Unfortunately, not fair comparison "Although we have every confidence in boattest.com’s findings, they were only performed on one boat, and unlike Motor Boat & Yachting’s sea trials, its tests are paid for by the manufacturer".

Need big scientific research, with many other participants.

For example, it is wide known, that 6-blade prop give near same power as 3-blade but at half rpm, so tip speed is also half.

Curious people, could google quiet helicopter, there are few open publications. Overall, same conclusions - more blades, lower rpm, quiet tip techniques, and try to avoid turbulence from one blade by other blade.