hearing degradation can take place over a long period of time and can result from the cumulative effects of repeated exposure...
Absolutely true! Two years ramping Boeings back in the early 80's took a big chunk out of my upper range....and that was with ear protectors!!!
Our club has very stiff noise rules, but for what ever reason exempted turbines (I'm not complaining). The test is done at 25 feet from the right side of the model with the soundmeter set for "A" rating, slow resonse. The max limit is 92 db. Just for fun, I tried one of my turbines on a test stand. It blew by 92 db at half throttle, then went way beyond that....
I think, as has been mentioned, the fairest test would be to do the sound test in the air, using a prop plane as a bench mark......