Hi,

living in a small and highly "regulated" country like Switzerland, we have gone great lenghts in optimizing our engines and planes with respect to noise. Laws in Switzerland are quite strict (and what isn't?)

What I found out, in random order (might not apply to you James or all others):
*) The engine ist not the cause #1 for noise problem: check for noise emission on retractable landing gears, control surfaces, canopies, etc. On a "sloppy" plane this can make out as much as 4 dBA, that's quite a lot! Of course this could not be an issue with most of the better built planes. Eliminate all noises not coming from the engine or exhaust - this is very important! Watch for any type of vibration (also from the inside)
*) Try using 3- or 4-blade props, this really helps. Again I can't say how much in dBA, but it's sensible. Bigger props help, too, and don't cost you as much power as you might think. The idea is to lower RPMs but maintain prop efficiency.
*) Props must be balanced, no matter what.
*) Engine noise can't be better dealt with than with a Hyde-type soft mount or silent-blocks (4 rubber grommets, 15x15mm). Always soft mount and don't listen to those who tell you that "you lose 10 to 20% of the power". This might be true (while I personally don't think so, but have no means to prove or refute the claim), but the available power ought to be enough for most of us. Hyde-type or other soft mounts make out as much as 3-4 dBA on most airframes.
*) On the pipe exhaust, a small piece (about 10 cm) of silicon tube helps to lower noise emission by approx 1-2 dBA

With my OS 140RX I read 89 dBA in 3m at my last contest in 2004. I believe 82 dBA in 7m should not be an issue.

Please note that no expensive measure was taken, what I described can be achieved with less than 30 Euros...

Regards,
-Fabrizio