Swirl, Woosh, and Warble: Everything You Need to Know About Modulation Pedals

Effects pedals have been a staple in touring rigs and studios for as long as they’ve been around. From Jimi Hendrix’s mind-bending use of modulation to today’s musicians who have perfected stacking pedals and are constantly discovering new sounds, effects pedals have changed the music landscape. To re-create the sounds of the studio, bands tour with pedalboards full of effects that can include delays, reverbs, distortions, compressors, and more. And for a beginner, those names can tell you a lot about what each effect does. For instance, a delay pedal makes copies of the input signal and repeats them for as long as you want or as long as the pedal is able. But what about the effects that aren’t as obvious as their names suggest? Modulation pedals may seem self-explanatory, but what is being modulated can vary based on the effect type.

In this article, I will dive deeply into the types of modulation effects, what they are modulating, and how the effects came to be. I have also added diagrams to further help you understand what the different modulation effects are doing, and I have included songs that use the effects. You may want to strap yourself in for this one because it’s about to get a little wobbly.

What Is a Modulation Effect?

Let’s start by explaining what modulation is as it relates to sound and music. The term “modulation” means “change,” and in music technology, it usually applies to a control signal being used to change an aspect or parameter of another signal. This can confuse people who are learning about effects because the vast category of modulation pedals covers various modulation effects that change different parameters. Modulation can encompass something as simple as changing the output volume of a guitar or, on the other hand, as complex as combining two signals with the output being their sum and difference while subtracting the original frequencies. Confusing, right? That is why modulation effects can be hard to describe, leaving many musicians to explain how the effects sound by reenacting the sounds with weird noises.

The Big Four of Modulation Effects

When talking about modulation effects, four effect types are mentioned before any other: tremolo, phaser, chorus, and flanger. These are widely regarded as the most popular modulation effects. But what is going on under the hoods of these effects, how do they sound, and how did they come to be? Don’t fret — I am here to answer all those questions and more.

Tremolo Pedals

First introduced in 1948 in a pedal from DeArmond, the tremolo pedal is widely considered the first modulation effect. Tremolo is the modulation of the amplitude of a sound using an LFO (low-frequency oscillator), which can result in a sound ranging from a slow and subtle change in volume to full-on psychedelic, choppy, and rhythmic sounds. Tremolo was a popular effect in the ’50s and ’60s due to the rise of Fender and Vox amps that had built-in tremolo circuits. But by the ’70s and ’80s, the effect had been forgotten, and other modulation effects such as chorus and phaser rose to become part of the mainstream sound. Regardless, tremolo is still an essential tool for musicians in modern music and can be heard in many genres ranging from blues to electronic dance music.

Tremolo pedals have two main control parameters: intensity (the depth to which the LFO will modulate the amplitude) and rate (the speed at which the LFO will modulate the amplitude). Many modern tremolo pedals may also feature a tap-tempo footswitch or an expression jack so you can adjust the rate without bending down and turning knobs.

The LFO shape is important when choosing a tremolo pedal because it dictates how the modulation will sound. There are three main LFO waveshapes found in tremolo pedals: sine, triangle, and square. As you see in the diagram below, their names correspond with the shapes of the waveforms.

The sine wave is the most common and gives you a classic tremolo as found on early tube amps. Triangle is a more sudden rise-and-fall tremolo with more pronounced peaks and slopes. Square is the most extreme of the three; its sound is closer to an on-off switch when the rate is turned up. Many avant-garde musicians prefer the square for how choppy it can get.

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Phaser Pedals

Phase shifting was first introduced in the mid-’60s as a studio trick that helped start the psychedelic movement. George Martin and Eddie Kramer both used phase shifting in their work with the Beatles and Jimi Hendrix, respectively.

Phase shifting is passing an input signal through a series of all-pass filters, which maintain the volume of the signal but alter its phase by varying amounts based on frequency. A number of these filters are put in series, called “stages” — which is why we have 4-stage phase shifters, 8-stage phase shifters, etc. By combining the phase-altered signal with the dry signal, you get peaks and cancellations in the audio output. By modulating the filters, you can get sweep effects. Although you can measure the depth of the phasing in milliseconds, the best way to think about it is in degrees. Using degrees as a measurement allows you to accurately determine how far out of phase the processed signal is relative to the dry signal.

With the popularity of phase shifting in the studio came the need to replicate the sound live in front of an audience. In 1967, a Japanese company named Shin-ei created the Uni-Vibe pedal — we will talk about this unit’s effect later, which will pave the way for phaser pedals. Rumor has it that the audio engineer behind the Uni-Vibe was trying to make a tremolo pedal but instead made a unique pedal that made its way into the hands of none other than Jimi Hendrix, who notably used it when performing “The Star-spangled Banner” at Woodstock.

Fast-forward to 1974, and a new company by the name of MXR releases a groundbreaking pedal: the Phase 90. The Phase 90 was ahead of its time not only in terms of the sound but also in terms of the size and control. This pedal only featured a rate control, giving musicians more time to play and requiring less time to worry about parameters. Before this, phaser effects were rackmount units or bulky, heavy pedals, but the MXR Phase 90 featured a small enclosure, making it easy to travel with. While the design was great, what made the Phase 90 revolutionary was its sound. Phaser units before it featured a depth control that allowed you to modulate the delay time between the dry and processed signals. The Phase 90 ditched the depth control and instead made the input signal automatically go out of phase by 90 degrees.

Guitarists from almost every genre jumped on the opportunity to play with an MXR Phase 90 as part of their rigs, including David Gilmour, Robin Trower, Jimmy Page, and more. But no one embodied the phaser pedal more than guitar virtuoso Eddie Van Halen, who was synonymous with using phasers. No song encompassed the sweeping sound better than Van Halen’s “Ain’t Talkin’ ’bout Love.”

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Chorus Pedals

The chorus effect is one of the oldest forms of modulation in music, its first introduction being a Hammond organ in the ’30s. However, the effect didn’t become popular until the late ’60s and early ’70s. The chorus effect is achieved by duplicating your signal, delaying the wet signal, and then modulating the pitch of the delayed signals. Chorus is usually classified as having a delay time between 15ms and 35ms, but those delay times vary depending on the pedal manufacturer and whether it has digital or analog circuitry. Chorus pedals typically feature two or three controls: depth, rate, and sometimes a mix knob. The combination of fast delay times and modulating pitch results in a sound that can range from a rich dream to seasickness with the depth and rate cranked up.

As stated above, chorus didn’t become popular until the late 1960s. George Martin and the Beatles can be credited with creating a studio version of a chorus effect during the recording of Abbey Road when they were trying to simulate the sound of multiple singers and instruments. From there, studios and musicians raced to re-create their own versions of the chorus effect.

In 1974, Roland released a groundbreaking amplifier with a built-in chorus circuit called the Jazz Chorus. The popularity of the amp was so great that, in 1976, BOSS took the chorus circuitry from the Jazz Chorus and introduced the first chorus pedal with the Chorus Ensemble (CE-1). The CE-1 changed the pedal market, and manufacturers flooded the market with other classic chorus effects, such as the TC Electronic Stereo Chorus+ and the Electro-Harmonix Small Clone. To this day, chorus is one of the most popular modulation effects, and chorus pedals are found on the boards of John Mayer, John Frusciante, and more.

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Flanger Pedals

Last but certainly not least is the flanger effect. Flanging is the process of recording the same signal on two tapes, playing them together, and then putting pressure on one of the reel flanges, thus briefly slowing down one of the machines. The short timing discrepancies result in a very pronounced comb-filter effect (see the flanger diagram below). Flanger pedals and effects units feature the greatest number of controls out of the modulation effects I have mentioned so far. The most common controls you will find on a flanger unit are manual, feedback, width, rate, mix, and regeneration. The sound is often described as “swooshing” and compared to a jet plane passing by you. A handful of manufacturers have gone as far as to feature footswitches called “Jet Plane” that give you that signature cranked regeneration and resonance flanger sound.

While the Beatles and George Martin were experimenting with different ways to create modulation, Eddie Kramer and Jimi Hendrix were taking things to a whole new level. During the recording of Electric Ladyland, Jimi wanted to do whatever it took to sound different and stand out from other musicians at that time. Kramer, being the studio genius he is, took that and decided to do whatever he could to make Jimi’s record stand out. Kramer implemented several techniques to create trippy sounds; it was the first recording to use stereo phase shifting. Another notable studio technique was to use multiple tape machines and change the Varispeed control to slow down one of the tapes. This technique can be heard on Hendrix’s “Voodoo Child (Slight Return)” as the whole track is flanged.

Until 1975, flanging was only an effect that could be achieved in the studio due to the amount of work and the equipment it took to create the sound. But that changed with the creation of the Eventide Instant Flanger rack unit, which brought flanging to a new era of musicians who could now produce that sound independently. MXR and Electro-Harmonix came out with versions of the flanger not long after, respectively releasing the MX-117 in 1976 and the Electric Mistress in 1977. Today, you can find more flanger pedals than you can imagine, all of which push the flanging sound to new heights and can cover effects from subtle flanging to crazy jet sweeps that make you feel like you’re about to take off.

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The Forgotten and Overlooked Modulation Effects

While phasers, tremolos, flangers, and choruses are common on pedalboards, a handful of other modulation effects exist that are not as well known. While they aren’t as popular as the big four, these modulation effects are great and can make you stand out among the hordes of other guitarists out there right now.

Uni-Vibe Pedals

As I mentioned earlier, the Uni-Vibe is considered the first phaser pedal, but to me and many others, the Uni-Vibe is far more than a simple phaser. And though the original Shin-ei Uni-Vibe features a chorus switch, there is no chorus effect going on. It is commonly, and incorrectly, held that this effect was created to simulate a Leslie speaker, but actually the designer was trying to replicate vexing radio modulations he was hearing. The effect does push your signal in and out of phase like a phaser, but where the Uni-Vibe differs from phaser pedals is its circuitry. The Uni-Vibe has a small light bulb in it that activates the LFO. The light bulb’s brightness fluctuates depending on the rate parameter, giving the Uni-Vibe an organic, rising and falling phase-shifting sound.

Many manufacturers have made versions of the Uni-Vibe that offer an array of different features, such as adding a tap-tempo footswitch, which I am a big fan of. One of my favorite Uni-Vibes is the JHS Unicorn V2. The Unicorn features a tap-tempo footswitch, giving musicians a way to dial in the rate without bending down and manually twisting the control.

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Vibrato Pedals

The vibrato effect is one of the most misunderstood modulation effects, often being confused with chorus or tremolo. Where the chorus effect duplicates the dry signal and modulates the pitch of the duplicated signal, vibrato modulates the pitch of the signal without duplicating it.

Before you could grab a vibrato pedal at your favorite music store, you could find the effect in early Magnatone amps, such as the Magnatone Custom 280. While vibrato pedals weren’t a common pedal to find on pedalboards, they did have a resurgence in the late 2010s and early 2020s, such as with Mac DeMarco, an artist who spawned a new era of DIY artists who heavily use vibrato on keyboards and guitars. Artists now use vibrato as a subtle effect that causes an uneasy or seasick feeling. Although it may sound nauseating, it is surprisingly soothing and gives instruments a sense of movement.

Many guitarists confuse the terms “vibrato” and “tremolo” — and I can’t blame them. Guitar manufacturers refer to whammy bars as tremolo arms, which is incorrect. The term “tremolo” in music technology refers to the modulation of volume, so instead of a tremolo arm/bar, we should be referring to it as a vibrato arm/bar since pushing or pulling the bar changes the pitch of the notes.

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Ring Modulator Pedals

Out of all the modulation effects and pedals in this article, ring modulation may be the craziest of the bunch. In my opinion, the ring modulation effect is the dark horse of effects, as many people try it and quickly realize how hard it is to use in a musical aspect. But the ones who do tame this beast end up creating unique and wonky sounds that are hard to replicate. A ring modulator combines two audio signals before outputting their sum and difference. For example, if you input a 1,000Hz wave and a 750Hz wave, you get the sum and difference out: 1,750Hz and 250Hz. These are called “sidebands.” It’s one thing with steady frequencies because you hear drones at different frequencies, but when ring modulating any moving or changing frequencies, you’re going to get wild, constantly changing, nonharmonic, and nearly unpredictable sum/difference output. Depending on what is ring modulating what, you can also get distortion-like effects or, with vocals, a robot-voice-type effect. There are myriad possibilities with this seemingly simple and crazy effect! To learn more, check out “A Simple Guide to Modulation: Ring Mod,” an inSync article that takes a deep dive into what ring modulation is and how to create popular ring-mod sounds. A popular example of a ring modulator in use is Incubus’s “Glass,” which sent a wave of musicians surging to get their hands on a DOD Gonkulator.

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Rotary Speaker Emulator Pedals

Leslie rotary speakers were first released in 1941 and originally made to enhance the sound of Donald Leslie’s Hammond organ. The rotary speaker creates its signature sound by using a rotating drum with a slot in it that spins around a fixed speaker for low frequencies and a spinning horn for the treble/high frequencies. These spin at different rates and don’t speed up or slow down in unison, resulting in a very complex, rich effect. At slow speeds, it can sound like a double phaser or a slow chorus. At faster speeds, it produces a cross between tremolo and chorus. Because there is a combination of volume modulation and physical soundwave changes (as the drum/speaker rotate away and toward you), you get a Doppler effect as the sound waves are pushed and pulled.

While the Leslie was the first rotary speaker, other manufacturers jumped on the chance to make their own; Fender produced the Vibratone rotating speaker cabinet from ’67 to ’72. Many artists, including George Harrison, Cream, and David Wright, have used the rotary speaker effect. Check out Cream’s use of it in their song “Badge.” The rotary speaker effect comes in at the 1:06 mark.

If you have ever seen a Leslie rotary speaker, then you know it is impractical to carry around on tour. It is a big cabinet with fragile parts, making it a hassle to move around. By the late ’60s, guitar-pedal manufacturers saw the need to create a pedal-size version of the rotary speaker, which allowed touring acts to re-create the sound without carrying around a cabinet. Today’s rotary speaker emulation pedals give you a great deal of controls, such as mic distance, horn level, and more, to get you as close as possible to the original Leslie rotary speaker for realistic rotary-speaker sounds.

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Envelope Filter Pedals

Although envelope filter pedals are synonymous with genres such as funk and R&B, other genres don’t utilize the effect to the same degree. The effect can be described as an autowah sound with a filter sweeping across the frequency spectrum. But unlike a wah pedal, which you have to manually modulate the filters by moving your foot up and down on an expression pedal, an envelope filter uses different control parameters based on the input signal to determine how much it sweeps across the frequency spectrum.

The way the envelope filter sounds is based on the intensity of how hard or soft you are hitting the strings. Soft picking will make the filter react slowly, while hitting the strings hard gives the envelope filter its signature pseudo-wah sound. Grateful Dead, John Mayer, Bootsy Collins, and many others are masters of the envelope-filter effect. In John Mayer’s “Wildfire,” you can hear him using an Electro-Harmonix Q-Tron envelope filter pedal during the solo.

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Multi-modulation Effects Pedals

Multi-effects pedals are nothing new and have been around since 1987, when BOSS introduced the GP-8. But we are talking about modulation effects, and creating a single box that can do multiple types of modulation effects — usually in the digital domain — is no easy task. As pedalboards have become more and more popular, pedalboard real estate and conserving space for only the essentials have become important, as well. Luckily for us, some of the smartest people on earth have created multi-modulation pedals in small enclosures, giving us our favorite effects in a pedalboard-friendly size. Companies such as Strymon, BOSS, and Universal Audio have released a few of the best multi-modulation effects pedals to date with the Mobius, MD-500, and Astra, respectively.

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Other Effects Pedals That Feature Modulation

Modulation can be found in pedals that aren’t specifically modulation pedals. Many time-based effects, such as reverb and delay, give users the option to add modulation to their wet signals, thus giving movement to their reverb or delay tails. The modulation typically used in time-based effects is a lush, slow-moving chorus with a preset rate. You can find pedals that feature other modulation types, such as flanged reverbs. Adding modulation to your reverbs and delays can spark creativity and add depth to your sound in such a way that normal time-based effects can’t. The TC Electronic Hall of Fame 2, Strymon Flint V2[HC1] , and Keeley Caverns V2 are great options to check out if you want to add modulation to your reverb and/or delay tones.

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Modulation is less commonly found in overdrive, distortion, and fuzz pedals. While modulation and distorted guitar tones are as old as rock ’n’ roll music, a two-in-one pedal hasn’t been as popular. The number of modulation types that can be paired with overdrive, distortion, and fuzz tones is immense, leading to creative soundscapes and interesting ear-candy bits that instantly engage and turn heads. One of my favorite modulation fuzz pedals is the Walrus Audio Janus, which features a tremolo and fuzz circuit. Plus, the Janus has two joysticks to control parameters. Talk about innovation!

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What Is Your Favorite Modulation Effect?

As you can see, there are numerous modulation effects pedals to choose from, and finding the right one for you can take time. But that’s the wonder of being a musician — finding a sound that opens doors to new soundscapes and ideas for you to play. We’d love to hear your favorite modulation pedal and why it became your go-to. If you have any questions about modulation pedals and which one would be great for you or if you want to learn about our wide selection of guitar pedals, then reach out to your knowledgeable Sweetwater Sales Engineer at (800) 222-4700. They’re always happy to help find the right gear for you!

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