Actually, hearing can be damaged by both continuous exposure to high noise levels or one time exposure to a very high noise level. If you've ever experienced ringing in the ears that lasted for 2 days or more, you've lost some of your hearing.

Now there are some mitigating factors that apply to a Self Defense shooting. One is that it does take a much higher intermittent exposure to cause the same level of damage as exposure to much lower noise levels for a prolonged period of time. Carpeting and funiture in a home will also reduce reflected noise and this will mitigate the damage, shoot a 38 spl. in your living area and you probably won't suffer any loss, shoot the same gun in a mostly empty basement with concrete walls and floor and you'll probably have some hearing loss. Shooting outside will also mitigage the effect of firing a gun without hearing protection. Barrel length will also reduce the emitted noise, my 6 1/2 inch 610 shooting 40 S&W is about equal in loudness to shooting a 4 inch 38 spl.. So, there are many factors that can act to mitigage any hearing damage.

However, these factors only go so far. Personally, I don't think the 357 Magnum is a good choice for a Defense caliber, it's just too loud out of any barrel shooter than 10 or 12 inches. Out of a typical carry revolver, the 357 Magnum will be loud enough that some hearing loss is nearly guaranteed. IMO, the best choice for a carry revolver is a modern 38 +P hollowpoint such as the Speer Gold Dot short barrel. For the Home, it's best to choose a gun with a longer barrel, such as the 6 inch 686, and load it with 38 +P hollowpoints for standard length barrels. You could also do as I did and get the long barreled 610 and load it up with 40 S&W hollowpoints, the first time I ever shot my 610 I was surprized at how quiet it was.

For semi's, you've seen the results posted above, the 45 ACP is the least damaging to the hearing in a major caliber. Personally, I chose the 40 S&W and it's an admittedly poor choice in terms of potential harm to the hearing. However, my excuse for making this choice is that I've found I shoot very well with the 40 caliber so in theory I may need fewer shots fired. Probably a pretty bone headed excuse, but I really do enjoy shooting my Sig P239 and won't give it up.

Finally there is one thing that can be done to insure your hearing will always be protected. Unfortunately, it is an expensive solution. That is electronic in the ear hearing protection. Cost for a fitted pair will run over 2000 dollars last time I looked but they are the ultimate answer. They are also probably a bit less costly than hearing aids, so while it's an expensive solution it may be a less expensive path to choose when you think long term.