"The ground squirrels mentioned converse at 50 khz, which is only just barely outside the measured audible range of domestic dogs, for instance – so perhaps some predators in the wild can actually hear the squirrrels, as evolutionary natural selection would probably favor this ability."
"A grid called a khz, range, of a dual beam made in its initial state of an oscillating."
"On the other hand, a few folks can in fact hear above 21 khz."