Ultrasonic Beacon Detector
Listens through your own microphone for tones above hearing, the 18 to 22 kHz band that cross-device tracking beacons use, and shows the live 15 to 24 kHz spectrum, the noise floor and every sustained tone it hears.
Audio is analysed and discarded inside this tab. The local speaker/mic test adds observations at individual frequencies; it does not calibrate microphone coverage.
Watch band 17 to 22 kHz, 15 dB over the floor, held 2 s.
Detector settings
Trigger, watch band and hold apply live. Microphone and resolution apply on the next start. Your choice is remembered on this device. Changes create a settings epoch; the same frequency can appear in more than one observation. Export before starting a new session.
Audio capture details
Detections
How the beacon detector works
Beacons work like this: an advert, broadcast or shop plays a tone above hearing, and an app with microphone permission reports hearing it, linking that place or screen to the handset. Regulators in the US and EU examined the practice in 2016 and 2017.
- Capture. The page asks for the microphone with echoCancellation, noiseSuppression and autoGainControl off, since each strips steady high tones. Browsers and some mobile OS audio stacks may ignore the request.
- Spectrum. A Web Audio analyser with 8192, 16384 or 32768-point FFT reads raw frames with no smoothing. The view runs from 15 kHz to 24 kHz. A 44.1 kHz stream ends at 22.05 kHz; a 48 kHz stream reaches 24 kHz.
- Floor. The floor is the median level across 15 to 24 kHz, so a single tone cannot lift its own baseline.
- Detection. A frequency in the watch band that sits above the floor by the trigger setting, and stays there for the hold time, becomes a detection. Hits are grouped into 100 Hz buckets, a gap under 0.4 s does not break a tone, and a tone that returns in the same bucket within one settings epoch is merged. Exports retain separate observation spans; these do not prove an uninterrupted signal.
- Mic reach test. Six quiet tones at 17, 18, 19, 20, 21 and 22 kHz play through your speaker for 0.7 s each. A tone counts as heard at 10 dB over the floor. Detection pauses while the test runs, so test tones never count.
How to read the result
- A flat trace does not rule out a source, even when local test tones are observed. Most phone and laptop mics roll off above 16 kHz, and speakers roll off too, so a miss can be the speaker.
- A detection means a steady tone was present. Switch-mode power supplies, LED drivers, motion sensors, rodent repellers, coil whine and fluorescent ballasts all emit steady tones here. The page never decodes a carrier.
- Beacon band marks a detection between 18 and 22 kHz. Multi-tone marks tones that overlapped in time; overlap does not establish a shared source.
- Drift is how far the strongest bin wandered while the tone was held. Bin movement does not identify a power supply, beacon or other source.
15 dB over the floor is about 5.6 times the amplitude, and a 2 second hold filters clicks and key presses; beacons repeat far longer. Both numbers are ours and adjustable. Tap a detection for a trace plan: walk toward suspected sources, switch devices off one at a time, and note what was playing.
Ultrasonic beacon FAQ
What is an ultrasonic tracking beacon?
A tone played above most people's hearing, usually around 18 to 22 kHz, by an advert, broadcast or shop. An app with microphone permission that hears it reports back, linking that place or screen to the handset. Regulators in the US and EU examined the practice in 2016 and 2017.
Why does the spectrum stay flat?
Most phone and laptop microphones roll off above 16 kHz and some audio paths strip the band. Run Test mic reach first: it plays six quiet tones from 17 to 22 kHz through your speaker and shows which ones the microphone picks up. If the 19 to 21 kHz tones are not heard, a flat trace means nothing.
Does a detection mean I am being tracked?
Not by itself. Switch-mode power supplies, LED drivers, motion sensors, rodent repellers, coil whine and fluorescent ballasts all emit steady tones in this band. The page never decodes a carrier, so a detection says a steady tone was present and leaves tracing the source to you.
How is a detection counted?
The noise floor is the median level across 15 to 24 kHz. A tone counts when a frequency in the watch band sits above that floor by the trigger setting, 15 dB by default, for the hold time, 2 seconds by default. Tones in the same 100 Hz bucket and settings epoch are merged; changing settings starts a new epoch. Earlier observations retain their original settings.
Is my audio recorded or uploaded?
No. Audio is analysed and discarded inside this tab and never uploaded. Only detector preferences persist on this device. Session evidence is held in tab memory and CSV/JSON exports are written by your browser; export before starting another session.
Raw audio is analysed inside this tab and is not retained or uploaded. Observation summaries stay in tab memory. Digital coverage follows the Web Audio API; reported processing settings come from Media Capture. Acoustic sensitivity and sound pressure are uncalibrated.