We've recently had testing done at the UK airport I work at.
From 30m away at a 45 degree angle a 737 with both engines at idle power produced around 80dB (as they were testing the noise level at a cabin used by ramp teams).
The point at which noise levels require hearing protection is (IIRC) 85dB. The reason ground crews wear hearing protection is because of the cumulative damaging effects of repeated exposure to louder than normal noise at constant frequency as we have to listen to it 20-30 times a day, every day whilst the passenger only hears it twice.