Inaudible is a fuzzy line.<p>I remember growing up always hearing around the house the hum of the CRT TV when my parents would turn the cablebox off and not realize they left the tv running.  Parents never heard anything.  You start to lose hearing at upper frequencies around as early as in your 30&#x27;s... when I talk to my friends about this, a surprising number of them (even the non-techies) remember this barely-audible-CRT-hum noise pollution phenomenon.<p>The upper range of human hearing is about 20khz.  From a design and manufacturing-costs point of view, it doesn&#x27;t make sense to design a speaker that can reproduce sounds much above human hearing.  If you&#x27;re gonna be optimizing the speaker cone for <i>anything</i>, you&#x27;re probably gonna set an upper bound around the upper range of human hearing, <i>maaaybe</i> go a little bit above if you&#x27;re high quality and want to reproduce everything.<p>So, your average TV speaker is probably going to reproduce some sounds above the upper range of human hearing, but not too far above.  The higher into the inaudible range you design your beacon, the more likely it&#x27;s <i>not</i> going to work, because not enough TV&#x27;s are going to be able to reproduce it and your system becomes unreliable.<p>If I was building such a beacon, there&#x27;s a good argument to be made to target your signal tone at or slightly <i>below</i> the upper range of human hearing, making it audible.<p>So (unless I don&#x27;t know some detail about speaker design and there&#x27;s a class of speakers that aren&#x27;t generally limited at around 20khz, in which case please share) I actually think it&#x27;s likely they&#x27;re using <i>almost</i>-inaudible tones.  In which case OP should smile knowing 1) he&#x27;s not yet losing his upper range of hearing, and 2) he&#x27;s now experiencing the new generation of the barely-audible-CRT-hum noise pollution phenomenon.<p>-------<p>[edit: yep, looks like that&#x27;s exactly what they&#x27;re doing.  Check out this short blog post from a comment further down about a guy spectograph-hunting for these:<p><a href="http:&#x2F;&#x2F;altmode.org&#x2F;2015&#x2F;11&#x2F;13&#x2F;searching-for-ultrasonic-beacons&#x2F;" rel="nofollow">http:&#x2F;&#x2F;altmode.org&#x2F;2015&#x2F;11&#x2F;13&#x2F;searching-for-ultrasonic-beaco...</a><p>The bottom of the post links to a patent for this tech.  &quot;It refers to the insertion of frequency-shift keying modulated data at 17.5 and 18.5 kHz.&quot;  Boom.  Right in the fuzzy area of the limits of human hearing. ]