You can also trigger wake words like "Ok Google" or "Alexa" by using _harmonics_ of normal human voices that are outside the range of audible sound. The key is that the mic can't differentiate between the harmonics and actual speech of normal human pitch and so the trigger is set off, but the sound isn't audible to humans.

I don't have a link to the paper handy (sorry!) but IIRC I found a white paper on ArXiv called "Dolphin Attack" or similar that demonstrated this. It was a fun read.