@prof, the hearing test and my learning of the loss of frequencies above 15k was back in my late 30’s, when I had my "Musician’s Ear Plugs" made. A wax mold is made of one’s ear canal, a soft rubber reproduction of it manufactured, which when inserted into one’s ear canal comes out just to the canal entrance. On that end of the plug is a small hole, into which the little plastic tab on the back of provided "attenuators" is inserted. The attenuators are flat discs about 1/2" in diameter and 1/8" thick, available in different values of dB attenuation---5dB, 10, 15, etc.
I’ve used the plugs ever since, and they are great. The attenuation is even across the frequencies, so the resulting sound heard has the same quality as non-ear plug, with only the SPL reduced. The only problem they have caused me is in regard to dynamics---it’s hard to gauge how hard to strike my drums and cymbals to achieve proper balance with the other instruments and vocals. I haven’t had my ears tested since, but my tinnitus has progressed as I’ve gotten older. When I lay down at night, the hissing in my ears (like white or pink noise) is quite noticeable. I put on a CD, and the hissing gets masked.