to do with EQing the signal after it’s been compressed.

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Tips On Using A Compressor Pedal

Compression is an important part of audio processing but it can take a while to understand completely.

Developing an in-depth comprehension of compression is beyond the scope of a relatively simple pedal article. However, there are a few tips worth sharing to help you get the most out of your compression pedal(s).

Here are a few thoughts to improve your experience and knowledge of compressor pedals:

Use A “Less-Is-More” Attitude

Over compressing any signal can really suck the life out of it.

If you plan on keeping the compressor pedal engaged (this is common for bass players), then ensuring that the gain-reduction isn’t too great will help keep some dynamic life in the signal and sound.

Just a touch of compression (-3 dB to -6 dB) is all we need to really hit the sweet spot when it comes to compression. It’s also important to adjust the attack and release accordingly.

Experiment With Signal Chain Placement

As we’ll discuss in a moment, compressors have the potential to imporve the signal wherever they are placed in the signal chain.

Try moving the compressor pedal around in order of your pedals and listen to where it sounds best. You could even pick up two compressors and have them in two key spots of your rig’s signal chain.

Engage It On Lead Lines To Increase Sustain And Volume

A big part of the allure of effects pedals is that we can control them with our feet while playing. Being able to turn our compressor pedal on and off can give it extra functionality.

One way to use a compressor pedal is for lead lines and solos on the guitar (or bass). Having a good amount of gain-reduction and a slower release does have the potential to kill the dynamics but the flip side of that is an increase in sustain. It’s up to you to find the sweet spot.

A bit of extra sustain sounds great on lead lines and guitar solos and can really bring the sound of the guitar to the forefront of a mix.

On top of that, the make-up gain (output level) of the compressor pedal can be turned up, making the compressor pedal a two-in-one with an extra boost in level along with the increase in sustain.

Crank It For Percussive Lines

Heavier compression sounds great for purcussive play (like in funk). Turning the compressor up a bit for those rhythmic extended chords can really solidify the guitar in the mix.

Similarly, if there’s a slapped bass-line, a heavier compressor with a slower attack can simulatenously increase the perceived transients and make for a thicker bass sound.

Try One Before The Preamp And In The Amp’s Effect Loop

Compressors can sound great anywhere in a chain. Why not try them before and after the guitar amp’s preamp if the amplifier has an effects loop.

A compressor before the guitar amp preamp can help to strengthen the guitar signal going in and cause some overdrive in the signal.

A compressor in the effects loop (after the guitar amp preamp) can simulate the effects of overdriving a tube (even if the amp is solid-state) by causing a pseudo-sag between the guitar amp’s preamp and power amp.

Of course, a compressor will also be interacting with whatever other pedals in before or after the amp’s preamp so take everything else into consideration as well!

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Where Should A Compressor Pedal Go In The Signal Chain?

Compressor pedals are dynamic processors so my gut reaction is to tell you to put them near the front of the pedal chain.

However, compressor pedals can sound great anywhere in the signal chain.

Think about it, compression is nearly always used in mixing. When mixing guitar and bass in the studio or in live settings, the signal has already gone through a list of units.

For example, a signal from a guitar could pass through the following:

  • Effect pedals before the amp
  • The guitar amplifier’s preamp
  • Effects in the amp’s effects loop
  • The guitar amplifier’s power amplifier
  • The cabinet’s speaker
  • The air
  • Into a microphone
  • Through a microphone preamp
  • Into the mixing console

That not to mention all the cable that connects the electronics mentioned above together.

After all of that, compression is used to incredible effect! It stands to reason, then, that a compressor pedal could go anywhere in the chain.

Really want a modulation effect to stand out? Try slapping a compressor just after it rather than cranking up the modulation effect.

Need more sustain just before the amp? Try putting the compressor pedal between the pedalboard and the amp.

Experiment with your particular rig to find the best results for you!

To learn more about ordering pedals in your rig/pedalboard, check out my article How To Order Guitar/Bass Pedals (Ultimate Signal Flow Guide).

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Pedals & Effects Similar To Compression

We’ve touched on the ∞:1 ratio of a limiter. Let’s discuss this process along with a few other processes related to compression.

Effects that are related to compression and are found in pedals include:

Limiter

A limiter effectively limits a signal from passing a certain amplitude.

As we’ve alluded to earlier, a limiter can be thought of as a compressor with a ratio of ∞:1. This means that the input signal could be infinitely higher than the set threshold but the “compressor” limiter will only ever output a maximum signal amplitude of the threshold value.

Limiting is typically reserved for mixing and mastering as a safety measure to keep the output of a mix from passing beyond a certain value (0 dBFS in digital mixes, for example).

As we can imagine, overdoing it with a limiter can lead to significant distortion. As we go increase the amount of gain reduction with a limiter, our output signal becomes more and more like a square wave. This puts us in the realm of fuzz pedals, though fuzz is a bit far removed from standard compression.

The Boss LMB-3 (link to check the price on Amazon) is defined as a limiter though it’s really a compressor.

Boss LMB-3

The Electro-Harmonix Nano Big Muff Pi (link to check the price on Amazon) is a world-famous fuzz pedal.

Electro-Harmonix Nano Big Muff Pi Fuzz Pedal

Electro-Harmonix is featured in My New Microphone’s Top 11 Best Guitar/Bass Effects Pedal Brands To Know & Use.

Overdrive

Fuzz might be a bit extreme but compression can certainly give us an overdrive effect.

Overdrive pedals are designed to recreate the sound of overdriving tube amplifier circuits.

Overdriving a tube results in this nice soft distortion and saturation in the the signal. What does the overdriven tube actually do to the signal? You guessed it. It compresses it!

A slight compression like this is totally achievable with a standard compressor. Just like the tube, a compressor with a decent amount gain-reduction can offer nice subtle overdriven harmonics to the guitar or bass tone.

The Ibanez TS9 Tube Screamer (link to check the price on Amazon) is a famous, often-copied-but-never-duplicated overdrive pedal.

Ibanez TS9 Tube Screamer Overdrive Pedal

To learn more about overdrive and fuzz, check out my article Guitar Pedals: Boost Vs. Overdrive Vs. Distortion Vs. Fuzz.

Expander

The expander is related to the compressor by the fact the two are opposites.

While the compressor reduces the dynamic range by attentuating the signal above a set threshold, the expander increases the dynamic range by attnuating the signal below a set threshold.

We won’t typically see expander pedals in guitar or bass rigs but the effect is worth mentioning here.

The Electro-Harmonix Steel Leather (link to check the price on Amazon) is one of the few expander pedals on the market.

Electro-Harmonix Steel Leather

Noise Gate

A noise gate is to an expander what a limiter is to a compressor.

Noise gates will effectively mute a signal if it is to drop below a certain threshold. When dialed in properly, a noise gate pedal will allow the guitar signal to pass through when the instrument is being played and mute the signal when the guitar is not being played.

Noise gates are incredibly useful in noisier setups or when noisier pedals are engaged in the signal chain. Remember the fuzz effect we discussed earlier?

The ISP Technologies Decimator II (link to check the price on Amazon) is a great example of a standalone noise gate pedal. This simple pedal has a single knob to adjust the threshold. If the input signal drops below the threshold, it gets gated. Simple as that!

ISP Technologies Decimator II

To learn about all pedal types, check out my article The Full List & Description Of Guitar Pedal Types.

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Can I run my guitar through a bass pedal or vice versa? Stompbox effects pedal are typically designed to affect guitar and/or bass guitar signal. Some pedals are designed and marketed to the guitar or bass specifically but will certainly work with either though results may very slightly due to the differences in sonic characters and frequency/harmonic profiles between the two instruments.

Related article: Do Guitar Effects Pedals Work With Bass Guitar?

What does a flanger pedal do? Flanger is a modulation audio effect whereby a signal is duplicated and the phase of one copy is continuouly being shifted. This changing phase causes a sweeping comb filter effect where peaks and notches are produced in the frequency spectrum or the signal’s EQ. Flanger pedals produce this effect.

To learn more about flanger pedals, check out my article What Are Flanger Pedals (Guitar/Bass FX) & How Do They Work?

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