Etymotic fitting guide (PDF)Hmm. I think for a multi-hour Springsteen concert I'll put NRR 20 in one ear and NRR 30 in the other ...
112 db Blues bar/Rock concert 5 MINS. 1.25 HRS.
Above 125 dB you are at risk for any period without maximum protection
Update 6/11: Well, it turned out to be a Zydeco New Orleans Folk Rock concert and we weren't all that close to any speakers. Hardly a real test - nothing like the Rock the Vote concerts. The NRR 20 material is a waxy glob that fills the external ear. Unbelievably geeky, but of course that didn't bother me. Unfortunately it made the music rather leaden. I opened a space on the right ear and that seemed to suffice without causing too much damage (I think).
As a concert it felt muddled. This type music really doesn't scale to a major venue -- it's way too intimate. On the other hand, it would have been great on Prairie Home Companion.
Sprinsteen must be getting bored of rock and roll, but it's really his natural home.