What is tremolo?

Tremolo in plain English

Put your hand on a volume fader and wiggle it up and down a few times a second while a chord rings out. That's tremolo. The chord keeps its pitch and its timing; it just gets louder and quieter on a regular cycle, and your ear hears that as a pulse or a throb.

A tremolo effect does the wiggling for you with an LFO, a wave that moves too slowly to hear as a tone, and here its only job is to set the volume from moment to moment. Every tremolo, from the circuit in an old guitar amp to the one in your DAW, comes down to a few choices about that wave: how fast it goes, how far it drops the volume, what shape it is, and whether the left and right speakers follow it together.

Because only the level moves, tremolo is the simplest of the modulation effects to understand. Chorus, flangers and phasers all mix in a delayed or filtered copy of the sound, which chorus vs flanger vs phaser covers. Tremolo has no copy at all.

Tremolo, vibrato and the Fender mix-up

In music, vibrato means a wobble in pitch and tremolo a wobble in volume. Guitar gear has had the two the wrong way round for decades. Fender's 1960s amps, the Deluxe Reverb and Twin Reverb among them, have a channel labelled Vibrato with Speed and Intensity knobs, and what it does is tremolo. The Stratocaster's tremolo arm does the opposite job: pressing it slackens the strings and bends the pitch, so it's really a vibrato.

Classical players use the word differently again. For a string player, tremolo means moving the bow back and forth as fast as possible on one note, so the note shimmers. It's a playing technique, though you can hear the family resemblance to the effect.

On this page, tremolo means the volume effect. The lab has a Vibrato card so you can hear the difference for yourself, and tremolo vs vibrato compares the two side by side.

Reading the lab

The lab opens on a chords loop at 120 BPM, because a rate is easier to judge against a steady beat. Dry is the loop untouched, Tremolo moves the volume and Vibrato moves the pitch. Each card keeps its own settings, so you can flip between them without losing anything.

The screen underneath on the left is the LFO, drawn as the volume it's setting from 0% to 100%, with a dot at the moment you're hearing. The wide screen at the top shows the whole loop, time left to right and frequency up the side. A tremolo darkens every frequency at once and then lets them all back, so it draws vertical stripes, where a filter sweep would draw a band that moves up and down.

The panel on the right describes your settings with numbers measured on the loop itself, including how far the volume dips in decibels (or Silent, when it dips to nothing) and the correlation between the two speakers. Every card plays at the same loudness as the dry loop. A deep tremolo makes a sound quieter on average, and without that matching the dry version would win every comparison by being louder.

The Shape buttons under the cards switch the tremolo between a sine, a triangle and a square, and the figure further down adds the ramp. You can swap the chords for your own sound by uploading a file or recording eight seconds, and it never leaves your device.

The controls

Rate

Rate is how many times a second the volume goes down and back up, in hertz. At 1 Hz you get one slow swell a second. At 5 Hz the sound throbs, which is roughly where a lot of guitar amp tremolos sit. Somewhere past 10 Hz the pulses blur into a flutter, because your ear can no longer follow each one as a separate event.

In their book Psychoacoustics, Hugo Fastl and Eberhard Zwicker report that listeners hear the strongest sense of fluctuation when the volume moves about four times a second, with the effect falling away at slower and faster rates. That fits with tremolo between 3 and 6 Hz sounding so obvious, and with a slow 0.5 Hz swell almost passing unnoticed under a busy mix.

Depth

Depth, sometimes called intensity, sets how far the volume falls at the bottom of each cycle. In the lab it's a straight percentage of the level, so a 50% depth halves the level at the bottom of each dip and 100% takes it all the way to silence. Halving the level is a 6 dB drop.

Plug-ins don't all scale depth the same way. Some measure it in decibels, and some only reach silence at the very top of the knob, so the same percentage can sound different from one tremolo to the next. Trust your ears more than the number on the knob.

Mix

The lab has a mix control because the other modulation effects need one, but on a tremolo it does almost the same job as depth. With the mix at 50%, half the sound stays at full level, so even a 100% depth only dips by 6 dB. Leave it at 100% and use depth to set how far the volume falls.

Syncing tremolo to the tempo

A tremolo running at a rate unrelated to the song drifts against the beat. Sometimes that's what you want, and the loose, wavering feel of amp tremolo on old records comes partly from it. On anything programmed, a pulse that lines up with the grid usually sits better, and most tremolo plug-ins have a sync switch that sets the rate as a note value instead of in hertz.

To work it out yourself, divide the tempo by 60 to get beats per second, then multiply by the number of pulses you want per beat. At 120 BPM that's 2 beats a second, so quarter notes are 2 Hz, eighth notes 4 Hz and sixteenths 8 Hz. At 90 BPM, eighths come out at 3 Hz.

The lab rounds its rate to a whole number of cycles per loop, and the chords loop is four bars long, so at 2, 4 and 8 Hz every pulse falls in the same place in the beat, bar after bar. The guitar loop runs at a different tempo, which is why the same 4 Hz sounds out of step with it.

Once the rate is synced, the phase decides where in the beat the pulse falls. If the LFO's loudest point lands just after the beat, the pulses feel late even though the rate is right. Many synced tremolos have a phase or offset control for moving the pulse within the beat.

LFO shapes: swell, pulse and chop

Once the rate and depth are set, the shape of the LFO decides the character. Two tremolos at the same rate and depth can sound completely different, one smooth and one choppy, because of the wave moving the volume.

A sine rises and falls smoothly and lingers near the top and the bottom, so it swells. Many amp tremolos start from a smooth wave like this, though their circuits bend it, and the lamp-based ones described below come out choppier. A triangle moves in straight lines and turns sharply at the top, so it spends less time at full volume and sounds slightly more pulsed. A square wave jumps between full volume and the bottom of the dip, which turns a held chord into a rhythmic chop, and set deep and synced to sixteenths it's what trance producers call a gate.

A ramp, also called a sawtooth, drops quickly and climbs back slowly. When it drops on each beat and rises in between, it's close to the shape a kick drum carves out of a pad through sidechain compression, which is why volume-shaping plug-ins get used in its place. Sidechain vs volume shaping compares the two.

A square tremolo with perfectly sharp edges clicks every time it switches, because the waveform jumps from one level to another between two samples. The figure gives each edge 2 ms, which is too quick to hear as a fade and slow enough to stop the click. Many plug-ins have a smoothing control for the same reason; Logic Pro's Tremolo calls it Smoothing.

Stereo tremolo and auto-pan

A mono tremolo turns both speakers down at the same moment, so the sound stays in place and pulses. Run a separate tremolo on each side and offset their LFOs and something else happens. At half a cycle apart, the left side is at full volume exactly when the right side is at the bottom of its dip, so the sound moves from one speaker to the other and back. That's auto-pan, built from the same parts.

The Spread knob in the lab sets that offset, from 0 (both sides together) to 100% (half a cycle apart). As you raise it, the correlation figure in the panel falls, because the two speakers carry more and more different signals. Settings in between give a stereo tremolo that throbs and moves a little at once. Stereo Rhodes Suitcase pianos, made from 1969, have the full side-to-side version built in, and its controls are labelled Vibrato, after Fender's amps.

How does tremolo work?

A tremolo multiplies the sound by the LFO. Every sample is scaled by a gain between 0 and 1 that the LFO sets, so at 0.5 the sample comes out at half its height and at 1 it comes out unchanged. The lab, the figure above and the tremolo in your DAW all work this way.

Hardware gets there by different routes. Some guitar amps vary the bias of their power valves, which turns the amp's gain up and down. Others, including Fender's mid-1960s designs, shine a small lamp on a light-dependent resistor, and the LFO flickers the lamp so the resistor turns the signal down in time with it. The lamp and the resistor don't respond instantly, which reshapes the wave, and that's part of why different amps throb differently at the same speed.

Multiplying by a moving gain also adds new frequencies. A 440 Hz tone under a 5 Hz tremolo comes out as 440 Hz plus two quieter tones at 435 and 445 Hz, called sidebands. At 5 Hz they're so close to the note that you hear them as the pulse itself. Speed the LFO up to 50 Hz and the sidebands sit 50 Hz either side, far enough apart to sound rough, and at a few hundred hertz they become new notes. That's amplitude modulation. Its close relative, ring modulation, multiplies by a wave that swings below zero too, which removes the original note and leaves only the sidebands.

Where tremolo works best

Tremolo needs something sustained to work on. A short drum hit is over before the LFO has moved very far, so it comes out a little louder or quieter depending on where in the cycle it lands. Held notes give the volume time to rise and fall, so guitar chords, electric piano, organ, pads and strings all show the pulse clearly.

On guitar it goes back to the 1950s, when Bo Diddley's sound leaned heavily on tremolo. The shuddering rhythm guitar on The Smiths' “How Soon Is Now?” is tremolo too. On synth pads and string sections, a tempo-synced tremolo turns a long chord into a rhythm part without anyone having to play one.

How to set up a tremolo, step by step

1. Put it on a held partPick a track with sustained notes, and put the tremolo after any compressor on it so the compressor can't even out the pulse.
2. Start deepSet the depth to around 80% so you can clearly hear what the rate is doing while you set it.
3. Choose the rateSync it to an eighth or sixteenth note for a programmed part, or leave it free at 4 to 6 Hz for an amp-style wobble. Listen against the drums for whether it locks in or drifts.
4. Choose the shapeA sine for a swell, a square for a chop and a ramp for pumping.
5. Bring the depth backLower it until the pulse sits in the mix without the part sounding like it's switching off. In a full arrangement, 40 to 60% is often plenty.
6. Decide on stereoLeave it mono if the part needs to stay in one place, or offset the two sides if you want it to move.

Common tremolo mistakes

A rate that almost matches the tempo

A tremolo at 4.2 Hz on a 120 BPM track starts in time and slowly slides out of it, so the pulse lands on the beat for a bar and between beats a few bars later. It sounds like a timing mistake. Either sync it exactly or move it far enough away that it obviously isn't trying to follow the beat.

Compressing after the tremolo

A compressor turns the loud parts of a sound down, and on a tremoloed part those are the tops of the pulses. Put a compressor or limiter after it and a 10 dB dip can shrink to 5 dB. If the tremolo sounds weaker in the mix than it did on its own, check what comes after it, including the mix bus.

Too much depth on a lead part

A vocal or lead line that drops 20 dB several times a second loses words and notes in the gaps. Unless the chop is the point, keep the depth on a lead under about 50%, which is a dip of 6 dB or less.

Comparing it at different levels

A deep tremolo lowers the average level, so switching it off makes the track jump up in volume, and the dry version can seem better just by being louder. Match the levels before you decide, the way the lab does.

Training your ears to hear tremolo

Tremolo and vibrato are easy to confuse at fast rates and shallow depths, because both make a held note wobble. The quiz under the lab plays the current sound through one or the other at matched loudness, with the screens covered, and asks which you heard. Listen for whether the note gets quieter or bends out of tune. There are eight rounds, and your best score is saved in your browser.

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