Rich, you missed somethings very important. A 3 dB increase in the level of continuous noise doubles the sound power {pressure}, however experimentation has determined that the frequency response of the human ear results in a perceived doubling of loudness with every 10 dB increase (from
http://en.wikipedia.org/wiki/Decibel). In other words, a 10dB increase that produces a doubling of the perceived sound level (twice a loud) is actually more than 8 times the amount of sound pressure. It is not the loudness that causes damage per se, but the pressure.
Indoor noises are louder can can cause more problems because the sound reflected back from walls, floors, ceilings essentially expands the duration of the event, as with gunshot noise, with the person actually hearing the report several times in very quick succession, reflected sound pressures decreasing with time of distance traveled. So with a single 150 db gunshot report indoors, the first many reflections of the sound that make it back to the shooter may all still be substantial enough to cause hearing damage. So a single report results in several insults via the reflected sound.
You mentioned the cumulative aspect, but presented nothing to sugstantiate it. Hearing damage is cumulative. While those with auditory exclusion during high stress events don't report hearing gunshots they fired or having the sensation of ringing ears afterward, they still suffered hearing damage. They will have suffered physical damage. This is because the hair cell's hair-like stereocilia get damaged or broken by loud noise. These aid in hearing and once damaged do not repair or regenerate.
You said you didn't understand the information you provided and wondered about things like inside v. outside, distance from firearm, and how different calibers can be louder than would be expected. This brings up several interesting points. The testing was probably outside so as to record just the singular event. As for distance, who knows? While distance is important, the pressure of sound decreasing with distance from the sounds, sounds are also directional. Shooting a Barrett 82A1 is not a bad experience for the shooter as the shooter is directly behind the source of the noise that the broadcast forward and then to the sides via the muzzle brake. From a position either side of the shooter by a foot or two, the sound is perceived as much louder and the pressure wave washing over the observer can be strongly felt and can be quite uncomfortable. Simply put, the shooter is in the position that receives the least amount of sound.
So how can small calibers have more sound than larger calibers? There are a whole host of reasons why. First, the size of the caliber isn't critical. We assume that bigger calibers will produce more noise, but this is not necessarily true. Things such as barrel length, powder speed, muzzle brakes, etc. all influence what sound is projected. For example, a shorter barrel may allow for the bullet to be gone long before the powder is finished burning and you get a goodly amount of the powder burning outside of the gun and generating noise outside of the gun which then produces more noise than would be the case for the same caliber, cartridge, load fired from a longer barrel in which the entire amount of powder is burned internally.
Rifles usually have slower powder than pistols. This means that the sound is generated over a longer period of time, hence the potential for more hearing damage because of the extended duration of the sound event.
So, a .25 acp and .223 round can have comparable sound levels? Maybe so. The sound levels reported do not include the aspect of duration. The .25acp with a short barrel and fast burning powder will no doubt be loud as much of the sound will be generated outside of the barrel. .223 with slower burning powder may reach a comparable level and not louder in part because the powder is burned over a longer period of time and much inside the barrel.
Now, a suppressor works by providing a place for those expanding gasses (and sound) exiting the end of the barrel to expand into an enclosed structure with baffles than then essentially release the gasses and sound at a reduced pressure, thereby lowering the report noise.
