From Hartzell I got a link to EASA database
"Whatever the reason, for the listings in the EASA noise level archives, you can bet that we had nothing to do with it. Here’s what I can tell you about it
Prior to 1988 noise was measured at 1000’ AGL (overflight). In FAA speak, this was FAR 36, Appendix G. EASA, or more specifically the Authorities that preceded EASA called in ICAO Annex 16, Chapter 6.
After 1988 both organizations changed to a noise measuring system that took take-off and climb performance into account. The better a plane climbed, the higher it would be at 8200’ from where the airplane started rolling, and the microphone is set up. This Take-off method is called FAR 36, appendix G, or ICAO Annex 16, Chapter 10.
I can tell you that Hartzell never tested a M20C with any prop on it for the FAA or EASA. Both of these prop TC/STC’s were validated in Europe well before EASA was established. Hence they were grand-fathered in without any noise testing on our part.
The note that EASA has pertaining to the three blade entry is “”9/24/201 Engine designation revised”.
Looking at the numbers themselves, the Chapter 6 numbers are on props that were TC’ed. Maybe these numbers came from Mooney from the TC process? I can only guess. The 7282 three blade is approved on an older STC. In 2010 we added eligibility for the Powerflow exhaust to the STC. The only thing I can think of is that when Powerflow applied to validate their STC it must have initiated some noise discussion. If you will note, 85.5 dB(a) is the maximum allowable under Chapter 10. That seems like allot for un untested unit that is the same diameter as the two blade??
So, to get to the bottom of your questions, The props are the same diameter. They should be about the same noise-wise, except the 3 blade climbs better (CHAPTER 10). The numbers in the archive compare two 74” props at the same RPM, using two different methods, one of which may be considerably closer to the mic, and the Chapter 10 listing is as high as it can be to pass. It seems almost arbitrary?? The three blade certainly is not 10 dB(a) louder than the two blade."