Eli5 – How do high fidelity earplugs work? What happens to the sound once it travels into the bore of the ear plug?
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Earplugs are used to reduce the total sound reaching the eardrum and prevent potential damage, or potential distraction.
A cotton bud helps too, while not nearly as much as is needed. A blob of play dough works exceptionally well, but is kinda risky cuz it could get stuck. Also, it is too good, that is, it attenuates sound more than needed, and will make everything sound muffled, and be an uncomfortable experience in general. I wouldn’t recommend it, but if you accidentally end up with a wad of playdough up your ear canal, you will find out how amazingly effective it is as a plug.
Between a simple cotton bud and a blob of play dough, there is an array of possible materials and shapes which will give you “just the right” amount of sound reduction across the frequency range. Ideally, for a “hifi” plug, you want the sound profile to have a similar frequency band shape as the source sound, albeit uniformly diminished in magnitude. This is achieved by various means: choosing the right material properties, throwing in controlled leakage, etc.
Tl,Dr: the “hifi” attribute is decided by the overall equalization of the sound reaching the eardrum. The plug doesn’t change the sound itself, it only applies a frequency dependent transmissibility factor onto the incoming sound, which is in turn determined by shape, holes and vents, and material of the plug.
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