Kuba Orlik> I can't remember how I first came across these near-ultrasonic 'beacons' ubiquitous in PA systems. I might have been scrolling through the audio spectrum while waiting for the underground train; or it might have been the screeching 'tinnitus-like' sensation I would often get near the loudspeakers at a local shopping centre.
https://www.windytan.com/2024/06/ultrasonic-investigations-in-shopping.html
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