An interesting bi-product.

I was due for, and reminded of, a due eyesight test. Since they offer a free "earsight" test I thought - "Well, why not?"
Having seen, shot, "CS5ed" out the glaring reflection which I hadn't seen I decided to have a play at home.
I came to the conclusion that they must test at a whisper. At home 4kHz was so deafening a SCREECH I had to zap the volume to nil, sharpish, then get rid of that example. I continued hearing, on "at home" volume up to 10kHz. Then I found one doing the range 12-20kHz - heard 19kHz and went totally deaf at 19.5kHz, as thought it had been switched off.
:¬O
I then found "How old are your ears?", but NOT my head-phones. Must hunt them down and have a play.
:¬)
BTW the elderly optics have been the same last 4 years.    ;¬)