Understanding Compressors & How to Use Them

Compression can sometimes be like a dirty little secret guarded by a clandestine spy agency. But the cloak-and-dagger isn’t about compression itself. The act of manipulating an audio signal’s dynamic range is hardly a potentially world-shattering operation that requires James Bond, the Black Widow, or Jason Bourne to keep its mysteries under wraps.

The big secret is that a fair amount of content creators—even those who have produced massive hits—don’t actually know exactly what they are doing when they use compression.  Shocked? Well, here’s another shocker—the absence of a comprehensive knowledge of compression techniques and parameters isn’t necessarily a bad thing.

Huh? No, not that kind of compressor.

No, we haven’t forgotten this article is supposed to shed some light on using compressors. But, first, let’s absorb some wisdom from producer Joe Meek, who is celebrated as one of the most significant, innovative, and influential audio engineers of all time. Meek (1929-1967) was obsessed with crafting a unique sonic imprint for each of his record productions, and one of his mantras was, “If it sounds good, it is good.”

Where Meek’s phrase informs an untutored application of compression is that adjusting parameters without the slightest idea of what those actions are doing to an audio signal can be overlooked if you end up with the sound you want.

However, as fun and exciting as bashing around knobs (physical or virtual) until genius descends might be, it’s not much of a winning exercise if you’re looking for repeatable results. In addition, a basic understanding of what’s what may even enhance your creativity, as you can approach a specific audio challenge or artistic desire with technical precision. For example, you’ll know that doing X will result in generating Y. Furthermore, acquiring practical recording knowledge frees you from that “secret” club of audio Luddites hiding their ignorance of compression. Even better, learning about compressors really isn’t that hard at all.

What is Compression

Some musicians tend to define compression by their perceptions of what it does—that it makes things louder, or fuller, or thicker, and so on. Technically speaking, all of that is not really what a compressor is about. Its basic gig is actually very simple, because compression is like an “AI-driven” volume control. As more input signal comes in, the compressor automatically reduces the output volume. The musical effect is the gulf between a signal’s loudest and softest parts is reduced, which produces a more consistent overall volume.

It may seem like a relatively small thing, but automated dynamics control is an immense benefit. Say, for example, a vocalist wants to sing verses at a whisper, then explode into a scream for the choruses, but stay at mid voice for the bridge? Keeping all of the different volume levels of the vocal clear and upfront against the music track would require a nightmare of multiple fader moves. But, when set correctly, compression can level out those peaks and valleys, making it far easier to sit the vocal into the mix. Note that we said “set correctly,” which is yet another reason why it’s a good thing to learn compression parameters and controls.

However, compressors aren’t always used simply to reduce a signal’s dynamic range. Some vintage compressors—such as the Fairchild 670 (introduced in 1960), Teletronix LA-2A (1965), Urei/UA 1176 (1966), and dbx 160A (1971)—are also revered for the tonal coloration they impart on audio signals, and those characteristics are often emulated in modern plug-ins looking to deliver some analog-styled sonic magic within a digital medium.

Four Basic Compression Parameters

Whether you’re using a rackmount/tabletop compressor, a guitar pedal, or a compression plug-in on your DAW, you should notice some fundamental parameter controls. We say “should,” because not all compressors include the basics, some plug-ins may offer many more parameters (thanks to the capability of digital technology to increase a processor’s feature set), and mixers and other audio gear occasionally offer single-knob compression controls. While the diverse range of controls available on different compressors seem maddeningly tailored to make a newbie’s head explode, if you know the “fundamental four,” you’ll be able to work your way through almost any compression unit or plug-in.

Threshold. Simply put, the threshold setting tells the compression circuitry when to start compressing the input signal. The setting is typically marked in decibels, and anything below the selected dB is uneffected, while signal peaks higher than the dB setting are compressed. If you’re new to the compression game, it may be confusing to see a setting such as -10dB and know how it relates to a signal in a musical or practical sense. Here’s a tip: Watch the compressor’s gain-reduction meter as you adjust the threshold. If, for example, you wanted to tame a loud snare drum, the meter will reveal how much the snare is being compressed based on the threshold level. You should also use your ears to evaluate the desired compression, of course, but the meter provides a good peak into what a particular threshold level is doing to the signal being compressed. You can also consider that the lower you set the threshold level, the greater the gain reduction—or, simply put, the smoother and more even the signal’s dynamic range will be. Select a higher threshold level, and more of the signal will remain uncompressed, and only higher peaks will get attenuated.

Ratio. This is the amount of gain reduction applied to a signal once it exceeds the threshold level you selected. Again, it may seem strange to see values such as 2:1 and 4:1 indicated for this control, but the number system is actually straightforward. A ratio of 2:1, for instance, specifies that a signal exceeding the threshold by 2dB will be reduced by 1dB. If that same signal smashes over the threshold by 8dB, the 2:1 ratio will attenuate it by 4dB. Gain reduction increases as the first numeral gets higher. For example, 2:1 and 3:1 ratios produce a relatively mild amount of compression, 5:1 is more significant, 8:1 and 10:1 ratios deliver potent compression, and 20:1 and above is considered “limiting”—which means a signal is prevented from ever exceeding the threshold level. On the other hand, a ratio of 1:1 is basically no compression at all. That setting is called “unity gain,” because the signal is output at the exact same level it came in. Once again, the ongoing theme here is to use your ears to assess which compression ratio (and threshold) best realizes your production concept.

Attack Time. Another way to refine the effect of compression on a signal is to determine how long it takes for that signal to become totally compressed after it goes above the selected threshold level. Obviously, there are different sonic ramifications to having a signal reach full compression almost immediately, and having ultimate compression occur over time. Play around with the ranges. Slow attack times are in the 10 to 100 milliseconds range (delivering lots of transients and hard-hitting attack), and faster attack times are often designated as between 0ms and 5ms (producing less attack and more heft). It’s often a good idea to start with slower attack times and assess the result, as generally speaking, if a signal appears to have the life sucked out of it by being excessively compression, it’s due to an attack time that’s too fast.

Release Time. No surprise here, but release time is how long it takes for a compressed signal to return its original, uncompressed state after it drops back below the selected threshold level. Slower release times of 40ms or more are often recommended, as ultra-fast release times can bring on distortion, or a condition often called “pumping and breathing,” where the compressor quickly releases the signal, then rapidly compresses it again when it retriggers the threshold level, and it does this over and over—an occurrence that’s common when compressing bass guitars, kick drums, and other low-frequency sounds. That said, a fast release time (50ms-100ms) can tame annoying spiky transients. Look for medium release times (100ms-200ms) for a more musical and organic-sound control of dynamic range. Slow release times (more than 300ms) can add awesome sustain to legato guitar passages, caterwauling feedback, and other sounds you want to extend a bit.

Wait. There’s More…

When using compressors, you can certainly get around quite nicely with knowledge of the big four parameters alone, but it doesn’t hurt to expand your horizons with just a few more compression-setting definitions.

Knee. You’ll often see an option to select “hard knee” or “soft knee” compression, and this is simply how aggressively a signal is compressed after it passes the threshold setting. Hard knee is like triggering action star Jason Statham into defensive action—the signal is clamped down immediately upon reaching the threshold level. Pick hard knee if you need to crush aggressive transients, such as those of a barking snare drum. Choosing soft knee gets you caressed by bunny rabbits, so to speak, as the onset of compression occurs ever-so-gently over time. For example, if a guitar part is composed of two staccato stabs and a big strum, soft-knee compression should let the funky skanking through loud and clear, while adding some thick, smooth sustain to the strum.

Sidechain. Setting up a compression sidechain is kind of a power-user operation, but it’s often a valuable studio trick, so you should know the basics. A sidechain uses the signal level of an instrument or sound to control the compressor’s gain reduction. (The input isn’t the source sound—it’s a signal on the side. Get it?) Why would you want to do that? Well, let’s say your guitarist has overdubbed ten parts and the all-important lead vocal is being overwhelmed by distorted guitars. You could send the ten guitar tracks to a stereo bus with a compressor in line, and the vocal track to a sidechain, so that every time the vocalist sings, the guitar bus is reduced in volume. Take that! Or, perhaps the frequency range of a bass and a kick drum are too close, and you want more impact to the groove. Send the kick drum to a sidechain, and whenever the kick punches in, the level of the bass is reduced to free up some boom room for the bass drum. Many DAWs and plug-ins have sidechain capabilities already built in, so it can be very easy and convenient to explode this type of compression.

Makeup gain. Compression, by nature, is about gain reduction, so the output level of a compressed signal is usually lower than the level that went in. That’s kind of a bummer if you’ve tamed those signal peaks and reduced the dynamic range expressly so that you could present an overall louder sound without “going into the red” of your mixer’s output meters. Happily, most compressors include a gain, level, or output control to “make up the lost gain” and boost the signal to where you want it to be. If you don’t see an output control, the compressor may have an automatic makeup gain feature.

Compression Types

Yes, there is more than one way to tame a signal, and you’ll notice that compressors come in different varieties. A VCA (Voltage-Controlled Amplifier) compressor does its job via an integrated circuit. VCA models offer precision, predictability, and repeatability. FET (Field Effect) compressors rely on transistors. They work amazingly fast and are often prized for imposing gorgeous grit and vibey coloration to signals. Opto compressors are controlled by a light-dependent resistor and a light source. They sound lush and musical, and are not as swift and aggressive as FET compressors, nor as clean and pristine as a VCA type. Tube compressors are usually part of every recording engineer’s dream team, due to the aural nostalgia of vintage ’60s sounds. Even better, it takes a lot for a tube compressor to strip the energy from a mix, as it compresses signals slow and easy enough to allow crucial signal transients to go unchecked. Not as well-known as the previous four types, but still a contender, PWM (pulse width modulation) compressors use high-frequency pulses to control the amplitude of a signal. These models offer transparent sound, and their attack and release times are as quick as a SR-71 Blackbird jet clocking Mach 3.

Avoid the Danger Zone

As compression reduces dynamic range between soft and loud sounds, you may notice things that were barely audible in a signal before compression are now more front and center when you increase the overall volume of the compressed track. This can be a good thing if you want more of a boom on a kick drum, or increased room ambience on a guitar track. But it can be a bummer if you’re distracted by boosted amp hum, environmental noises, fret squeaks, pick and finger clicks, lip smacks, plosives, and other potential audio gremlins. It never hurts to solo a compressed track and take a critical listen from start to finish to ensure there’s nothing audible you don’t want heard.

You May Ask Yourself…

Does a track need compression? Sometimes, a wide dynamic range can deliver the absolute perfect expression for a part. Crush that signal down, and something beautiful may be lost. Always listen critically and make processing decisions that truly improve musical communication and impact. There is no “have to” in music production, so don’t compress a signal simply for the sake of compressing it, or because you read somewhere that electric-bass tracks should always be compressed, or some other such nonsense. Always find an explicit purpose for inviting compression to the party.

Diving for Dynamics

Ready to dig in? Experimentation is half the fun of signal processing, so don’t be afraid to break rules, jump into the abyss, or do something another creator might consider irrational or just plain dumb. If you get to something that blows your mind, document the settings for your home-studio trick bag, but remember—just because something worked on a particular instrument in a specific mix at a certain time, does not mean it’s going to always provide the exact same sound on a different day. That volatile unpredictability in the studio—often caused by almost imperceptible changes in gear settings, or the performance idiosyncrasies of different artists, or the distinctive sonic signatures of various instruments, or perhaps even the position of the moon on certain nights—can serve to fire up your creativity. If you let it. Always be ready to accept the “surprise gifts” that drop into your lap as you’re chasing sounds.

Whether you eventually toss out convention or not, however, it helps to start out with some basic case studies so you’re not working completely in a web of oblivion. Here are some very rudimentary settings to get you on your way.

Vocals. Typically, you want a lead vocal beautifully upfront and soaring over the instrumental track to ensure lyrical intelligibility—and the artist’s emotional expression—impacts listeners. However, there isn’t a magic compression setting that works for all singers and all styles. There’s simply too much dynamic diversity between vocal techniques, the vocal arrangement (loud and soft passages), different stylists (rock, jazz, rap, etc.), and even the music track (some musical sections may be interfering with hearing the vocal clearly more than others). To get a vocal upfront, try starting with a -9dB threshold, a 2:1 ratio, an attack time of 1ms or less, and a release time of approximately 50ms.

Electric Guitar. Overdriven and distorted guitars already have some compression in effect, due to the nature of saturation—whether the grind is from a tube or solid-state amp, a stompbox, or a digital emulation. But guitars still need to present right in your face—even when clean toned—so try these settings to get the party started: threshold at -3dB, ratio between 2:1 and 4:1, attack at around 25ms, and release set at about 200ms.

Acoustic Guitar. Getting fingerpicked, single-note melodies to appear smooth and balanced may require compression, of course, but even strummed rhythm parts can benefit from sounding more “blended together” (all strings displaying an even, sustained attack) and shimmery. Try a threshold of -6dB, a ratio of 3:1 or 4:1, a 150ms attack, and a 400ms release.

Bass. A great bass line should anchor a song, keep the groove percolating, and glue the ensemble together, so you don’t want the low end wavering dynamically, or not sounding rich and warm. Keep the bass levels even by opening with settings of a -4dB threshold, a ratio of 3:1, attack of 40ms, and a release of 180ms. Remember, as the dynamic range is reduced by compressing the bass, when you turn the bass level back up in the mix, you may notice muddiness, boominess, fret noises, and other less-than-stellar sounds you may not have noticed before adding compression. You may need to deploy EQ or other processing to get the signals sounding marvelous.

Drums. Unless you’re recording cafe jazz, drums should explode out of the playback speakers. You can use fairly aggressive compression settings to bring on the wallop, but the challenge is that reducing the dynamic gap between loud and soft drum sounds may bring a whole lot of signal bleed into the forefront of your drum tracks. Potential problems may arise from now having too much snare bleeding into the kick-mic track, hi-hats bleeding into the snare, overhead cymbals bleeding into everything, and so on. One trick is to send the kick, snare, hi-hat, toms, and overheads to a separate stereo bus on your mixer. Now, lightly compress or totally crush the stereo drum track—your choice or try both and everything in-between—and fade the compressed sounds back into the main mix along with the (uncompressed) individual drum tracks. Now, you have the organic drum tracks and a compressed stereo version to blend together—much like determining a wet/dry mix on a reverb—so it becomes a bit of a “have your cake and eat it too” scenario. Alternately, you can easily compress the individual drum tracks. Start each piece of the kit at a threshold of -4dB, a ratio of between 4:1 and 8:1, a 5ms to 15ms attack, and a 50ms to 200ms release. You may want to back off the overheads a tad, as every part of the drum set bleeds into those mics and compression will bring them all together into a percussion pie. Perhaps try a threshold of -2dB, a 2:1 ratio, a 20ms attack, and a 300ms or higher release.

Mastering. Successfully mastering audio requires big ears, a ton of experience, and a dollop or two of taste. You can find scores of articles online about common self-mastering mistakes, and I don’t think they are written by audio professionals looking for mastering gigs to scare home-studio musicians away from a DIY approach. Those pieces simply illustrate how hard it is to do mastering well, and why good tracking engineers are not always awesome mastering technicians. So, perhaps think twice about mastering your next CD, vinyl, or streaming project. But if you want to get a music production mastered for playback on YouTube, TikTok, Instagram, and other social networks, then start with light compression. You want the track to sound loud and proud on laptops, mobile devices, and earbuds, but you don’t want the music to appear dull, lifeless, or thick. Begin with a -2dB threshold and a 2:1 ratio. Rapid attacks tend to make a stereo track smooth and relatively natural, while slower attacks bring on punch and impact. Release times of 300ms and up can help prevent listeners from hearing the compression pump and breathe—something you very much don’t want in a stereo master track for public consumption.

A Word About Effortless Compression

If you’re into high-end, pop-up-style restaurants created by Michelin-starred chefs, you’ve likely heard the phrase, “Trust the cook”—which translates to “let the chef curate your meal, sit back, and enjoy the feast.” Using a compressor can sometimes be managed in much the same way, as some mixers and preamps offer a one-knob compression control. Here, you are letting the equipment manufacturer determine the appropriate settings, and all you need to do is turn the knob to select less or more of what they’ve devised for you.

We’ve done that very thing in our Fishman Platinum Pro EQ/DI, Aura Spectrum DI, and ToneDEQ preamps. Our engineers developed an analog, soft-knee compressor optimized for acoustic guitars that smooths out performance dynamics and ensures every note rings out clear and distinctly.

There are times—such as during live performances—when simply turning one control to get a near-perfect sound is an excellent way to relieve that hurry-up-and-get-on-stage stress and get your head in the right place to deliver a stunning show. So, while you’ve hopefully learned some practical knowledge about compression in this article, we’re more than happy to do the job for you when you’re using one of our acoustic preamps.

Make Your Mood

There are obvious technical and practical reasons to deploy compression. Compression can fix or alleviate audio problems, clarify performances, tame wild dynamic shifts, and ensure your recordings sound professional. But don’t get so wrapped up in tech operations that you miss the joy of creating vibes, surprises, and atmospheres with different compressors.

There are so many compression plug-ins available that you can experiment with a galaxy of vintage and modern options. Look for variations in the plug-ins at hand, as some compressors are super transparent no matter how hard you hit a signal, while others can be used to add meat and grit. Try different models on guitars, vocals, drums, bass, and other instruments—don’t just use one compressor on everything, as you may miss out on transforming certain tracks into something magical. Oh, and remember our mantra: Trust your ears!