TDOA acoustic localization · property maintenance
A hallway sensor network that hears a smoke detector's low-battery chirp and tells maintenance which door to knock on — before anyone walks the building.
A low-battery smoke detector chirps once every 39 seconds, around the clock, until someone finds it. Hallways echo. The chirp from 214 sounds exactly like the chirp from 218. The standard method is to walk the hall, wait almost a minute, walk further, wait again — 20–45 minutes per complaint, longer if the tenant can't say which floor.
There is no affordable product that solves this. Industrial acoustic localization exists — for factories and pipelines, at $5,000–$50,000.
A handful of coin-sized listeners along the hallway ceiling hear the chirp continuously. Because sound takes measurable time to travel — about 1,130 feet per second — the same chirp reaches each listener at a very slightly different instant. That instant, measured against a shared radio clock broadcast once a second, is enough to say which stretch of hallway it came from.
No building WiFi involved. The nodes talk to each other directly over ESP-NOW at the radio level — IT policy and client isolation are irrelevant.
The 127µs figure is one ~10-minute test against a real detector, not a repeated or field result. Current hardware is 2 sensor nodes on a bench — the system has not yet run in an actual hallway.
A plain status ledger, not a sales sheet — this is what's verified, what's off, and what's still open.
Every node is already an always-on ESP32 with power, radio, and a microphone continuously sampling its surroundings. For close to nothing extra per node, that's a foundation for motion sensing (a $1–2 PIR add-on), temperature and humidity (a few-dollar I2C sensor), and ambient noise-level trending — the mic is already listening, it just isn't logged yet. None of it is built, but it's a meaningful product angle worth scoping alongside chirp detection: one sensor network installed for one purpose, reporting on several.
"I've personally walked hundreds of hallways looking for a chirping detector. At some point you start doing the math on how much of your life that is. This system does the math for you." — Ray, apartment maintenance technician, designer & builder of ChirpLocator