Hearing Protection: Manufacturing noise-cancelling earmuffs with an NRR (noise reduction) of 40db

When it comes to hearing protection, you have a few main problems: high frequencies, low frequencies, and duration. Presently, there isn’t a single earmuff on the market that has achieved a rating of NRR (40 decibels of noise reduction). I have tried many of these earmuffs and they are all hollow inside, are made out of extremely cheap materials, are extremely lightweight, and they are hollow.

I am looking for a material to use to fill the hollow part of earmuffs to achieve a higher NRR. In particular, I would like a material, or layering of materials, that can help attenuate bass, which all earmuffs and earplugs struggle with. I do not care about the final weight of the material, although obviously if we fill the cavity with gold it will be expensive and bad for your neck.

I was considering a gel or a glue that might be malleable enough to absorb sound, and cheap. What are your thoughts? Ideally the total weight should be less than 1 pound.

This is one reason why aviation ear protection is different. It is tuned for lower frequencies. Passive attenuation of low frequencies requires mass. ANR tuned for lower frequencies and/or mass are what you need.

I’ve seen very thick noise damping rubbery sheets of something used in the walls of a friend’s home theater, but I don’t know what it’s called, where you would find it, or whether it would work.

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All I know is I really wish I took it More seriously in my youth. years of loud punk rock music when I was young, followed by 20 years in factories and manufacturing plants have done my hearing in. Nothing like the constant ringing of tinnitus to remind you that you were stupid when you were younger.

When I worked at the foundry we had to have yearly hearing checks, and after the fourth straight year of appreciable hearing loss I decided I wasn’t long for that world. The back room was a double hearing protection area, you had to have earplugs in with high quality earmuffs over the top. One of those areas that’s so loud that your pants vibrate against your legs.

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I’m now wondering about an elastomer between the outer cup and an inner layer. It would be interesting to compare attenuation over a frequency sweep with two layers of plastic separated by an elastomer with an outer plastic layer separated from an inner tin (soft and not dangerous like lead) layer by an elastomer.

Elastomeric foam is often used in aviation headsets; a solid elastomer might have stronger low-frequency attenuation. You could experiment with elastomeric caulk for this purpose; in the US this includes products like 3M marine adhesive sealant 5200 and I expect similar products are available worldwide. I believe that the best headsets for low-frequency attenuation use gel ear seals.

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I also have tinnitus, and hearing protection that doesn’t attenuate low frequencies gives you an additional kind of tinnitus for low frequencies because it puts them into sharp relief. You then have an internally generated extremely high pitched sound, nothing in the middle, and the only thing that actually makes it to your ear is whatever low frequency sounds are in the vicinity.

You can pair earplugs, bose noise cancelling headphones (inserted in the ear, on top of the earplugs) and earmuffs. It’s an uncomfortable fit, and will sharpen any remaining bass that the noise cancelling doesn’t get to.

For tinnitus, you can pair earplugs, in-ear headphones, and earmuffs (or earmuffs with embedded speakers), and play active white fan noise. This masks your tinnitus, and if you pick a fan recording (download from YouTube) that has some bass in it, it will effectively mask environmental bass. It takes time to get used to.

These are both extreme options, with nothing in the middle. However, for a tinnitus sufferer, active noise is actually good for your tinnitus, because the other sounds have a lateral inhibition effect on the neurons of your tinnitus frequency, which are hyperactive. Unfortunately, listening to white fan noise for too long can become a dissociative. (I have logged thousands of hours), and wearing headphones over earplugs can be uncomfortable. Wearing earplugs too often can also irritate the ear canal, not to mention the risk of putting something in your ear so many times.

If we could do a better job at attenuating bass, it would actually be a big win because bass in your white fan noise is kind of annoying, especially for sleep.

I am currently using this earmuff from 3m which has an NRR of 30. It has multiple layers, hard to say how many. The foam layer is removable, and could tolerate a marine sealant layer (that sealant takes a week to cure!) under the foam.

https://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Pro-Grade-Earmuff

In terms of low-frequency aircraft earmuffs, I don’t see anything aside from a helmet that might be specialized for low frequency sounds. This could also be the crux of the issue - low frequency sounds arriving through the skull from all directions, instead of just through the ear canal :frowning:

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Normal engine RPM in a piston plane is 2200-2800RPM, and the more powerful planes with louder engines are in the low end of that range because they swing a longer propeller, and the speed of the propeller is limited by the speed of sound at the tip of the propeller. Most planes don’t have any gear reduction because of the weight of the gear reduction. 2200RPM is 36Hz.

General aviation headsets are tuned to reduce engine and propeller noise with active noise cancellation.

Sure, that has nothing to do with bone conduction; can’t help you there. But they really are tuned for low frequencies.

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Bone conduction can only be ameliorated by white fan noise with enough bass to mask the contribution of the skeletal system, and ambient bass through the ear canal can only be reduced by active noise canceling if it is either embedded in the earmuff and there is no earplug, or if the headset is the earplug, ie a nice pair of bose in-ear noise cancelling earplugs.

However, as earplugs bose headsets on their own are not as good as real earplugs. However, they actually do provide some noise cancellation as just earplugs, and putting them on top of earplugs prevents both the noise canceling and fan noise contributions from working.

Thus, it actually makes sense to separate the active noise canceling out of the earmuff and to use the bose in-ear headphones, which means that the focus should be on adding 5-10 db of low-frequency-focused noise cancelling to the earmuff, so that you can get the best of all worlds.

I don’t see any reason for these earmuffs to have a cavity except to accommodate in-ear active noise canceling, so it is probably there by design, meaning the design shouldn’t fill the entire cavity, otherwise you won’t be able to fit your in-ear plugs under your earmuffs.

According to a study I read, pressing your earmuffs closer to your head does not improve the noise cancellation, thus, the foam cushions resting on your head aren’t worth replacing for a gel.

This puts the focus on your proposed solution - a more solid elastomer such as marine sealant covered with a soft metal. Tin comes to mind, although I note that Flare Audio has an earplug which uses aluminum and titanium, so aluminum foil on top of the sealant, and then covered by the elastomer foam, may be the most readily available hack.

The prospect of achieving an additional 5-10 db of additional noise reduction using this method strikes me as unlikely. Wouldn’t there be at least one earmuff on the market with a higher noise reduction rating if it worked? I am getting the impression that industry moved in the direction of active noise cancellation for a reason - presumably the $400,000 F35 helmet does actually work, and anything less is not worth the money / effort.

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Perhaps the best solution here: if you have the money, is aircraft-spec noise cancelling earmuffs covering bose in-ear noise cancelling headphones, each thus tuned to a different range of bass (aircraft + inner city).

If you are really hard core, you could also give yourself a craniotomy so you have 50% less skull for bone conduction. You could replace the missing part of the skull with transparent aluminum.

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