How to phase align drums

Drum phase alignment in plain English

A drum kit recorded with several mics is one instrument heard from several places at once. The kick mic sits inside the kick drum, the snare mic a few centimetres above the snare, and the overheads a metre or so above the whole kit. Every mic hears every drum, so the snare mic catches some kick and hi-hat and the overheads catch everything. Engineers call the drums a mic wasn't aimed at spill, or bleed.

Each mic hears a drum at a slightly different moment, because the sound has further to travel to some of them. When you mix the tracks, those late copies of each hit add to the early ones. For some frequencies the copies are in step and the drum gets louder, and for others they're half a cycle apart and cancel. That pattern of notches is a comb filter, and on a kit it usually takes out the low end and the body first, which is why badly aligned drums sound thin and hollow.

Phase alignment is the job of lining those tracks back up. Some fixes are about time, sliding a track a couple of milliseconds earlier. Others are about polarity, flipping a track upside down because its mic faces the drum from the other side. The phase lesson explains phase itself and the comb filtering lesson covers where the notches land in general; this page is about doing it on a kit.

Reading the lab

The lab builds a kit in your browser and records it through four mics placed at real distances: one 10 cm inside the kick drum, one 5 cm above the snare, one under the snare and one overhead 95 cm above the snare. Every delay in it comes from the speed of sound. The four tracks then go through a small mixer, with a channel for each mic and a mono mix of all of them playing through both speakers.

The top screen is the view you'd get zoomed right in on a DAW timeline. It shows one snare hit, or one kick hit if you press Kick hit, as each mic hears it after your settings, with the mix in pink at the bottom. The dashed line is where the hit reaches its close mic, and each lane says how many milliseconds behind that line its copy arrives and whether it's upside down.

The comb scope underneath is the most useful screen. The lab sends a click through every mic as the snare and again as the kick, and measures what your mix does to the tone of each one. The 0 dB line is the sound you'd get with every mic lined up and the same way up, so any dip below it is something your mix is cancelling. The snare's notches are labelled in Hz, and hovering gives you the level of both drums at any frequency.

Each mic has an In button and a Ø button for polarity, and the knobs nudge the overhead and the bottom mic in whole samples or change their level. Press A/B to hear the four tracks as they were recorded. The lab plays every setting at the same loudness, because lined-up mics add up louder, and a louder version tends to win any comparison. The Right now panel describes what your settings are doing, using the numbers measured from the clicks.

Mic distance and delay

Sound travels at about 343 metres a second at room temperature, which is 34.3 cm every millisecond, or 2.9 ms for every metre. That's all you need to work out how late a mic will be: measure how much further it is from the drum than the close mic, and divide by 34.3 cm.

Take the lab's overhead. It's 95 cm above the snare and the top mic is 5 cm above it, so the overhead is 90 cm further away. 90 ÷ 34.3 gives 2.62 ms. Mix the two and the first notch lands at 1 ÷ (2 × delay), which is 1 ÷ 0.00524 s, or about 191 Hz. That's in the body of most snares, which is why a late overhead makes a snare sound papery. The lab measures its first notch at about 196 Hz, a little higher, because the snare also reaches the kick mic and that spill shifts the pattern slightly.

DAWs move tracks in samples, so it helps to know what a millisecond is in samples: 44.1 at 44.1 kHz and 48 at 48 kHz. One sample at 48 kHz is about 0.02 ms, or 7 mm of distance, which is finer than you'll ever need for a kit.

How much the notches hurt depends on how loud the late mic is. An overhead that's 10 dB under the snare mic makes notches only about 3 dB deep, while one at the same level can cancel a frequency outright. On most kits the overheads are loud, because they carry the cymbals, so their late copy of the snare is loud enough to hear.

Which mic to line up to

Pick one track to stay put and move the others to it. On most kits that's the snare top mic. The overheads are usually set up around the snare, and a snare that loses its body is the first thing people notice when drums are out of phase.

The trouble is that one overhead track can only be lined up with one drum. In the lab the overhead is 2.62 ms behind the snare mic but 3.5 ms behind the kick mic, because it's further from the kick. Nudge it 2.62 ms earlier and the snare is spot on while the kick is still about 0.9 ms off, which puts the kick's first notch near 470 Hz. The kick's weight lives under 100 Hz, though, and an offset that small barely touches it: the lab reads the kick's low end within a quarter of a decibel of lined up. What you lose is a little of the beater click.

Line it up with the kick instead and the snare's 0.9 ms offset moves its first notch to about 600 Hz, which you'll hear more, since that's where much of a snare's tone sits. So the snare usually wins. On a kit where the kick is what's falling apart, line up to the kick and accept a little damage to the snare, or meet in the middle.

With a pair of overheads, treat each one as its own track. If the snare is closer to one than the other, each needs its own nudge, and moving the pair as one block leaves one of them out. Better still, measure the two overheads to the snare with a tape or a piece of string when you set them up, so they're the same distance away before you press record. The Recorderman technique does that for the kick as well, setting both overheads equally far from the snare and from the kick.

Snare top and bottom mics

A mic under the snare picks up the wires, the rattle that makes a snare sound like a snare, which a top mic doesn't catch much of. It also points at the drum from the opposite side. When the stick hits, it pushes the top head down, away from the top mic, and the air inside the shell pushes the bottom head down too, towards the bottom mic. So the top mic records a pull at the moment the bottom mic records a push, and the two tracks are upside down compared with each other.

Mix them like that and the bottom mic cancels the body of the snare wherever the two overlap. In the lab, bringing the bottom mic up at −6 dB takes the snare's body down by about 5 dB and leaves it sounding like wires and not much else. One press of Ø on the bottom mic fixes nearly all of it, and it's the one polarity flip you'll make on almost every multi-miked kit.

The flip leaves a small timing difference. The bottom head moves a fraction of a millisecond after the top one, and the two mics are rarely at exactly the same distance, so in the lab the bottom mic's hit still lands about 0.4 ms late. Zoom in until you can see the start of a hit on both tracks and nudge the bottom mic earlier until they match. It's a smaller improvement than the flip, and it shows up mostly in the upper mids.

One more thing happens when you flip the bottom mic: everything else it heard flips too. Its kick spill was in step with the kick mic, and now it isn't, which in the lab costs the kick almost a decibel of low end. Keeping the bottom mic well under the top mic, where it usually sits anyway, keeps that small.

Kick in and kick out mics

A kick drum is often recorded with two mics: one inside the shell near the beater for the click, and one outside the front head for the boom. They're tens of centimetres apart, so the outside mic is always late. Say it's 50 cm further from the beater than the inside mic. That's 1.46 ms, which puts the first notch near 340 Hz and the next around 1 kHz, right through the knock of the kick.

Line them up the same way as the snare mics: pick the inside mic as the reference, zoom in on a hit, and slide the outside mic earlier until the starts line up. Then try the Ø button on the outside mic and keep whichever setting has more low end. Both mics point back towards the drummer, so on paper they start the same way up, but the front head moves later and less simply than the beater head, and only listening tells you which way round is fuller. Do the timing first, since flipping a late track only moves its notches.

Room mics are meant to be late

A room mic three or four metres in front of the kit hears it about 9 to 12 ms after the close mics. Lining it up would take away the very thing it's there for, the gap between the drum and the room's answer that tells your ears how big the space is. So leave room mics where they were recorded, keep them a good way under the close mics, and check them in mono for the low end.

At 10 ms the notches are only 100 Hz apart, and a room mic also hears the kit bouncing off every wall at different times, which smears those notches across the spectrum. That mostly reads as space. The place a room mic still hurts is the low end, because a 10 ms delay puts its first notch at 50 Hz, under the kick. A high-pass filter on the room mic around 100 Hz usually sorts that out without touching the timing.

How to phase align drums, step by step

This order works for most kits. Do it with the drums soloed, and check each pair of mics in mono, which makes cancellation much easier to hear.

1. Check the session is in timeMake sure plug-in delay compensation is on and every drum track starts at the same point. A track that came in through outboard gear or a different interface can be late before any mic distance comes into it.
2. Choose a referenceLeave the snare top mic where it is. Everything else moves to it, or to the kick in mic for the kick's own mics.
3. Flip the mics that face the other wayFlip the snare bottom mic. Then try the Ø button on each other close mic against its partner and keep whichever way sounds fuller.
4. Line up the overheadsFind a loud, clean snare hit and zoom in until you can see its start on the snare mic and the overheads. Measure the gap and slide the overheads earlier by that much. In most DAWs you can drag the region with snapping off or nudge it by a set number of samples, and Ableton Live's Track Delay takes negative values, which pulls a whole track earlier.
5. Check the kick and the tomsListen to the kick and each tom against the overheads. If one has lost its weight, decide whether to move the overheads a little towards it or leave the snare in charge.
6. Listen, then comparePlay the whole kit in mono and in stereo, and flip back to the original timing. If the aligned version doesn't sound fuller at the same level, undo it.

Trust your ears over the waveforms. Each mic hears the drum from a different angle, so the starts of the hits rarely look identical, and a nudge that looks perfect on screen can sound a sample or two worse than one beside it. Plug-ins such as Sound Radix Auto-Align and MAutoAlign measure the delay from the audio itself, which is more accurate than lining up waveforms by eye.

Common drum phase mistakes

Fixing a delay with the polarity switch

It's tempting to leave a late overhead where it is and flip it until it sounds better. Sometimes it does sound better, by luck: in the lab, flipping the late overhead brings the snare's body back, but the first notch moves to about 370 Hz and the kick loses about 3 dB of low end. The delay is still there and the notches have only moved. Line up the timing first, then decide on polarity.

Pulling the room mics into line

Room mics that are lined up with the close mics stop sounding like a room. Leave them late, and turn them down or high-pass them if they're causing trouble.

Judging by level

Lined-up mics add together, so the aligned kit is louder, and louder usually sounds better for a moment. Match the levels before you compare. The lab does this for you; in a DAW, pull the drum bus fader down a decibel or two on the aligned version.

Aligning the overheads to the kick by accident

If you line up the overheads using a kick hit, the snare ends up early and you've swapped one notch for another. Use a snare hit unless you've decided the kick is the drum to protect.

Forgetting that spill moves too

Every nudge and every flip applies to the whole track, including the drums that mic wasn't aimed at. After each change, listen to the whole kit as well as the drum you were fixing.

Train your ears

The quiz under the lab mixes the kit with hidden settings and covers the screens. Each round asks one of two things: whether the snare bottom mic is in phase with the top mic, or whether the overhead has been lined up with the snare or left as recorded. Every round plays at the same loudness, so listen for the body of the snare around 200 Hz and the weight of the kick. It's eight rounds, and your best score is saved in your browser.

Questions people ask

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