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We have genetically modified the humans so that:

  • They can normally hear as well as humans normally hear.

  • Like action movie characters after big gunfights in enclosed spaces, despite prolonged exposure to noises that are immediately injurious to real human hearing, their hearing is back to normal within a few seconds.

  • They look still just like humans from the outside.

We have access to mad science genetic engineering. We don't have to care about how we made the human genome do these things. As long as they're things that any genome does, or might plausibly do, we can use mad science to make the human genome do it in the human ear. But we have no magic and no synthetic solutions like prosthetic parts or indwelling colonies of nanobots.

What kinds of changes could we have induced in the human phenotype which would accomplish our design goals?

Same question put differently: What physiological changes must occur to the human ear to ensure loud noises don't damage the ability to hear while preserving the quality of hearing an average human enjoys?

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  • $\begingroup$ Are you asking, "What physiological changes must occur to the human ear to ensure loud noises don't damage the ability to hear while preserving the quality of hearing an average human enjoys?" $\endgroup$
    – JBH
    Commented Mar 11 at 17:28
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    $\begingroup$ @jbh yes, that's right. $\endgroup$
    – g s
    Commented Mar 11 at 17:59
  • $\begingroup$ I don't have issues with loud noise since my ears are often clogged by thick ear waxs. $\endgroup$
    – user6760
    Commented Mar 12 at 7:25

2 Answers 2

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There are two possible answers to this question, to understand why we need to look at the anatomy involved in hearing.

The human ear is a very interesting piece of biological machinery.

enter image description here

Sounds first impinge on the tympanic membrane, better known as the eardrum, which cause vibrations in a chain of bones, the ossicles, which themselves press on another membrane at the cochlea. The point of this chain is to allow impedance matching of the eardrum, which vibrates best in air, to the cochleal window, which vibrates best in fluid.

Inside the cochlea, there are a line of hairs that act like "tuning forks," vibrating at specific frequencies. These hairs are connected to nerves, which transmit their sensations to the brain through the acoustic nerve. In effect, the cochlea is performing a short-time Fourier transform on the input, and the nerves are encoding the output of this operation, but that's neither here nor there.

Now, hearing loss due to noise is caused when these hairs in the cochlea are damaged due to the force of the input sound. You can think of it as the same thing as a strong gust of wind blowing over a tree; the loudness of the sound creates a pressure wave that "snaps" the hairs in the cochlea, making them unable to vibrate. In response, the attached nerves become hypersensitized due to having no input and begin firing randomly. This results in the ringing sensation of tinnitus.

So to make your species resistant to loud noises, you want to do one of two things: prevent loud noises from reaching the cochlea, and/or make it so the hairs can regenerate in the cochlea.

For the first part, we already have a method of limiting sound: the acoustic reflex.

enter image description here

As part of the hearing organ, there are two muscles whose primary purpose is to act as part of this reflex: the tensor tempani muscle and the stapedius muscle.

The tensor tempani is attached to the eardrum and can tense up to stop it from vibrating. This muscle is why your ear can feel like it is "closing up" in response to a loud noise. It is the tensor tempani stiffening to prevent vibrations from properly coupling with the eardrum. Essentially it is changing the impedance of the eardrum so that it is no longer matched properly to the air. As an aside, this impedance matching is why you're technically deaf underwater: you don't hear through your eardrum, but instead through the bone conduction of your skull. It's also why everything sounds different underwater (if your ears were properly impedance matched to the water it would sound just as clear as in air).

The stapedius muscle is the smallest muscle in the human body at around 1 mm. Like the tensor tempani, it stiffens in response to loud noises. However, instead of stopping the eardrum from vibrating, it instead stops the ossicles from vibrating, which ultimately stops energy from reaching the cochleal window.

So, to answer your question, to be more resistant to loud noises, you want a beefed up version of this acoustic reflex. In your species, let's say the tensor tempani is capable of "folding" over the eardrum rather than simply tightening it the way it does in humans. This completely damps vibrations through the eardrum.

Similarly, let's say the stapedius muscle doesn't simply stiffen the ossicle chain, but actually dislocates the ossicles from each other, preventing them from transmitting power at all.

These two changes would be a very powerful means of limiting the damage from loud noises, but unfortunately, gunshots are extremely loud.

In air, an unsuppressed gunshot is around 170-180dB, depending on the caliber, powder load, and muzzle device employed by the firearm. To avoid long term hearing damage, it's recommended that this level get reduced to below 100 dB.

In the real world, heavy ear protection is used for this, but unfortunately it doesn't seem to be enough. Research has shown that even with proper hearing protection, constant exposure to gunshots can and does still lead to hearing damage over time.

This damage appears to come from the fact that there are alternative ways for energy to reach the cochlea. It has been found that sound waves can actually travel through the mouth, or up the nose, and reach the inner ear 'from behind' through the Eustachian tubes and bone conduction.

So even with a beefed up acoustic reflex, your tinkerers will need for their cochleae to be resistant to noise-induced damage in the first place.

This is where the second part comes in: regeneration of the cochlea.

Let's say your species, in addition to having a very strong acoustic reflex, also has the ability to regenerate the hairs in their cochlae.

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  • $\begingroup$ Very nice explanation. If it is reflex controlled you also need to consider the speed or the reaction. I would expect it takes a few hundred milliseconds for the protection to kick in so for a gun shot you might still get ear damage. The regeneration should help there. $\endgroup$
    – quarague
    Commented Mar 12 at 8:16
  • $\begingroup$ @quarague: A few hundred milliseconds seems high. Humans can detect a delay between pressing a button and a light turning on as low as 60ms. Since the reflex here would be local, I'd expect 60ms is a high estimate. Still... the (first) pressure wave has already passed by that point. $\endgroup$ Commented Mar 12 at 12:32
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    $\begingroup$ @MatthieuM. Detect yes but react I would say no. You can distinguish between the light turning on instanteneous and with 60ms delay but you would only be aware of which of the two happened a few 100 milliseconds after the light is on. If you touch a burning stove it takes a few 100 milliseconds until you hand starts moving away so I wouldn't expect this to be very different. $\endgroup$
    – quarague
    Commented Mar 12 at 12:37
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    $\begingroup$ @quarague: I am admittedly not well versed in biology, however Startle Response gives a 14ms delay between stimulus and clenching of jaw. The "hand of stove" is a stomatic reflex, which involves a round-trip to the spine, literally an arm's length away (and some) from the hand itself and also requires mobilizing quite a few muscles to jerk it away. I wouldn't use it as a benchmark. $\endgroup$ Commented Mar 12 at 13:46
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    $\begingroup$ @quarague the acoustic reflex happens in less than 100 ms. There are electronic earpro devices that cut out when they detect a loud noise and their response time is similar. $\endgroup$
    – stix
    Commented Mar 12 at 16:47
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Perhaps not what you wanted exactly, but porpoises can change the volume they hear at.

https://www.science.org/content/article/whales-and-dolphins-can-naturally-muffle-loud-sounds-potentially-protecting-them-sonar

I couldn't find a source for this, but I believe elephants have some protection from loud noises, using dense tissue in the ears. You could just create hearing aids that block only loud sounds though.

So, add these adaptations to humans, and you should probably be fine.

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    $\begingroup$ A downside is that this requires knowing that a loud sound is coming and preparing for it - it's basically an internalized equivalent of putting your hands over your ears. A loud sound without warning will still deafen. $\endgroup$ Commented Mar 11 at 19:29
  • $\begingroup$ @NuclearHoagie This solution would be ok for the story, however, as those people are aware of when they are going into a gunfight and can activate their protections accordingly. $\endgroup$
    – Mermaker
    Commented Mar 12 at 13:08

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