So one of our readers asked us this question the other day: Do sound canceling headphones function as hearing protection in extremely loud environments, such as near jet engines? If not, does the ambient noise ‘stack’ with the sound cancellation wave and cause more ear damage?
Destructive interference, such as that found in ANC earphones, can, in theory, completely cancel the sound waves it interferes with, hence “destructive”. The problem with super loud noises, however, is that they are beyond the capabilities of the headphones. They simply can’t reproduce the right amplitude and frequency of sound needed to cancel the part of the sound wave that is going to harm your hearing. They might help, but they are far inferior to proper safety ear plugs.
The other issue is that for very loud sounds, the sound doesn’t only reach your eardrums through your ear hole. When you are working in close proximity to large jackhammers and similar equipment, its recommended you use both ear plugs and over the ear muffs. In the case of your headphones, they might not cancel out the reverberations travelling through your skull.
The human skull and body has a limit to how much sound it attenuates. I seem to remember that the attenuation of the head is somewhere in the region of 40 decibels. Very loud noises can still cause hearing damage by transmitting the sound to the eardrums straight through your skull or body. So for super-loud environments, sound protection that covers the whole head is required. I don’t think full-body spacesuit type protection is employed for sound attenuation, but helmets are. See the example here.
There’s ample evidence that the OSHA guidelines for sound exposure are insufficient in some conditions. E.g. helicopter pilots are known to begin suffering from tinnitus even though they are (after protection is applied) exposed to continuous noise which is below the OSHA guidelines.
Your pinna acts as a sort of sound focuser that artificially amplifies certain frequencies. Muffs lessen this effect, but stopping high sound pressure level low frequency material is similar to stopping gamma rays: multiple feet of concrete or lead walls, etc. A good earplug on it’s own should provide almost the same protection as a muff, but a muff in addition couldn’t hurt.