Thanks for the thread bump, lots of useful advice here.
A couple of years ago I asked a similar question on the ALMS fan forum (RIP) since I was about to get stationed in a very noisy area of the Toronto IndyCar circuit. After much good advice I decided that the easiest/best solution for me was what I was already doing - 3M foam earplugs (now replaced with generic drugstore ones). Rated at 33, they were better than all of the over-ear defenders I could find, at least those that weren't breathtakingly expensive, and much easier to wear under a wide-brimmed hat!
No use at all if you want to listen to RLM or race control, mind you, but plenty quiet enough for today's turbocharged Indycars, even trackside.
Worst experience for me would have been the Trans Am series cars on the Toronto IndyCar track a few years back... man those things are LOUD! The Canadian Tire NASCAR stock cars were brutal too, but I wasn't as close for those...
