Some AirPods Pro 2 and AirPods Pro 3 owners hear a faint, high-pitched tone coming from a closed charging case. It usually happens in a quiet room, a little while after the earbuds have been put away.
A researcher who goes by ateliti99 has now taken that sound apart. In a post on Substack, ateliti99 pulled the audio from a short Reddit video and ran it through a spectrogram. Apple already confirms the chime in its AirPods hearing health guide, so the sound is intentional. The explanation for why it exists is ateliti99’s own interpretation, and Apple has not published the details of how the check works.
The chime is six pure tones played in 0.6 seconds
The Reddit clip runs 27 seconds, the lid closes with a loud click, and about 16 seconds later, while the case sits still, a short staircase of tones plays. Each tone lasts 100 milliseconds.
| Tone | Frequency | Duration |
|---|---|---|
| 1 | 1 kHz | 100 ms |
| 2 | 2 kHz | 100 ms |
| 3 | 3 kHz | 100 ms |
| 4 | 4 kHz | 100 ms |
| 5 | 6 kHz | 100 ms |
| 6 | 8 kHz | 100 ms |
The measurements show a very clean signal:
- Every tone lands within about 1 Hz of its round value.
- Only one pitch plays at a time, and each tone switches to the next in under 5 ms.
- The tones sit barely above the room’s background noise.
- The sequence plays once in the whole 27 seconds.
Those frequencies are standard test points in a clinical hearing test. Full pure-tone audiometry also uses 125, 250 and 500 Hz, which are hard to capture on a phone or reproduce through earbuds.
A closed AirPods Pro case works like a tiny artificial ear
A hearing test plays tones at different pitches, quieter and quieter, until you stop hearing them. The result only means something if the device knows exactly how loud each tone is at your eardrum. If an earphone loses 5 dB at 4 kHz, the test would wrongly conclude your hearing at that pitch is 5 dB worse than it really is.
Clinics and factories solve this with an artificial ear, a metal cavity that behaves like an average adult ear canal. The standards behind it include IEC 60318-4 for the ear simulator and ISO 389-2 for reference levels. ISO 8253-1 also calls for regular checks, because earphones drift over time.
AirPods cannot visit a lab every few months, but ateliti99 points out that each pair carries its own test chamber. The closed case maps onto the lab setup like this:
| Lab equipment | AirPods stand-in |
|---|---|
| Calibrated earphone | The earbud’s speaker |
| Artificial ear | The sealed pocket inside the closed case |
| Measuring microphone | The earbud’s own microphones |
| Analyzer | The chip, comparing against values stored at the factory |
The conditions also explain when you hear it. The lid has to be closed so the pocket has the same shape every time. The case has to be still, since bumps would spoil the reading, and the room has to be quiet because the tones are so soft. Apple says the chime plays periodically rather than every time the lid closes.
ateliti99 notes it shows up most at night, which fits. The earbuds are put away, the case sits untouched and the room is at its quietest. A check like this could catch several problems:
- A speaker or microphone losing sensitivity with age or humidity.
- A grille blocked by dust or earwax.
- A damaged speaker that distorts the tones.
- A mismatch between microphones, which affects noise cancellation, Transparency mode and calls.
The chime arrived with the iOS 18.1 hearing health features
Apple’s support guide says AirPods may periodically play a quiet chime while sitting in the charging case. The company added the self-check alongside Hearing Test and Hearing Aid in iOS 18.1. The FDA authorized the first over-the-counter hearing aid software on September 12, 2024.
Bose and Google have described the same idea in patents
Apple is not the only company with this approach on paper. A Bose patent plays test signals from the earpiece driver into the storage case cavity, measures them with the earpiece microphones, then adjusts microphone and driver sensitivity.
A Google patent covers a tone, a frequency sweep or pink noise played inside a closed case to calibrate speaker, microphone and noise cancellation gains. A 2021 study in Trends in Hearing also found that true wireless earbuds needed careful calibration to give reliable hearing test results.
Our Take
Apple’s own wording ties the chime to keeping the microphones and speakers accurate, and it gives hearing test results as the example. Six standard audiometry frequencies in a sealed case point the same way.
It also fits how Apple has been building out hearing features. AirPods Pro 2 already got a full hearing health upgrade with a hearing test, protection and a hearing aid mode, and AirPods Pro 3 added a real-time acoustic seal test for noise cancellation. A health feature that depends on accurate audio needs a way to keep the hardware honest.
If you are choosing between models, our AirPods 5 vs AirPods Pro 3 comparison covers what each one offers.
