A recurring beep from an unpacked box room was difficult to trace by ear, so Simon Willison used Claude Code to build Beep Locator: a web app that turns phones, tablets, and computers into networked microphones. The Python server coordinates listener devices through WebSockets, while browser code records recent audio and detects candidate beeps using frequency analysis. When one is detected, a device plays a chirp that helps align the recordings; the server then compares the sound’s arrival across listeners to estimate its location. Devices can also play sounds to help determine their positions, though this proved unreliable enough that Willison added a mode for manually placing them roughly on a map.
The app supports clapping as a test, playback of recordings to identify the target sound, and confirmation of which beep to track. It then uses repeated detections to refine the estimated location and suggest where another listener might improve accuracy. In use, it narrowed the beep’s source to within about 40 centimetres, leading Willison to an old carbon monoxide detector with depleted batteries. The project draws on the 2007 BeepBeep research paper on acoustic ranging with ordinary devices. Its broader demonstration is that a coding agent can gain practical “ears” by analyzing microphone audio through code: Claude could inspect sound data, update listener software live, and iterate on the system while it ran.
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