While not specifically about HRA, the issue of noise induced hearing loss is an important one to both sound quality and quality of life. To that end, here is an informational piece by Craig Anderton:
The Guardians of Quality Sound
Quality sound does matter, and all of us who love music—whether consumers or professionals—need to take care of the most important piece of home entertainment or hi-fi gear we’ll ever own: our ears. Ears are remarkable, with a dynamic range that exceeds the best audiophile equipment. Given what they put up with, they’re remarkably rugged—but to retain good hearing over the years, it’s important to care for our ears and not take them for granted.
Wearing hearing protection at concerts and under other loud circumstances is obvious, but as I learned recently to my dismay, noise is attacking us all the time—whether we know it or not.
I was on a tight deadline to finish a recording project, and had taken a trans-Atlantic flight back to my studio. I arrived in the morning, rested for a bit, and started work in the afternoon. The next day, I opened the project and thought it sounded overly bright and not what I remembered. I was trying to trace down a technical reason for the brightness, but to no avail. So I pulled back the treble just a bit, and everything fell into place.
After the project was finished, I didn't think of it any more until the next time I tried to mix a record after a long plane flight. I realized that it was the prolonged exposure to moderately high noise levels that was the problem. I now have a rule: no critical listening within at least 24 hours after a flight.
I also have another rule—I now wear hearing protection on planes. However, never use those foam plugs that expand to fill your ear; if there's a sudden loss of cabin pressure, your eardrum could pop. Use protection that doesn't make an airtight seal (like most earbuds, noise-cancelling headphones, or in a pinch, a wadded-up tissue). Another “secret weapon” for airline travel is Earplanes, which you can buy at drug stores like Walgreen’s; they not only reduce noise by up to 20dB, but can relieve discomfort caused by air pressure changes.
Even walking down some city streets justifies hearing protection. Ears are at their most vulnerable when exposed to noise for extended periods of time, which can happen just by taking a long walk in a big city. Another potential source of trouble is driving with the window open, and the wind whistling past your ears—that can put more pressure on your ears than you realize. One piano-playing friend lost quite a bit of hearing in his left ear due to his heavy driving schedule. And flying when you have a serious cold or sinus infection is asking for trouble and potential long-term damage. The price of changing a flight is far less than the cost of losing your hearing.
There are quite a few “designer” earplugs that reduce noise, but don't produce a muffled sound. Carry a pair around whenever you're traveling, because you never know when something is going to assault your ears. And don’t forget to schedule a trip to an audiologist from time to time. After all, your enjoyment in listening to music for the rest of your life depends on taking care of your ears.
Author/musician Craig Anderton is currently Chief Magic Officer of Gibson Brands, Inc.