// 03 RESTRICTED
earsence
Ballistocardiography from AirPods. Each heartbeat moves the head by one to three milli-g and the headphone IMU sees it: band-pass 0.6–3.3 Hz on eight motion channels, autocorrelation plus a Goertzel spectrum to resolve octave errors, keep the rate the most channels agree on. Alongside it, head pose, breathing, and a still / self-motion / walking / vehicle classifier built on the vibration floor. Ground truth is the Pro 3's own PPG sensor, logged into the same CSV by an iPhone companion through an iOS 26 workout session — a reference that shares no physics with the estimate.
| Still | Moving | |
|---|---|---|
| capture | 150 s · 0.007 g rms | 78.6 s · 0.129 g rms |
| PPG reference | 61–65 bpm, median 63 | 113 falling to 68, settling near 75 bpm |
| windows past the gate | 10 of 12 | 0 of 5 |
| mean abs error, passed | 1.1 bpm | — |
| within 3 bpm, passed | 10 of 10 | — |
| error, rejected windows | 3.9 and 10.8 bpm | 28.1 bpm mean, 57.0 worst |
| peak confidence | 0.34 | 0.04 |
| channel consensus | 4–7 of 8 within 1.1–3.2 bpm | 2–4 of 8 |
| verdict | 63.5 bpm ± 1.3 | refused — not enough confident windows |
The gate is the finding. Confidence collapses above 0.010 g accelerometer RMS, and the fair (≥ 0.22) and good (≥ 0.30) bars were set from this data after an invented 0.45 bar had thrown away a reading accurate to about a beat a minute. Consensus alone does not qualify a reading: on a 21 s walk at 0.13 g, five of eight channels agreed within 3.3 bpm and the gate still held confidence at 0.00. Synthetic self-test: 12 of 12 clean cases from 48 to 120 bpm within 1.4 bpm at a 3 mg heartbeat; at 5 mg of noise the worst miss is 4.3 bpm. Limits, on the record: one wearer, one session, one resting rate, and 50 Hz sampling quantises beats to 20 ms, so this is an average rate per window and never HRV.