Long-term hearing damage is a real threat to motorcyclists, and I recently spent some time on the phone with Etymotic audiologist Dr. Patty Johnson AuD to discuss the best ways to protect our hearing. During our conversation, I unleashed the full spectrum of questions I'd cooked up during years spent riding motorcycles and worrying about my ears.
Background photo by Juanjo Viagran
The initial motivation for me to contact an audio expert, however, wasn't motorcycle-related. I'd been researching in-ear—rather than over-the ear—aviation headsets, and had read on an aviation forum that even if in-ear headsets sound quiet, vibrations from sound waves can travel through the bones around your ears and still cause damage. So that's where I started the interview.
RideApart: During some research, I read that it’s possible to sustain hearing damage even if you’re wearing earplugs. This is apparently because noise can travel through bones, which can damage your ears—even if a noise doesn’t perceptibly seem loud to you at the time. Is that true?
Dr. Johnson: That depends. If a sound is loud enough, yes, you can damage your hearing even while wearing earplugs. One of the best examples is people who work on an aircraft carrier, where the levels are really high—150 - 160 dB. People working there are using foam earplugs inserted deeply plus earmuffs. Even with that double protection, the exposure is so high that if they really want to limit risk, they have to limit time in the environment.
Sound is energy and is a wave. It’s pushing on your body, not just your ear. It does get transmitted through the skull at that high a level. If you had maximum protection—no ear canal at all—that would protect your inner ear by about 60dB. If the sound is 160 dB and you had no outer ear at all, then 100 dB would get to your inner ear. Aircraft carriers, IEDs, and gunfire are extraordinarily loud. There are some sounds that are so loud that you cannot prevent damage with just hearing protection, again, you also have to limit exposure time.
My Thoughts: The Etymotics' Noise Exposure Chart got me thinking. How can it be that you can safely listen to a chainsaw for only one minute each week—for a total of 520 minutes over a 1-decade period—but not for, say, 520 minutes in a row?
RideApart: Your hearing chart suggests that hearing damage isn’t cumulative if you limit exposure to a certain amount, and that ears can heal from day to day. Is that the case?
Dr. Johnson: Yes. I think what you’re getting at is how the damage occurs. When you have a big percussive shockwave that hits your ear like an explosion, it is possible to rupture the eardrum. Behind the eardrum is the middle ear. It has three little bones that pass the vibration onto the inner ear. In an explosion, they can become disarticulated. The wave is so strong that it permanently damages the stereocilia in the inner ear, and they cannot recover.
The situation you’re referring to is a situation of metabolic exhaustion or overload. The inner ear, like any organ in our body, requires nutrients to function. When we’re exposed to high-level of sound of over long durations, the ear uses up nutrients that are in the fluid in the inner ear. The hair cells—the stereocilia—kind of lay down. If you give the ear rest—quiet—the ear can recharge and recover, and the hair cells stand back up. Ears ring after a loud sound exposure, and that’s because you did some temporary damage and the sterocilia layed down. After a period of rest, they recover and stand upright. That’s the metabolic story.
But there’s another piece of the story that has been coming out recently, from researchers doing work on animals. I’m most familiar with a study by Dr. Kujawa and Dr. Liberman at Harvard. They gave animals a noise dose that causes temporary hearing loss. What they find is that after the animals have recovered, the stereocilia go back up, and the hearing levels recover. When they dissect the animals’ inner ears later, however, they see damage to the nerve fibers that go from the stereocilia to the brain. There is a swelling at the base of the hair cell and the nerve fiber deteriorates and detaches from the hair cell. While the ear may have mechanically recovered, there is a loss off transmission of some of the sound to the brain. It is subtle, but is progressive over time. Dr. Kujawa and Dr. Liberman think is that there is damage is beyond the level of the inner ear and it progresses over time. We first start to notice it when we have trouble hearing people in a noisy place—because of the damaged nerve fibers.
As an audiologist, this work has completely changed the way I look at noise-induced hearing loss. We used to think that temporary hearing changes were just temporary. The animal models suggest that there are other things going on that we can’t measure, and mean that it is important that we protect our hearing even from “temporary” damage.
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