A detection is one thing: energy in a narrow bin between 17kHz and 22kHz that stayed above the surrounding noise floor for longer than two seconds.
Ultrasonic energy is common. Switch-mode power supplies, LED drivers, CRT-era displays, rodent repellers, motion sensors, laptop coil whine and some fluorescent ballasts all put tones up there. Air conditioning and traffic push broadband noise into the band too.
We do not decode. Confirming a cross-device tracking watermark means demodulating the carrier and recovering the payload, which is specific to each vendor's scheme. This page measures the spectrum and stops there. A detection is a reason to look, not a verdict.
The floor is the median level across every bin from 15 to 24kHz, not the mean. A median ignores the spike you are hunting for, so one loud tone cannot raise the very floor it is measured against.
15dB above that median is roughly 5.6 times the amplitude. Room noise in this band wanders by a few dB frame to frame, so a smaller margin fires on the wandering. A real watermark carrier sits well clear of it.
Two seconds is there because beacons repeat. A watermark has to survive being heard through a room, so it is emitted continuously or in repeating cycles for far longer than that. A single frame over threshold is a click, a key press or a chair scraping.
Both numbers are adjustable in the trigger control, and both are ours rather than a standard.
The capture asks for echoCancellation, noiseSuppression and autoGainControl all set to false. Each of the three is built to remove exactly the kind of steady, non-speech, high-frequency signal we are trying to see.
Browsers treat these as requests. A platform can ignore them, and some mobile audio stacks apply processing below the browser. If your trace is flat above 16kHz on a device you know has a wideband microphone, suspect the OS audio path.
Nyquist matters. A stream sampled at 44.1kHz can represent nothing above 22.05kHz, so the top of the beacon band is at the very edge of what is visible. At 48kHz you see to 24kHz. The tile shows what you actually got.
An advert or a broadcast plays a tone above hearing. An app on a phone in the room, holding microphone permission, hears it and reports the match. That links a television, a shop or a poster to a specific handset, and through the handset to an advertising profile.
The technique drew regulatory attention in the United States and Europe around 2016 and 2017, largely over apps listening without a clear disclosure. It has not gone away, and retail audio, conference systems and smart-TV content recognition all still use audio watermarking.