How AirPods track your head position

There is an accelerometer and a gyroscope in each earbud. iOS blends their output into a running estimate of how your head is turned and tilted, publishes it to apps through Core Motion, and that estimate is what Spatial Audio and every AirPods posture app read.

None of this is a secret or a workaround. Apple documented the interface publicly in iOS 14 and any developer can use it. What almost nobody explains is what the numbers actually are, which is a shame, because the shape of the data explains most of what these apps get right and all of what they get wrong.

What is physically in there

Two sensors do the work, and they are bad at opposite things.

The accelerometer measures acceleration, including the constant tug of gravity. Sit still and the only acceleration it sees is that tug, which gives it a permanent arrow pointing down. From the direction of that arrow you can work out how the earbud is tilted. The catch is that it cannot tell gravity apart from you moving. Stand up quickly and the accelerometer briefly thinks the world tipped.

The gyroscope measures rotation speed rather than position. It is excellent at the fast stuff the accelerometer fumbles, and it does not care about gravity at all. Its weakness is that position has to be built up by adding rotation rates together over time, and every small error in that sum stays in the total. Left alone for a few minutes, a gyroscope quietly wanders off.

Fuse the two and each one covers for the other. The accelerometer keeps the estimate anchored to the real direction of down, and the gyroscope keeps it responsive between those anchor points. That fusion happens before your app ever sees the data.

Pitch, roll and yaw

The orientation an app receives is described with three angles.

• Pitch is nodding. Chin toward the chest, then back up.
• Roll is tipping your head sideways toward a shoulder.
• Yaw is turning to look left or right.

A film soundtrack needs all three. A posture app mostly wants pitch. Forward head posture, the thing that creeps up on you at a desk, shows up as the head tipping down and forward, and pitch captures that cleanly. Roll is useful for catching the phone-in-bed lean. Yaw is nearly useless for posture, and there is a good reason why.

The part everyone skips: it is relative, not absolute

Pitch and roll get to lean on gravity, which points the same way in every room on earth. That gives them a fixed reference, and they stay honest over hours.

Yaw has nothing equivalent. There is no compass in play, so the reference for left and right is simply wherever your head was pointing when tracking started. Over a long session it slides. Anyone who has watched a spatial audio soundstage slowly rotate away from their laptop has seen exactly this.

Which brings up the more interesting limitation. Even with a rock solid pitch reading, an app does not know what your upright looks like. Everyone sits differently. Some people work with a screen above eye level, some with a laptop flat on a kitchen table. A pitch of twelve degrees below level is a slouch for one person and a perfectly normal working position for another.

That is what calibration is for. Sit the way you want to sit, hold it for a few seconds, and the app records your pitch as zero. Every reading afterwards is measured against your own baseline instead of some textbook ideal. An app that skips this step and hard-codes an angle is guessing, and it will nag you for sitting normally.

What an app receives, and how often

Each update carries the orientation, the rotation rate straight off the gyroscope, and acceleration split into the gravity component and the part caused by you moving. Updates arrive many times a second, far faster than posture needs.

Posture is slow. Nobody slouches in a tenth of a second. Most apps take only a fraction of the available updates and then smooth what they take, because a raw stream will spike every time you sneeze, laugh, or reach for a mug. Poise samples about ten times a second, then requires the slouch to hold for a set time before it does anything, which is the difference between a useful nudge and an app you delete on day two.

What head tracking cannot see

Worth being blunt about this, because the marketing in this category often is not.

The sensors are in your ears. They report how your head sits and nothing else. Rounded shoulders, a collapsed lower back, a pelvis tipped back in a soft chair, legs crossed the same way for four hours straight: all invisible, unless your head happens to move as a consequence. Head tracking reads forward head posture well. It does not read posture.

It also cannot tell you why your head went down. Looking at your phone and slumping over a keyboard produce a similar pitch. Apps handle that with extra signals rather than better sensors. Poise checks whether your screen is on and whether your head is sitting in the range typical of phone use, and holds off if both are true, since being nudged for reading a message is annoying and pointless.

And it says nothing about health. An angle is an angle. It is not a diagnosis, and a number of degrees on a screen is not a measure of how your neck is doing.

Battery and privacy, briefly

The gyroscope is already running whenever head tracked Spatial Audio is switched on, so an app reading the same stream costs less than people expect. There is a cost, but it is a dent across a working day rather than a collapse.

On privacy, the motion stream goes from your AirPods to your iPhone and stops there. iOS hands it to the app on your phone and sends it nowhere else. What happens next is the app's own choice, so it is worth reading the privacy policy of anything you leave running all day. Poise processes the raw stream and every angle derived from it on the device and never transmits either, which you can read in full in our privacy policy.

Common questions

Does head tracking work without music playing?

Yes. Motion and audio are separate. An app can follow your head while you work in silence. Some apps keep an audio session open in the background so a sound can play the instant it is needed, which is why your phone may list one as playing audio when you hear nothing.

Why does the tracking feel like it drifts?

Left and right drift is expected and comes from yaw having no fixed reference. Up and down drift is rarer, and usually means your baseline is stale rather than the sensors misbehaving. If you have changed desk, chair or monitor height, recalibrate.

Do both earbuds have sensors?

Yes, and iOS uses whichever is available. Wearing one is enough.

Can a Mac read the same data?

Not through this interface. It is part of Core Motion on iOS and iPadOS. macOS is not included, so posture apps in this category are iPhone apps even when you spend your whole day at a Mac.

Why this matters if you are choosing an app

Every AirPods posture app on the App Store reads the same stream from the same sensors. The hardware is not what separates them. What separates them is what happens after the numbers arrive: whether they ask for a baseline, how long they make you hold a slouch before saying anything, whether they can tell your phone from your keyboard, and whether they keep running when you switch to another app.

Poise calibrates to your own upright, waits until a slouch has actually held, stays out of the way while you are on your phone, and runs quietly in the background all day.

Written and published by Poise, which makes a posture app for AirPods. The technical description here follows Apple's Core Motion developer documentation and our own work building on it. Poise is a general wellness app and not a medical device, and nothing here is health advice. Questions or corrections: poise-app.com/contact.