While I was testing in the military, it was found that I could audio frequencies as high as 55 KHz, what they call "dog ears", which came in handy despite the fact they couldn't make me a sonar technician (which they really, really wanted to do). I could not only hear the clicks and various normal audible sounds of electronics as they powered on but also the harmonic "singing" in the higher freqs so I always "knew" when it was in proper operation. Really handy when high power electronics (RADAR, navigation, etc. equipment) was involved. I did receive a lot of strange looks over the years though.