Thanks. Well, a little encouragement so I'll carry on a bit. It is an important subject, and I do say 'WHAT' a lot myself. Hearing damage sort of sneaks up on you.
Hearing loss is more a loss of threshold than of volume. A person with damage can't detect quite sounds, but once a sound is heard, the sound is perceived about as loud as to any other person.
The threshold in human hearing isn't the same for all frequencies. There is something called a 'threshold shift,' which is a frequency range where hearing is more sensitive. The shift is around 2k - 4k cycles. As might be suspected, these frequencies are those needed to hear speech clearly, and also are the frequencies that are most damaging to hearing.
The idea to make this relevant to equipment is that the sound source as well as volume must be considered. A sound source very rich in threshold shift frequencies may not be loud enough to trigger strong protective contractions of an ear muscle, but still may do considerable hearing damage. Small chain saws are very rich in these frequencies, and tractors are less so. My dad was a commercial pilot who wore 2 hearing aids for over 15 years. Jet engines are very rich in threshold frequencies, although ambient sound levels in cockpits aren't excessive.
Awhile ago, MarC I think, mentioned electronic ear muffs. They're supposed to work by receiving sound, inverting the phase, and playing it inside the muffs so it partially cancels sound coming from outside. I don't know how well the muffs work, but I do know the technique works in general. I used to selectively invert the phases of my stage monitors to clean up bass wash and control feedback.
I imagine electronic muffs could be made with controls to adjust the amount of cut and the frequencies affected. Such a device could be really useful in allowing an equipment operator to a type of select protection that is appropriate for both the level and the type of sound.
