Apple locked hearing assistance inside AirPods. So I built an open-source version for any earbuds.

In 2024, Apple shipped something genuinely great: AirPods Pro can run a clinical-style hearing test and then act as hearing assistance, tuned to your ears. People love it. There's just one catch — it needs an iPhone to set up, recent AirPods to run, and if you're on Android you get nothing. Meanwhile, the average pair of prescription hearing aids costs about $4,700 , and surveys show a $1,500 device is simply out of reach for more than half the people who need one. There are a billion-plus Android phones out there, most of them sitting next to a pair of ordinary earbuds that already contain everything you physically need: a microphone, a DAC, and speakers. The gap seemed absurd. So I've spent the past weeks building OpenHearing — a free, GPLv3 Android app that does the whole pipeline: Hearing check — a pure-tone test using the modified Hughson–Westlake staircase (the same adaptive up-down procedure audiologists use), per ear, per frequency. Or skip it and type in the numbers from a real audiogram. Sound profile — the results are fitted into a per-ear gain curve (half-gain rule for v1; NAL-NL2 is a pluggable strategy for later). Real-time assist — mic in, per-ear DSP, earbuds out. Quiet speech gets louder. Works with whatever earbuds you already own. No root, no special hardware. It is very deliberately not a medical device — no diagnosis, no treatment claims, big disclaimer before anything plays a tone. Think of it as the open, inspectable "gateway" tier below real hearing care. This post is about the three engineering decisions that turned out to matter most. 1. The safety-critical DSP is pure Kotlin — and that's the whole point An app that amplifies sound directly into human ears has exactly one unforgivable failure mode: being loud when it shouldn't be. So the entire signal chain is plain Kotlin with zero Android dependencies, hidden behind a tiny I/O interface. AudioRecord / AudioTrack is a dumb shell; everything that can hurt someone is JVM-testable: input → EQ