Originally Posted by Silica

The issue is not whether the sound is transmitted via bone conduction. The real issue is whether that the sound is damaging to the inner ear. The sound will be transmitted to the ear at any point on the skull. For example, strike a tuning fork and place it in the middle of your forehead; you will hear the tone in both ears. The loud noise produced by gunfire will demonstrate bone conduction regardless of whether you dampen the mastoid with a muff. Do not get me wrong, I am a strong advocate of hearing protection. I would recommend wearing both muffs and in-ear protection anytime that the circumstances allow. The real value in double protection is not through any effect on bone conduction, but on the additional reduction of sound pressure into the ear canal and to the tympanic membrane. I guess it's a nice way to motivate people to wear double protection, but it's based on a myth.

There is no scientific evidence that bone conduction of noise alone (SPL around 140dB) produces any permanent hearing threshold shift. If you know of such evidence, I would very much appreciate learning of the source.