Hi, I'm from Belgium and here sound requirements are very severe. I am experimenting for some 12 or so years now with all kinds of gear to soften the noise.
Noise is a very complex thing, for example there is a difference in the measured loudness and the degree of irritation it has on humans. So one can read equal amounts of db's but the one kind of sound will irritate people tremendously while the other does so very much less.
I think this has to do with what Diablo states.
This is one thing.
Another thing is indeed standardised measurement. Ofcourse when one has to meet some requirements one has to apply standardised measurements. They give no indication in how much the noise is going to be irritating people, it just is a way of comparing things.
What kind of noise is irritating, it is meanly .60 engines at 13000 + rpm or with other words high monotone frequenties. Even loudnesses of under 30 dba are quite nerve scattering.
4strokes are as loud as 2 strokes but they don't do so much damage because of the lower frequentie.The larger the engine the lower the rpm hence less enerving.
How to keep noise down? 3 things
a) keep rpm down
b) elastic engine mounts
c) last but not least a good muffler.....so NO slimline pitts and co. but canisters.
I know they are difficult to get rid of under the canopy but they are absolute must in this 3-points scheme.
Maybe you will say, I had a canister and I had only so much db's less than whith my "one volume straight out" muffler. That may be the case. It's a 3 points thing !!! take one away and it won't work.
Some practical things. Comparing the noise of props is only useful at equal rpm. My experiance is that some props are definitely quiter than others but that is mostly expressed in the higher rpm's... whitch we try to avoid.
RPM we avoid also in fly-style, so throttle down in dives.
Speed maniaks will have a hard time to keep the sound down....RPM.
Some planes will make more noise than others with the same engine set-up due to all kinds of amplification . Vibration will make everything sing that has play. Work away play in weels and controls and it will be that much quiter again......
So it's a matter of many things at the same time but don't worry.. it will still make noise and hopefully not enough to make your neighbours nervous.

At the end Je fais du bruit..donc je suis (freely translated" I make noise so I exist")