You need to have a high sitting landing gear and calm weather. You can try by raising the model slowly from the ground and see the result in the noise meter. I think the noise measurement is quite stupid in the present FAI rules. I don't say it isn't needed, I think it should be made some other way. We are now measuring the wrong thing, not the real noise which is IMHO when the plane is in the air. Even electric planes fail the noise check on bad weather when strong wind comes from side. You can get terrible sounds from the propeller.

You might want to try some foam in your belly pan. A friend of mine has been working with acoustics and he told me you should use melamine sheet or similar open cell foam, that is the most efficent way to absorb noise. Glue some foam all over the bellypan, it weights nothing. I will try that trick next season.

Make a tunnel from foam in belly pan your cooling air intake so that the air flows directly to the engine. This helps little with the noise also and you will gain a bit more power for this as the engine runs cooler. Avoid openings around the engine if you can.

The smaller hole you have in the exhaust extension tube the quieter it gets. If you have a short 15mm extension try a long 10mm one and you will see the difference in the noise meter. There are commerically available external silencers you might want to try.

Try foam filter on the intake. It protects your engine against dirt, absorbs noise, has very little effect on the power, but is hard to get to stay on.

You might check that you have soft enough rubber isolators on pipe and header mounts. Put some foam tape between the belly and the fuse and make sure they aren't hitting each other or touchning anything else that moves like the header.

16x12 might be a bit light for the engine, if you get the RPM close to 9000 you will get some noise as the tip speed raises. Try 16,5X12W I like it a lot on 140DZ, another one might be 17x12N, I havent tried that one yet. Remember that with a four blader you loose some efficency and you double the tips (noise sources). This gives you an increase in the noise in that means, but you gain some as the tip speed goes down with the RPM. Different blade shapes have different noise levels... I think you should fly the prop you like best in the air and try everything else first.


Don't hit the 'secret weapon' and use a noise kill switch. I know many people do this in competitions but it is cheating and therefore stupid, isn't it? There is a more simple way: Warm up the engine a bit hotter than usually before the sound check and you will pass. When the engine gets hot it looses some power and you are ready for the measurement.