My wife has extremely good hearing. Good enough that the 19 kHz flyback transformer noise from some older CRT TVs bothered her. Recently I bought an induction cooktop and when I turned it on she screamed in pain. It injured her ear enough that it has been sore for 3 weeks. At first I thought it was just that particular unit, so I got another one and she said the beeper was painfully loud. I took out the beeper and she still felt the pain, 25 feet away. Me, I don't hear a thing, but I am 70 dB down or more from 4 kHz up.
I know that induction heating is in the 20KHz and up frequency range. So I thought I would try to find a mic that would work up to 30-40 kHz, hook up to my scope and see what is going on. So the mic does not need to have a flat response, it just needs to capture the frequency. Inexpensive is good, too. It is true I could measure the frequency directly with my scope, but I thought a mic might help identify where the high freq vibration is coming from.