[oh_sigh]

Or from any human standing in front of the TV.

A better system would be for advertisers or shows to embed an ultrasonic signal in their shows which say the magic words, which google home/alexa watch out for. Of course, that would only prevent unintentional triggerings, so it wouldn't help in the circumstance where the tv show/advertiser actively wants to annoy you.

[cestith]

Then you're depending on the cheap 2w speaker built into the TV to reliably and accurately produce a 20 KHz signal.