I'm not sure about decibel levels, but a couple of years ago or so NHRA invited some of the seismology people to Denver to check on readings. The guys were a little skeptical that their equipment would measure anything at all. They placed the equipment at the starting line and when a couple of Top Fuelers launched it registered somewhere around a 3 if memory serves me right.
The decibel question could be a good one for Mike Dunn.