This is in support of old ears: Keep ALL sounds below 1 kHz. It's the consonant sounds that go away first, and that pretty much means sounds above 1 kHz. That's 1,000 cycles per second, approximately the beep sound oif most microwave ovens.
What?! Microwave ovens beep? Well, I never!
That's not a joke, by the way. My wife can't hear her iPhone's lost/alert sound that the FindMy app on her iPad makes it play when it's lost. So that's useless, and she has to get me to phone her phone. But it all evens out in the end, because I can't hear my iPhone's lost/alert sound at any distance, either. And that's with hearing aids.
If you're smirking--just wait a few years.