To nit-pick: Anything under 20kHZ isn't ultrasonic. :) (Not that either you or the blog author called this high-frequency signalling "ultrasound", but it does appear to be what people are calling it. :/ )
But it seems to fall into the range used by those "mosquito" devices. Meaning it is on the edge of what most humans can hear at young age, and lost with age.
