noise and how it is measured

[Dave Collis 2296]

Gentlemen:

As with a lot of clubs, both here in Canada and in the USA, we have lost our field after 40+ years. A major concern is noisey planes.

Our club rule was 95 DB at 10 ft. My question to you is how do you measure your sound pressure readings, and what DB levels have you to achieve?

In my own case I fly a 1/2 scale Spacewalker with a 3W 200. Due to lack of space,(no room for cannisters) my homebuilt muffler has to be in cowl, under the motor. With a lot of playing around with various chambers etc, and going to a 3 bl. prop(32/10) I was able to achieve the req'd 95DB.

I would, however like to lower this, again with different silencer boxes, and maybe routing the carb intake into the fuse.

I don't know if the LMA has it's own sound requirements, or if there is a national sound level or local club level. Any light you may be able to shed on the matter would be greatly appreciated.

Cheers, Dave.

[Tony Collins 1073]

Hi Dave
The BMFA Have a Manual in PDF form and the information that you require regarding the BMFA noise standards can be found here http://www.bmfa.org/handbook/Handbook2007.pdf The relevant info starts at page 62.

Can't quite figure how some of our models can be held 1.2 metres above the ground at full throttle though.
The actual method of testing is described on page 65.

Good luck. Tony Collins 1073

[Bob Thompson1894]

I think that needs updating a bit, Tony! What they dont take into account is 'perceived noise', Where a slow revving 4-stroke or petrol engine is much less offensive than a screaming 2-stroke glo- on the other hand, the high-pitched 2 stroke has a sound which only carries a short distance, whereas a large petrol can be heard some way off. We had a problem at Goosedale where the council set up a decibel meter in a neighbours garden- it would not register anything above the traffic noise. However, we got enough complaints to cause the then management to ban anything over a 120 four stroke. The best silencers I have come across were the german Zahn team, who used the 'two Gaz canisters' idea together with carbon 3-blade props. Quieter than most electrics!

[Dave Collis 2296]

Thank you, gents.
Is there any info available on how to properly construct an effective silencer, re. chamber sizes relative to cyl. capacity, diam of outlet pipes etc?

[Bob Thompson1894]

there is a formula regarding outlet sizes to engine size, but I can not remember what it is, I'm sure somebody does! The Gaz canister idea is two of the lightweight camping canisters soldered or brazed together, bottom to bottm, with holes drilled into the (concave) bottoms staggered to each other so no holes line up, then an inlet brazed into one, and an outlet into the other, this idea first mooted (I think) by 3W. The canisters can be placed in the fuselage and connected by some Mick Reeves flexipipe. I hope this is of help. VERY effective silencer, the loudest thing then would be prop noise.

[Chris Lane]

I know its old but there is a Model Engineer write-up by Edgar T Westbury on http://www.metalstop.com/technical/Silence.pdf which explains the topic nicely. There is also an article on silver soldering and silencer making in AMI for December 2006 which uses empty spray cans - a bit slimmer than Gaz canisters! Neither of these are about TUNED exhausts which are different animals entirely. Chris [2938]

[Dave Collis 2296]

Thanks, Chris.

Interesting reading, gives me something to go on.

Cheers, Dave.