@jimfenton recorded some audio from Cartoon Network in an attempt to find these beacons: <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>FSK at 17.5 and 18.5 kHZ is one possibility, but wasn&#x27;t detected in this experiment.