Effects Basics: Compressors, Limiters, Expanders, Noise Gates, & Sustainers

Tech Tip: Effects Basics—Part 4

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Learn what compressors, limiters, expanders, noise gates and sustain effects do and how to use them to produce better sound onstage and in the studio

So far we’ve learned about bass and guitar effects that are hard to miss. From mile-wide reverbs to filth-laden distortion stomps, there’s nothing subtle about such effects when they’re laid on thick.

In this final instalment we’ll be looking at compressors and their relatives, sustain effects, limiters, expanders and noise gates. Used well, they can have an enormous impact on your sound without calling attention to themselves.

But before we jump in, here’s what we’ve covered in our Effects Basics so far for those who need to catch up:

In Part 1 we explored time-based effects like reverb, delay and echo. Part 2 delved into modulation effects such as tremolo, phasers, flangers and chorus pedals. In Part 3 we discussed overdrive, distortion, boost and fuzz effects.

What does a compressor do anyway?

In a nutshell, a compressor reduces the dynamic range of audio so that the quietest and loudest parts are closer together volume-wise. This is achieved by reducing the loudest signals and boosting the quietest signals. While the concept is simple in theory, applying compression to enhance sound is more subtle and takes practice and careful listening. In a future tech tip we’ll help demystify that process, but for now, let’s look at the controls you’ll typically find on compressors:

Threshold: This determines how loud a signal must be before compression is applied.

Ratio: This establishes how much compression is applied. For instance, a ratio control set to 5:1 means the compressor will only kick in once the input signal crosses a threshold of 5dB and will then increase the output signal level by 1dB. Ratio-setting controls are sometimes labeled as “Slope.”

Attack: This determines how quickly the compressor begins acting on a signal once it has reached the threshold level.

Release: This determines how quickly the compressor stops acting on the signal after it has dropped below the threshold. Both attack and release can have a big impact on tone and are very commonly applied on drums to control their sustain.

Hard Knee/Soft Knee: You can think of these settings, which work in tandem with Attack and Release, like an actual knee. A hard knee setting resembles the steep angle of your knee in a kneeling position. It compresses the signal practically instantaneously once the threshold is crossed. A soft knee setting has a more gentle angle slowing down the speed at which compression is applied. Generally the lower the settings for attack, release, and knee, the more natural the sound.

Makeup Gain: This control allows you to add gain back into the signal after compression. Depending on the compressor, this gain may be applied automatically or dialed in by the user.

The impact of compression

Let’s put the action of compression in more visual terms. Think of the signal from your guitar’s pickups as a series of waves representing the highest and lowest volume it generates—in other words the guitar’s dynamics. The upper horizontal line represents the highest level your signal can reach before clipping (distortion) occurs. We'll call this the ceiling. The lower line represents the quietest or lowest part of your signal that is either inaudible or is drowned out by the rest of the band. We'll call this the floor.

Compressed Inaudible Signal

By applying the right compression parameters to the above signal, you can eliminate clipping of the louder parts while increasing the gain of the quieter parts so they can be heard in the mix. The signal would now look more like this:

Compressed Audible Signal

Compression can be used on any single instrument or on the mix as a whole. Many bands and studios use compression on vocals because of the dynamic range of the human voice. Very high notes take more power to belt out and will sound louder than low notes that sound softer because of the lack of power used to produce them. Using a compressor will give vocals a more consistent range of volume.

Compression also works well for live applications where the singer is constantly moving his or her head toward and away from the microphone. Bass players use compression to make smoother-sounding transitions between notes.

And as noted above, compression is very frequently used to control the wide ranging dynamics of a drum kit. By setting the right parameters, the distortion-inducing impact of the kick drum can be better balanced with more subtle sounds such as brushes on a snare or cymbal.

Since the focus of this series is on those effects readily accessible to the guitarist or bassists, let’s take a look at couple of compressor stompboxes designed for that purpose.

The BOSS CS-3 Compression Sustainer Pedal, as the name implies, offers both functions in a single pedal. Though it may lack the control sophistication of studio compressors, it has the advantage of being tailored for use with guitars. Unlike the control options of a studio compressor, the Level, Tone, Attack and Sustain knobs aim to help the guitarist dial in a pleasing sound. It works with your amp and guitar pickups’ sonic assets and liabilities to produce smoother more even tone across all registers. For recording, the CS-3 can help balance bass and treble response to create a tighter, more modern sound.


Though its impact is fairly subtle, try switching the BOSS CS-3 in and out of your signal chain to appreciate its impact on clarity and punch.

The Wampler Ego Compressor is designed to overcome the objections many guitarists have where compression is concerned: an overall dull sound plus a lack of picking dynamics and expression. It can provide the “squished, quacky” sound beloved by country guitarists while letting factors such as pick attack shine through. A Blend knob allows you to find the sweet spot between compression and dynamics. Used in conjunction with your guitar and amp’s tone controls, you can rein in dynamics for smooth-sounding recording while maintaining sparkle and punch.


Effects designer Brian Wampler puts his Ego Compressor through its paces, demonstrating how its unique control surface balances dynamics compression with playing expression.

You can explore the entire Musician’s Friend selection of compression and sustain effects pedals here.

Expanders—The flip side of compression

Expanders extend your dynamic range in both directions making loud sounds louder and quiet sounds quieter. This comes in handy when you want to bring out subtle fingerpicking licks while diminishing the noise of fingertips squeaking on your strings. With vocals, an expander can open up nuances in the singer’s technique while keeping more subtle sounds such as breathing to a minimum. Used carefully, an expander can hold down background sounds in your mix without negatively impacting the more subtle musical passages you want to capture.

Pedal-based expanders such as the Electro-Harmonix Steel Leather Bass Expander are popular with bassists who want a lot of attack in their playing for producing percussive effects.

Limiters—Keeping a lid on volume

Limiters are quite similar to compressors. Unlike a compressor however, a limiter completely cuts the signal off at a preset threshold. They can prevent your sound system from hitting levels that will damage speakers. Limiters will not let your signal go above the maximum level you set. So unlike a compressor, the limiter only deals with the louder levels of your signal.

The BOSS LMB-3 Bass Limiter Enhancer incorporates compressor-like controls including Threshold and Ratio knobs to help bassists fine-tune their overall sound. Its Enhance control allows you to dial in more punch that’s ideal for slap- and funk-style bass work.


The BOSS LMB-3 reins in the extreme dynamics of the electric bass while producing plenty of presence for punching through the mix without overloading your sound system.

Beyond pedal-based effects, you’ll find a large selection of rackmount compressors and limiters at Musician’s Friend. They have the advantage of allowing you to manage the dynamics of all your instruments including drums as well as vocals.

Gates—Sonic fence posts to keep your mix clean

A noise gate works like a limiter but at the other end of the signal’s dynamic spectrum. A gate cuts the signal off below a set level. Unlike a compressor that increases lower level sounds, a gate eliminates those sounds completely. Like an expander, it is used to keep unwanted background noise out of the mix.

One very common use of gates is in miking drums. When drums are miked individually, you don’t want adjacent drums sounds bleeding into the wrong microphone. The gate cuts off low-level signals bleeding over from other drums and thus helps clean up the mix. This is effective in keeping the cymbal sounds from bleeding into the tom tracks. It is used most dynamically on the snare drum mic and hi-hat mic because of their close proximity.

Gates work well with guitar for eliminating the hiss and unwanted noise heard when the instrument is not being played. Guitarists with elaborate signal chains often use a gate at the end closest to the amp or recording console to eliminate unwanted noise introduced by the various effects in the chain.

Explore the entire range of noise gate and suppressor pedals here.

A word about pedal order

Where you place compressors, limiters, expanders and noise gates in your signal chain can have a big impact on their performance and how they interact with your other effects.


We recently prevailed on shredder extraordinaire Steve Vai to share his insights on perfect pedal order.

Want to delve deeper into effects? Our Guitar and Bass Effects Buying Guide looks at the entire spectrum of stompboxes and effects multiprocessors.

Tags: Electric Guitars


# CL 2015-05-31 17:22
# CL 2015-05-31 17:21
Recheck the ratio definition. 5:1 has nothing to do with the threshold. A ratio of 1:5 means all sound levels above the threshold are multiplied by 1/5 and thus reduced or compressed above the threshold.
# Marshall B 2015-01-15 03:29
Thank You Guys For This Video. Learned more in one hour about setting up a rig than in 20years of "playing around".

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