What is auto-pan?

Auto-pan in plain English

Every channel in a mixer has a pan control, which decides how much of a sound goes to the left speaker and how much to the right. Turn it fully left and the sound comes only from the left; leave it in the middle and both speakers play it equally, so it seems to come from a point between them. Now imagine someone turning that knob slowly left and right, over and over, in time with the song. That is auto-pan.

The hand on the knob is an LFO, a low frequency oscillator: a wave too slow to hear that moves a setting up and down for you. In an auto-pan it moves the pan position. Rate sets how many trips the sound makes each second, and depth sets how far from the middle each trip goes. Most auto-pan plug-ins also let you change the shape of the LFO, which changes how the sound moves between the two ends.

An auto-pan is made from the same thing as a tremolo, a volume that an LFO moves up and down, and the lab at the top of this page has both: the Tremolo card moves the two speakers together and the Auto-pan card moves them in opposite directions.

How auto-pan works

A tremolo uses an LFO to turn the volume of a sound up and down. Give each speaker its own tremolo and you get to choose how the two line up. If both LFOs are in step, both speakers get quieter at the same moment and the sound pulses in place. Put the right speaker's LFO half a cycle behind the left one and the speakers move in opposite directions: while the left fades down, the right comes up. Your ear hears the level moving from one speaker to the other as the sound changing position.

So an auto-pan is two tremolos half a cycle apart. The offset between them is a phase difference, and half a cycle is 180°. Plenty of plug-ins expose that offset as a control, usually called phase, stereo phase or spread, which is why some tremolo plug-ins can double as auto-panners and some auto-panners have a tremolo mode.

At 180° the two volumes in the lab always add up to the same total, so the sound moves without the overall level changing. At 90°, a quarter of a cycle, they don't, and the sound swings partway across while it also pulses. At 0° it is plain tremolo.

Reading the lab

The lab is the modulation lab used across the modulation lessons. Its Spread knob is the phase offset: 0% puts both speakers in step and 100% puts them half a cycle apart, which is a full auto-pan. The Tremolo card starts with spread at 0% and the Auto-pan card at 100%. Feedback and mix are locked because an auto-pan doesn't use them. The buttons that appear under the cards set the LFO shape, the pan law and mono playback, and they come up in the sections below.

The most useful screen on this page is the LFO screen, the first one under the wide screen. It draws the volume of the left channel as a solid line and, once spread is above zero, the right channel as a dashed line, over about two and a half cycles centred on the moment you're hearing. At 100% spread the two lines are mirror images, so where one is at the top the other is at the bottom.

The wide screen above it compares the left channel with the original across the whole loop, so an auto-pan shows up as the picture getting brighter and darker in bands, once per cycle. The Right now panel reads out how far each side dips in decibels (or Silent, when it reaches nothing), the rate and the correlation between left and right, which drops as the two sides move further apart. All of it is measured from the loop as the lab renders it.

Every setting plays at the same loudness as the dry sound, so nothing wins by being louder. Headphones make the movement easiest to hear. You can upload your own sound or record eight seconds of one, and nothing leaves your device.

The controls

Auto-pan plug-ins vary in what they call things, but the same few controls turn up on nearly all of them. Ableton Live's Auto Pan, for example, has Amount, Rate and Phase controls and a choice of LFO waveforms, and FL Studio has a dedicated auto-panner called Fruity PanOMatic.

Rate

Rate is how fast the LFO goes round, and one cycle takes the sound from one side, across to the other and back again. It's set either in hertz, meaning cycles per second, or as a note value locked to the tempo of your project. At 1 Hz the sound makes one round trip a second; at 0.25 Hz one trip takes four seconds.

Tempo sync is usually the better choice, because a free-running rate drifts against the beat and the sound ends up on a different side every time the chorus comes round. If your auto-pan only offers hertz, you can work the rate out from the BPM. Divide the BPM by 60 to get beats per second, then divide that by the number of beats one cycle should last. At 120 BPM that's 2 beats a second, so a cycle per quarter note is 2 Hz and a cycle per four-beat bar is 0.5 Hz.

The lab rounds its rate to a whole number of cycles per loop so the sound and the screens stay lined up. The chords loop is four bars at 120 BPM, so every rate in the middle column except the dotted eighth lands exactly on the beat there.

Slow rates, up to about 1 Hz, sound like the part is moving around the room. Somewhere around 3 to 5 Hz you stop hearing a position and start hearing a rhythm, a flutter that bounces between your ears. Push the LFO past about 20 Hz and the volume moves so fast that you hear new tones beside the original instead of a wobble, which is the idea behind ring modulation.

Depth

Depth, often called amount or width, sets how far the sound travels from the middle. At 100% it goes all the way to each speaker and, for a moment, is completely missing from the other one. At lower settings the quieter side only dips part of the way, so the sound leans left and right of centre without ever reaching a speaker.

In the lab, 40% depth means each side drops to 60% of its level at the bottom of its cycle, which is 4.4 dB down, while the other side is at full level. A 4.4 dB difference between the speakers moves the image only a little way off centre, and that small, regular lean is what most auto-pans in a finished mix sound like. Full depth is an effect people notice, while 30 to 50% keeps a part from sitting still without anyone pointing at it.

LFO shape

The shape of the LFO waveform decides how the sound moves between the two ends. These are the shapes you'll usually be offered, though not every plug-in has all of them, and the lab's Shape buttons cover the first three:

• A sine slows down at each side and speeds up through the middle, like a swing. It's the most natural-sounding choice and the usual default.
• A triangle moves at a constant speed and turns round sharply at each end, so the sound spends less time at the sides than with a sine.
• A square jumps straight from one side to the other and holds there. Alternate beats land on alternate sides, like two parts answering each other, and a smoothing control on most plug-ins rounds off the jump so it doesn't click. The lab's square has its jumps rounded off over about 8% of a cycle.
• Random, or sample and hold, picks a new position at each step of the rate, so the sound lands somewhere different every time.

Ableton's Auto Pan also has a Shape control that pushes its waveform towards a square, so the sound holds at the sides for longer and moves between them more quickly.

Phase, or spread

Phase sets the offset between the left and right LFOs. At 180° you get a true auto-pan, at 0° a tremolo, and the angles between give a mix of the two. The lab's Spread knob goes from 0 to 100%, which is 0° to 180°, so 50% is a quarter of a cycle. Some plug-ins also have an offset or start-phase control, which decides where the LFO is in its cycle when the song starts, so you can choose whether the first downbeat lands on the left or the right.

Pan law and what happens in mono

When a sound sits in the middle, both speakers play it. If each played it at full level, a centred sound would come out louder than the same sound panned hard to one side, because two speakers are adding up. A pan law turns both sides down a little at the centre to make up for it, and how much it turns them down changes what an auto-pan does.

The lab starts on a linear crossfade: at the centre each side is at half level, which is −6 dB, and the two levels always add up to exactly one. A −3 dB pan law, also called equal power and a common default in DAWs, keeps each side at about 71% at the centre instead, so the total power stays constant as the sound moves. The Pan law buttons switch between the two. The difference shows most when the mix is heard in mono, where the two channels are added together into one, and the Mono button does that sum.

That mono column is the reason an auto-pan should never be the only thing making a part interesting. A single smart speaker plays in mono, as do plenty of club systems, and a phone's two speakers sit so close together that they come near it. There an auto-pan either vanishes or becomes a faint tremolo. Check your mix in mono with the effect on; the mono compatibility lesson shows how, and the panning laws lesson explains why −3 dB is the usual compromise.

Auto-pan on mono and stereo sounds

On a mono sound, like a single vocal, one guitar mic or a mono synth, an auto-pan does exactly what you expect and the whole sound moves as one. A stereo sound is different, because its left and right channels hold different things. An auto-pan that works by turning each channel up and down, as the lab's does, behaves like a balance control on a stereo sound: the left half gets quieter while the right half gets louder, and neither half ever crosses to the other side.

The lead loop in the lab shows this well. It's so wide that its left and right channels have a correlation of about −0.18, meaning they partly move in opposite directions, so auto-panning it fades two different halves in and out in turn. The chords loop, at about 0.97, is close to mono and moves as one sound. If you want a wide part to travel across the speakers, narrow it or sum it to mono first, then put the auto-pan after that.

Where auto-pan works

Auto-pan suits parts that repeat and sit behind the main ones. Hi-hats, shakers and tambourines panned in quarter or eighth notes add movement without stealing attention from the vocal, as long as the depth stays under about 50%. Arpeggiated synths and plucks are another good fit, and syncing the pan to the arpeggio makes alternate notes land on alternate sides.

Pads and long chords take slow rates well, a bar or two per cycle, which keeps a held sound from feeling static over eight or sixteen bars. On a reverb or delay return, a slow auto-pan makes the tail drift around while the dry part stays in the middle.

The Fender Rhodes Suitcase piano had a built-in effect, labelled vibrato, that bounced the sound between the left and right speakers of its amp. Despite the name, that's an auto-pan. Put a stereo tremolo on an electric piano patch today and you get the same movement back. A rotary speaker moves sound around too, but it also bends the pitch and changes the tone as the horn spins, which an auto-pan can't do.

Auto-pan also works as a one-off event. Automating the depth up for the bar before a drop, or panning a riser from side to side as it climbs, gives a transition some movement and then hands the space back to the mix.

How to set up auto-pan, step by step

1. Pick a part that can moveChoose something above the bass, ideally mono, that repeats, such as a shaker, an arpeggio or a pad. Leave the low end and the lead vocal alone.
2. Sync the rate to the tempoStart with a quarter note for percussion and arpeggios, or one bar for pads. If the plug-in only has hertz, use the table above.
3. Start with the depth at 100%Full depth makes it obvious where the sound is at each point in the bar, which helps you choose the rate.
4. Choose the shapeKeep the sine for smooth movement. Try a square if you want alternate beats on alternate sides.
5. Bring the depth back downLower it until the movement is only just there when you listen to the whole mix, often 30 to 50%. You want a part with some life in it that still stays in its own place.
6. Check it in mono and on headphonesIn mono the part shouldn't vanish or pulse in a way that distracts. On headphones it shouldn't feel tiring after a minute.

Starting settings for common parts

These are places to begin. The depth figures assume a plug-in where 100% means the sound reaches each speaker.

Common auto-pan mistakes

Full depth on everything

A part that keeps leaving one ear completely pulls attention away from the vocal, and on headphones it gets tiring fast. Most auto-pans in a finished mix are much shallower than they sound when you first set them up.

Moving the low end

Kick, bass and sub carry the weight of a track. If they move from side to side the whole mix feels like it's tipping, and on a mono system the movement turns into a level wobble nobody can explain. Keep everything under about 150 Hz in the middle.

A free-running rate

An unsynced rate drifts against the beat, so the pan lands somewhere different on every bar and the part sounds sloppy. Sync it, or work the hertz value out from the tempo.

Several parts panning in step

If the hats, the arpeggio and the pad all go left on the same beat, the whole mix swings left with them. Give each its own rate, or set one plug-in's start phase half a cycle from the other so they cross.

Forgetting mono

An auto-pan that's doing all the work of making a part interesting will either disappear or turn into a pulse in mono. Check it there before you call the mix finished.

Auto-pan, tremolo and pan automation

Tremolo moves the volume of both sides together; auto-pan moves them in opposite directions. Pan automation is the hand-drawn version, where you draw the pan position yourself in your DAW. That suits a one-off move, like a guitar sliding across for a fill, while an auto-pan repeats and so suits anything that should keep moving in time.

Ping-pong delay and the Haas effect change where a sound seems to be as well, but they work with copies. A ping-pong delay sends echoes to alternate sides while the original stays put, and the Haas effect moves a sound's apparent position with a short delay, usually under 30 ms, between the channels. Auto-pan makes no copies and adds no delay, since only the levels change. The LFO lesson covers the oscillator behind it and the other settings it can move.

Train your ears

The quiz under the lab plays the current sound with either a tremolo or an auto-pan, at matched loudness with the screens covered. Both get the same random rate and depth, so the only difference is whether the two speakers dip together or in turn, and you say whether the sound is pulsing in place or moving from side to side. Eight rounds take a few minutes, and your best score is saved in your browser.

Questions people ask

Keep going