Wrist Sniffer

About

Warning - the app requires paired Apple Watch to function properly. Wrist Sniffer turns your Apple Watch into an ambient ultrasonic-beacon detector. The 17–22 kHz band is increasingly used by cross-device tracking systems - Silverpush, Lisnr, SoundPays, and similar - to pair phones to TV ads, in-store displays, kiosks, and event sound systems without your knowledge. Your wrist mic is closer to the open air than a pocketed phone, which makes the Watch a better sensor for these signals. Open the app, tap Scan, and Wrist Sniffer listens for the distinctive spectral shape of a beacon: a narrowband tone or keyed pattern sitting well above the speech and HVAC floor, in a band human ears cannot hear. When the detector trips, the Watch taps you with a haptic and logs a spectral fingerprint to the paired iPhone for later inspection. WHAT IT DOES • Real-time FFT analysis of microphone input, on-device • Three-band energy detector (5–10 kHz reference, 14–17 kHz guard, 17–22 kHz target) with configurable thresholds and hysteresis • Haptic alerts on the Watch the moment a beacon-shaped signal is detected • Each scan runs in an extended-runtime session so it keeps listening with the wrist down or the screen asleep, up to the watchOS session limit (roughly one hour) - useful for walking through a venue • Spectrogram viewer and detection log on the iPhone • JSON export of detection events for offline analysis WHAT IT IS NOT • Not silently background-continuous. watchOS does not allow indefinit background microphone use, and Wrist Sniffer does not try to. A scan only runs while a session you explicitly started is active, and that session ends when you stop it or when watchOS hits its time cap. • Not a classifier of who placed the beacon. The detector flags beacon-shaped energy; distinguishing one vendor from another is an ongoing engineering effort and the detection log gives you the raw fingerprint to investigate. PRIVACY • Microphone input is analysed entirely on-device, on the Watch and the paired iPhone. • No audio, no spectra, no detection events leave your devices unless you explicitly export them. • No analytics SDKs. No third-party trackers. No accounts. No servers. • Optional location tagging is opt-in and stored only locally. HARDWARE Designed for Apple Watch Series 9 and Apple Watch Ultra 2 or newer, paired with an iPhone running iOS 17 or later. Performance on older Watch hardware is limited by the microphone's high-frequency response and is not supported. A NOTE ON LIMITS Ultrasonic detection on a wrist microphone is a real signal-processing problem, not magic. Anti-aliasing filters, mic placement, body shadowing, and ambient noise all matter. Wrist Sniffer reports what it sees and gives you the raw spectra to second-guess it. It is a tool for the curious - a field instrument, not a guarantee.