What is a phaser?

A phaser in plain English

A phaser takes a copy of your sound, shifts the timing of each frequency in it by a slightly different amount, and mixes that copy back in with the original. Wherever a frequency in the copy ends up exactly half a cycle behind the same frequency in the original, the peaks of one line up with the troughs of the other and the two cancel. That leaves a few dips in the sound called notches.

On its own a set of fixed notches would sound like a slightly odd EQ, so a phaser adds an LFO, a wave too slow to hear, that keeps moving them up and down. Your ear follows them as they sweep, and that moving cut is the whoosh people mean by phasing, closer to a vowel slowly changing than to anything metallic.

The phaser belongs to the same family as chorus and flanger, which all mix a moving copy with the original. To hear all three side by side, go to chorus vs flanger vs phaser. This page stays with the phaser and goes further into how it works.

Reading the lab

The lab has three cards: the dry sound, the phaser, and a flanger for comparison. The wide screen under them shows the whole loop, with time running left to right and frequency running up the side. Each spot compares the sound after the phaser with the original. An even colour means that frequency came through unchanged, a dark band means it's being cut and white means it came out louder, so a phaser draws two or three broad dark bands that rise and fall together.

Below it on the left is the LFO, drawn as the frequency of the middle notch over time, with a dot at the moment you're hearing. The middle screen is the spectrum at that moment: the faint line is the original, the coloured shape is the sound after the phaser, and the white line is the shape of the phaser itself, with its notches marked along the bottom. The panel on the right explains your settings using numbers measured from the loop.

Every option plays at the same loudness as the original, so the phaser can't win just by being louder. You can use your own sound as well: upload a file, or press Record and play something for eight seconds. Nothing leaves your device.

How a phaser works

The allpass filter

An ordinary filter changes the level of some frequencies. An allpass filter lets every frequency through at exactly the same level and changes its phase, which is how far through its cycle a wave is at a given instant. A simple allpass filter shifts low frequencies hardly at all and high frequencies by up to half a cycle, with a quarter of a cycle at the frequency you set it to. Listen to its output alone and it sounds almost exactly like the input, because your ear barely notices a phase shift on its own.

The change only becomes audible when you mix the shifted copy with the original. A frequency shifted by a quarter of a cycle partly cancels, dropping by about 3 dB at a 50% mix, while one shifted by half a cycle is an upside-down copy of itself and cancels completely. Shift it by a full cycle and it lines up with the original again, so it comes through at full level.

Stages and notches

One allpass filter never quite reaches half a cycle of shift, so on its own it can't cut a notch. Phasers chain several in a row, and each filter in the chain is called a stage. Two stages together reach half a cycle at one frequency, which gives one notch. Four stages reach half a cycle at one frequency and one and a half cycles at another, which gives two, so every two stages make one notch.

Suppose every stage is tuned to 1 kHz. This is where the notches land as you add stages:

The notches are spread unevenly, squeezed together around the middle and further apart at each end, and their spacing has nothing to do with the notes being played. That's a large part of why a phaser sounds smooth. More stages means more notches and a busier, more obvious sweep, while four stages sound gentle and open.

The phaser in the lab has six stages, so it always cuts three notches, with the outer two about 3.7 times below and above the middle one. Some hardware phasers tune their stages to slightly different frequencies, which moves the notches around and gives each design its own sound.

What the LFO moves

The LFO turns the frequency of every stage up and down together, so all the notches slide at once and keep their spacing. In the lab, the depth control sets how far: at 70% the middle notch moves between 160 Hz and about 1.8 kHz, and at 100% it covers five octaves, from 160 Hz to about 5.1 kHz. The lab sweeps in octaves, so the notch spends as long getting from 200 to 400 Hz as from 2 to 4 kHz, which sounds even to the ear.

Phaser vs flanger

A flanger also mixes a moving copy with the original, but it makes the copy with a short delay of under about 10 ms. A delay holds back every frequency by the same amount of time, so any frequency whose half cycle fits into the delay an odd number of times cancels. That gives a long row of evenly spaced notches called a comb filter: a 1 ms delay cuts notches at 500 Hz, 1.5 kHz, 2.5 kHz and every 1 kHz after that, eighteen of them below 18 kHz.

The phaser gets three unevenly spaced notches out of the same job. Because the flanger's notches are evenly spaced, they cut into the harmonics of a note in a regular pattern, and the result sounds metallic, like a jet passing overhead. The phaser's few wide notches sound softer and rounder. The figure below draws both from the lab's own maths, and the slider moves them through their sweeps.

The controls

Phaser plug-ins and pedals share a small set of controls. Logic's Phaser, FL Studio's Fruity Phaser and the phaser in your own DAW label them slightly differently, and the jobs are the same. Some guitar pedals have a single speed knob and fix everything else.

Rate

Rate is the speed of the LFO in hertz, which is cycles per second. At 0.25 Hz a full sweep up and back down takes four seconds, and at 1 Hz it takes one. Slow rates between 0.1 and 0.5 Hz let you hear each sweep as a gesture, which suits pads, keys and rhythm guitar. Somewhere above 3 Hz the sweep gets too quick to follow and becomes a warble, closer to the fast setting of a rotary speaker.

The lab rounds the rate to a whole number of cycles per loop. The chords loop is 8 seconds, which is four bars at 120 BPM, so 0.25 Hz gives exactly one sweep every two bars. Many plug-ins can sync the rate to tempo in note values, which keeps the sweep landing in the same place every bar.

Depth

Depth sets how far the notches travel. Low depth keeps the sweep narrow, so the movement is subtle and stays in one part of the spectrum. High depth sends the notches from the low mids right up into the treble for the full, obvious whoosh. If your phaser has a separate frequency or manual control, that sets where the sweep is centred, and depth sets how far either side of it the notches go.

Feedback

Feedback, often labelled resonance, regeneration or colour, sends part of the end of the filter chain back into the start. The frequencies between the notches build up into peaks, and the sweep gets a ringing, vocal quality, a bit like someone slowly saying "ooh" and "aah". In the lab, 70% feedback at a 50% mix lifts the peaks by about 4.5 dB, and the notches, which cut almost all the way down with no feedback, only go about 11 dB deep. What you hear sweeping is the contrast between peak and notch.

Feedback also pulls the notches upwards a little, the top one most. Some phasers let you set it negative, which flips the copy before it's fed back and moves the peaks to different places; the lab only goes positive.

Mix

Mix is the balance between the original and the filtered copy, and on a phaser it decides whether there's an effect at all. The notches only exist because the two cancel, so they're deepest at 50%, where the copy and the original are the same level. At a 25% mix the same notches only go down about 6 dB. At 100% you hear only the allpass copy, which on a slow sweep sounds almost dry.

That copy does differ from the original in one small way. When the LFO moves the filters, it changes how far behind each frequency is from moment to moment, and that bends the pitch very slightly. At slow rates the bend is too small to notice; at fast rates you hear it as vibrato. The Uni-Vibe pedal Jimi Hendrix used has a switch between chorus and vibrato that works this way, with the vibrato setting leaving the original out.

Many guitar phasers have no mix knob, because they're fixed at an even blend.

Stereo spread

Stereo phasers can run the left and right LFOs out of step, under a name like spread, stereo phase or width. At 100% in the lab they're half a cycle apart, so when the notches are at their lowest in the left speaker they're at their highest in the right. Each side sounds different, which makes the sound wider and makes it seem to move around your head on headphones. The correlation figure in the lab's panel measures how far apart the two sides have drifted: 1 means identical, and lower means wider.

Where the phaser came from

From the late 1950s, engineers made a sweeping effect by playing the same recording on two tape machines and slowing one down by hand, so the two drifted in and out of time. People called the result phasing or flanging, and it needed two machines and someone to work them. Phase shifter circuits did a similar job electronically, with a chain of allpass stages in place of the second tape machine. They were far simpler to fit in a box, and they gave a softer sweep with fewer notches, which became a sound of its own.

The Uni-Vibe, made by the Japanese company Shin-ei in the late 1960s, was one of the first. It used a lamp and four light-sensitive cells to move its stages and was designed to imitate a rotary speaker, and Hendrix played through one on "Machine Gun". In the 1970s the MXR Phase 90 put a four-stage phaser in a small box with a single speed knob, and Eddie Van Halen used one on his early recordings. The Electro-Harmonix Small Stone, another four-stage pedal from the same decade, has a Color switch that turns feedback on for a sharper sweep. If your phaser plug-in has a stages control, try four and then eight, and you'll hear the sweep get busier.

How to use a phaser on guitars, keys and synths

Guitar

A phaser suits rhythm guitar. A slow sweep on a repeating funk or rock part keeps it moving without changing the notes, and it sits behind a vocal better than a flanger would. On a clean guitar the sweep is soft and liquid. Put the phaser after a distortion pedal and there are more harmonics for the notches to cut into, so the sweep comes through stronger. Pick the Guitar loop in the lab to try it.

Keys and electric piano

Electric piano and string machines take a slow phaser very well. A rate of 0.1 to 0.3 Hz with plenty of depth and only a little feedback gives a gentle rolling movement over long chords, which is what the chords loop in the lab is set up to show.

Synth pads and leads

A stereo phaser on a pad adds width and slow movement at the same time. On a bright synth lead or a noisy sound effect, high feedback and full depth give a strong, vocal sweep that works well going into a drop. A sawtooth, with its long row of harmonics, shows the notches much more clearly than a soft tone close to a sine, which has few harmonics for them to cut.

Drums

Noisy sounds like hi-hats, cymbals and snares have energy right across the spectrum, so every notch has something to cut and the sweep is easy to hear. A slow phaser on a drum loop, synced to two or four bars, gives it a long rise and fall. Keep the depth away from the kick, or the sweep will thin it out each time the lowest notch passes through.

Vocals

On a lead vocal a phaser is a special effect, best kept for a phrase or a word. Automate the mix up for that moment and back down again so the rest of the vocal stays clean.

How to set up a phaser, step by step

1. Start with an even mixSet the mix to 50%, or use the phaser on a send at 100% wet with the return about as loud as the dry channel, so the notches are as deep as they get while you set everything else.
2. Set the rate slowBegin around 0.2 Hz, or sync it to one sweep every bar or two. You can speed it up once you know what the sweep is doing.
3. Bring the depth up until you hear the sweepThen decide how wide you want it. Keep it narrow if the part only needs gentle movement, or open it up for the full whoosh.
4. Add feedback for characterA little makes the sweep more vocal and a lot makes it ring. Stop before it starts to whistle on sustained notes.
5. Check the low endListen to the bass notes of the part as the sweep reaches the bottom. If they dip, lower the depth or filter the low end out of the phaser.
6. Listen in the mix and in monoIf you used stereo spread, switch your master to mono and make sure the part doesn't drop away or sound hollow.

Starting settings

These are places to begin, chosen so the phaser is clearly there. The depth and feedback figures are for the lab, so on another plug-in treat them as low, medium or high.

Common phaser mistakes

The mix at 100% on an insert

If a phaser seems to be doing nothing, check the mix before anything else. With no original underneath there's nothing for the copy to cancel against. On a send, the plug-in should be 100% wet, because the dry signal is already coming through the channel.

A sweep that cuts the bass

With a deep sweep, the lowest notch passes through the bottom of the sound every cycle, and a bass line or a kick loses its weight each time. Narrow the depth so the notches stay higher, or put the phaser on a send with a high-pass filter in front of it.

A rate that ignores the tempo

A free-running sweep lands at a different point in every bar, which can make a loop-based track feel unsettled. At 120 BPM a bar is two seconds long, so 0.5 Hz is one sweep a bar, 0.25 Hz one every two bars and 0.125 Hz one every four.

Too much feedback on held notes

High feedback makes the peaks between the notches ring. On short notes that's character, but on a long pad or a sustained vocal the peaks can whistle as they pass through. Back it off until the sweep stops drawing attention to one frequency.

Forgetting about mono

A phaser with wide stereo spread can sound big on headphones and thin on a single speaker, because the left and right partly cancel when they're added together. The mono compatibility lesson shows what to listen for.

Training your ears to hear a phaser

Phaser and flanger are easy to mix up, so the quiz under the lab plays the current sound through one of them with random settings, the screens covered and the loudness matched, and you say which one you heard. Listen to the edge of the sweep: a hard, metallic quality points to the flanger, and a softer, rounder one to the phaser. Eight rounds take a few minutes, and your best score is saved in your browser. Start on the chords, then switch to the drums, where the difference is easier to hear.

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