44 AMP said:
I suppose its all in how you measure (and describe) it. All the energy is what comes out of the muzzle. Reflected sound adds to what the ear "hears", but not to the actual "sound" (energy) of the muzzle blast itself. Just as the position of the muzzle, relative to the ear changes what the ear hears, but doesn't change what comes out of the muzzle at all.
You're right that all the energy comes originally from the muzzle blast, but the point is that
some of the energy is reflected back into the room, instead of dissipating into open air, as it does outside. The reflections add to the acoustic energy
reaching your ears. In the case of an impulse noise such a gunshot, you get a set of reflected impulses which are less intense than the direct sound -- but they can still be intense enough to damage your hearing, depending on the size of the room and the reflective properties of the walls.
If you're firing a handgun with a muzzle blast that measures 165 dB in a small room with reflective walls, ceiling, and floor, that's going to produce six reflected impulses within a few milliseconds of the direct sound; if the reflected impulses each measure around 150 dB, which is entirely possible, your ears have now been exposed to seven potentially damaging acoustic events instead of just one, and to the extent that the impulses overlap when they reach your ears, they'll act like a single, more intense acoustic event, and the SPL will be the
sum of their acoustic energy, and so even higher, with more potential for hearing damage. Ouch.
The fact that you're not aware of the reflected sounds as a series of echoes doesn't mean they're not there. The auditory system filters echoes within a few milliseconds of the original sound, and you perceive only the original sound source. This is what enables you to locate sound sources in a reverberant environment. But this filtering happens at the level of the brain, so there's no protective effect in the cochlea. (This is called the precedence effect, and isn't the same thing as auditory exclusion.)
It's no different from a ricochet. A bullet that bounces off a hard surface can still have enough kinetic energy to injure you, and reflected sound can have enough acoustic energy to damage your ears.
The damaging effect of noise is cumulative: exposure to several intense acoustic events is more harmful than exposure to just one, and a longer event will do more damage than a short one of the same intensity. It makes no difference to your ears if these events are direct, from the original sound source, or reflected.
