1) Important to distinguish between overall hearing loss and frequency-dependent hearing loss. In other words, a person can have the subjective impression that everything is hunky-dory but be losing audibility of high end. Once upon a time when there were no LED screens and it was all CRT, the "flyback" transformers on TVs would provide a 15.75 kHz whine. If it was a high-end TV the whine would be difficult to hear, but regular consumer-grade TVs and monitors would produce it. At the time, I would ask my students if they could hear it or had ever noticed it. Only a small handful would indicate so. Those folks should not be impressed with speakers or headphones that go clear out to 24khz. 16khz bandwidth is probably good enough for everything of interest to humans (dogs and bats are another story), but when useful bandwidth starts dropping to 10khz and below, you can find yourself saying "What did he say?" a lot more often.
One of the things that tends to accompany our penchant for distortion is sustained SPL levels at higher frequencies.
2) One needs to also distinguish between temporary and permanent loss, though I imagine frequent bouts of temporary loss are likely associated with some forms of permanent loss. We know that exposure to bouts of high-SPL sound for concert-length periods can result in temporary hearing loss, with useful bandwidth returning after 24-48hrs. That's not great, but it's not permanent.
3) Shooting ranges for Olympic-type sharpshooting frequently provide earphones that are connected to triggers. Before the hammer hits the bullet, a loud tone in the earphones prompts the various mechanisms in the ear to damp the sensitivity of the ear drum, and interconnecting bones, as well as provide back-pressure through the Eustachian tube, so that the cochlea ends up being bombarded less. Hearing loss is the result of death to hair cells in the basilar membrane, and generally not damage to the bones, So what the warning tone ends up doing is reducing the SPL applied to the oval window of the cochlea. Unfortunately, there's nothing at a gig to prompt such a prophylactic response in one's ears when a mic'd kick drum booms out of 18" subwoofers. You will likely apply "natural" damping of the ear mechanics as you sit/stand and listen - and that's part of what can prompt temporary hearing loss, as that "prevention" remains in place for a while - but there can be plenty of VERY high-SPL bursts that come without warning or preparation that can do damage. And keep in mind that contemporary music has a dangerous mixture of both excessive sustained high end, and enhanced dynamics. Vinyl used to have a dynamic range of around 60db. Digital has a much wider dynamic range, making for sudden high-contrasts. At lower SPL, that merely more engaging and emotive. At higher SPL that can become dangerous.
4) There have been many discussions regarding limiting of SPL levels from personal listening devices, since so much of our music listening is through earphones these days. I'm sure many of you have sat on a bus, train, airplane, or subway and been forced to listen to the "tsss-tsss-tsss-tsss" of someone else's headphones from 20 feet away. Unfortunately, unless mobile listening devices come with proprietary headphones/earphones, there is little way to anticipate what the electronic output of the mobile device translates into in terms of SPL at the ear. After all, one can always engineer a more efficient driver for the earphones that can turn sensible SPL into dangerous SPL. Even with proprietary earphones, though (such as what Apple is making/marketing), there isn't much stopping anyone from producing more and more efficient earphones, or even add-on devices that could, for instance, further amplify a wireless music signal.
So, what to do? One idea that occurred to me - certainly not foolproof - is to make noise-cancelling earphones the default. The idea is that the SPL people listen at is partly a function of what the music is competing with. If noise-cancellation turns the 60db ambient level into 20db, then it won't take high SPL to make the music audible over top of that ambient level. It's like the difference between talking to someone at a club vs talking at the library or church; in the one environment nothing less than yelling will do, while in the other whispering is sufficient for intelligibility. Certainly the technology for making noise-cancellation the default is widely available. All it needs is TRRS plugs and jacks, which are generally used for microphone-equipped earphones. leave the mics on, invert it electronically and feed it to the ear-pieces, and bingo-bango noise-cancellation. It's a thought.
