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Wearing both ear muffs (whether passive or active) and earplugs simultaneously can't yield more than 36 dB noise reduction ratio (NRR), because:

it does not prevent the sound to reach your inner ear from the flesh, or your skull…

which the same answer estimates to attenuate by around -32.64 dB, or a bit higher with the skull.

Does there exist a noise-cancelling or noise-isolating helmet with a noise reduction ratio (NRR) larger than 36 dB? Or some other kind of device that aims to overcome this 36dB limitation.

I don't intend this to be a shopping question but I just want to know if the concept exists or is feasible. I tried googling but I couldn't find it, so I might not have used the proper keywords for the search.

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If by "wearable" you mean something that's just sitting on your head, the answer is probably "no".

There are two ways that sound can get into the human body:

  1. Airborne sound: transmitted through the air and received through the ears (outer ear -> middle ear -> inner ear -> cochlea ->brain).
  2. Structure borne sound: vibrations transmitted through floor, chair, walls into the body and that transmitted through the body into the cochlea (were your sound receptors are). This can technically include sound that comes through the air but gets into your body through some other means than the ears, but this is rarely the case

Reduction of airborne sound is relatively easy to measure since there are are only two ingress points, where the sound can be quantified. Even that has a fair bit of complexity: the sound attenuation is usually strongly dependent on frequency and the definition of NRR makes some very simplifying assumptions around that.

The current limits of airborne sound reduction through plugs and over the ear contraptions are indeed around 30dB limited especially at low frequencies by head-to-head, ear-to-ear, and placement variations. Everyone's head is different and getting a consistently good air seal at very low frequency is hard and also requires a LOT of clamping force which limits the wearability.

These are also the attenuation levels were structure borne sound start to become relevant and then things get much more complex. Even measuring the exposure is very difficult: in order to do this correctly you would have to drill a hole in the petrous bone close to the cochlea and then screw in a very sensitive accelerometer. That's a bit on the invasive side.

The conduction is also complicated. Bones are very good conductors of sound but the floppy bits (flesh, skin, organs, fat, ...) not so much. So the question becomes how does the vibration get into the body in the first place and how and where does it get into the skeleton. You are much more likely to pick up vibration from the floor if you are standing barefoot as compared to wearing fluffy slippers. If you sit down, it all depends on how your rear end couples to the seating surface. This is one of the few cases where a little more junk in the trunk is actually a good thing :-)

So any type of structure borne sound reduction would have to be specific to the contact and conduction mechanism. I'm guessing a heavy duty diving suit would help with most of that especially if you wear fluffy slippers on top of it, but I don't think there is a one-size-fits-all off-the-shelf solution.

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  • $\begingroup$ Thanks for the great answer! "in order to do this correctly you would have to drill a hole in the petrous bone close to the cochlea and then screw in a very sensitive accelerometer. That's a bit on the invasive side." can't we just take a dead body? $\endgroup$
    – Franck Dernoncourt
    Commented Apr 4 at 20:52
  • $\begingroup$ @FranckDernoncourt if you want to reduce the noise the dead body hears... the point is that if you want to achieve a better than 30dB reduction with active techniques, you need measurements that are accurate to better than 0.1%. You can't used data from another time and place. $\endgroup$
    – hobbs
    Commented Apr 5 at 12:27
  • $\begingroup$ @FranckDernoncourt: The ethics, regulations and oversight of doing any type of experimentation with a dead body are extensive, complicated and expensive (for good reasons). You are not going to get one at "corpses-are-us". In this case it would also be of limited use (for a variety of technical reasons). $\endgroup$
    – Hilmar
    Commented Apr 5 at 12:45
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Not intending to contradict Hilmar's very thorough answer in any way, I just want to throw in some additional thoughts:

  • Why would anybody need this? >30dB is a very decent amount of attenuation and will be able to reduce anything above the pain threshold to something not harmful. (Except for some very freaky cases, where damaged ears would be one's least problem.)

  • Why would anybody want this? In "normal" sonic environments, assuming an ideal noise reduction aside the threshold, 35dB will take you to some 50dBSPL. That's already quite quiet (pardon the pun). Have you been to a quiet room? You will start to feel funny below 30dBSPL and you will start to feel unwell below 15dBSPL. Some of the quietest rooms there are take you to or even beyond 0dBSPL. In these rooms you will go bat shit crazy in no time, as you will hear each and every sound your body makes. Blood rushing through arteries, your heartbeat, joints cracking, cartilage crunching and squeaking, intestines moving. Your breath will sound like a hurricane. (That's all apart from the fact, that with the complete abscence of acoustic reflections [as these rooms are built to simulate a perfect free field] in this kind of room, it feels way weird, if you say something out loud.)

  • Our hearing system works adaptively over a very large range. An environment of 30dBSPL will seem no quiter than one of 50dBSPL after some exposure time. So there is no inherent sense in getting ever quiter. Noise cancellation is done for two purposes: reduce harmful levels to harmless levels (no need to be very quite) and improve SNR in conversation/pure listening (no need to increase the attenuation anymore, once the intelligibility hits a certain point). In none of these two, very hight attenuations are required or even useful.

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  • $\begingroup$ Thanks! "Why would anybody need this?" Eg sleep, work or study in a very noisy environment. $\endgroup$
    – Franck Dernoncourt
    Commented Apr 5 at 7:00
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    $\begingroup$ For these purposes 36dB is usually plenty, if achieved over the whole hearing frequency range. The cases where it's not enought seem rather exotic, like sleeping on an airport runway or studying next to an operating excavator. $\endgroup$
    – Max
    Commented Apr 5 at 7:16
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    $\begingroup$ "You will start to feel funny below 30dBSPL". Sorry, that's simply not true. 30 dBSPL is a reasonably quiet bedroom and recording studios are typically around 20 dBSPL. That all feels perfectly normal and very pleasant. "... beyond 0dBSPL. In these rooms you will go bat shit crazy " Also a stark exaggeration. Some people react mildly to it, but that has also to do with the funky looking interior and the fact that you are often standing on a vibrating trampoline. I've spend countless hours in rooms like this and so far no bat shit has materialized :-) $\endgroup$
    – Hilmar
    Commented Apr 5 at 12:39
  • $\begingroup$ OK, it strongly depends on the person, I feel queasy below 15dBSPL. $\endgroup$
    – Max
    Commented Apr 5 at 13:06

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