My old ears don’t work much above 15K anyway. I cannot hear a thing at 20K. I discovered that all of the females in the house are quite sensitive to it though. Now, I am not advocating this in any way and I certainly would never do such a thing. But… if an older gentlemen such as myself is experiencing annoying female traffic near the bench when trying to get very important electronic work done. Well, he might just construct a simple ultrasonic repellent using his signal generator, test amplifier and a spare tweeter. I have it on good authority that some of the younger ones can be repelled with frequencies in the 21~22 kHz range but 19~20 is effective with all of them young enough to have the energy to come drag you away from your bench.