I saw a Boeing presentation some time ago that discussed the Flight Deck Noise Reduction program on the 737NG... you know the BS that added vortex generators to the base of the forward windows, allegedly changed the routing of ECS piping and mufflers? There were no concrete FACTS on the actual noise level (just a bunch of colored squiggly lines in a chart with no dB values), but a summary slide mentioned the "potential" for 73dB. I think they're off by 10dB. The noise reduction program was supposed to reduce the noise by 3 points, so a "baseline" of 76?
The reduction is supposed to be 1dB from ECS modifications, and 2dB from the boundary layer noise (vortex generators, insulation, etc). In my experience, the modified aircraft are just as bloody noisy as the unmodified ones so I think it's a crock!!
If anyone is active in the Bluecoat forum, I believe Bill Bulfer made some notes on flight deck noise by using a Radio Shack measuring device in the 737-300, 500 and -800 6 or 7 years ago. It has been some time, but I seem to remember at Mach .80, the -500 was quietest at 86 dB, and the -800 was noisier still at 87 db? I may be remembering incorrectly, but I am damn sure the -500 was quieter because it surprised me. You might try contacting him, he may have some more definitive numbers for you.
And to hell with the bloody Steve Jobs product already.