mufflers
 

Hi All

With all of the complaints about noise at some fields, Has any one looked at making mufflers that reduce's "gas" engine noise 50 cc and up to less than 90 db's ? or modifying existing brands to comply. I know R & D is expensive and time sensitive. and I relise that prop and vibration are also problems but i think they should be in another thread. I am aware that tuned pipes are good but they are not practical in scale or arf's with out a lot of modifications.

I am sure there some great engineering folks out there that might just have the ansures. Back a long time a go when I worked for a jet engine test cell manufacture (no names hear) there was a engineering group that did the exhaust systems designs and when standing out side of a cell you could feel/hear an engine running at full throutle but the sound was more like a big swoosh of fast moving air and not as load as a big truck going by, and mabe that kind of technologycould help.

Other than the snuf plug that TROYBUILT MODELS is working on, it is the only thing out there and from what I read it will only get about 2db's reduction. But 2db's is a lot. But for some of the engines it is not enough, and mabe with some tweeks it might be a good ansure JUST MY THINKING.

So what ideas or suggestions have you got that you may have tried ?

Cheers Bob T
AMA13377

RE: mufflers
 

Well the canister mufflers work the best at quieting a engine down. A good canister muffler can get the engine exhaust noise down to below 90 db.
But you still have to use a good low noise prop too. Some engines can have a lot of intake noise at the carb too, that may require a air filter or having the carb enclosed in a cowl or something too.

I made a homemade baffle insert for one of my gas engines and in my case it worked out really well as the muffler was pretty poor for noise reduction to start with. So the baffle insert really did drop the noise level down significantly to almost 90 db.

The souvenir type of aluminum beer cans can work quite similar to canister mufflers if you set them up pretty good too. There is a thread here in the forum about them. But I am not sure if one can get them under 90 db though.

A J'Tec Snuffler muffler can work well to help reduce the engine exhaust noise too. They sell the Snufflers separate so one can go that route easily enough.


http://i248.photobucket.com/albums/g..._insert_02.jpg

http://i248.photobucket.com/albums/g..._engine_03.jpg

This DB reading was standing on top of the plane with the engine running at about 4 to 5 feet away from the engine. Normally one stands about 20 feet away from the plane to make the measurement, so the engine would be a little below 90 db in that case.

http://i248.photobucket.com/albums/g...e_level_02.jpg


Here is a single Snuffler muffler on my 30cc engine for example. They have dual ones as well.
http://i248.photobucket.com/albums/g..._engine_03.jpg

RE: mufflers
 

Jet engine test cells have a large expansion volume with baffles and a tall exhaust stack is directed upward at the rear to disperse the sound away from the ground. Also, the huge volume of water that is sprayed to cool the inside of the exhaust stack helps to reduce the noise. The walls are thick and made from lots of stone and concrete. Even so, the noise of a powerful military jet engine running on max afterburner is barely tolerable in the control rooms where I spent many hours conducting my jet engine evaluation test programs. Control rooms that are remotely located from the test cells are quieter but it is still noisy as hell outdoors.

So, just like a jet engine test cell, the more effective model engine mufflers have thicker walls, large expansion chambers and contain baffles. Now all we need to do is add a water spray system. :D

RE: mufflers
 

Normal USA practice is to measure at 9 feet.
Doubling the distance reduces dB(a) readings by 6 dB, so it would be 84dB(A) which is very silent.

RE: mufflers
 

I agree and also no one mention yet about sound quality,i've used the wrong term to try and describe what type of sound most people find disageeable at any level on a sunday morning..the neighbour mowing his lawn with ANY gas powered two stroke.We hav'nt even got to the field yet if you have houses around where you fly.If you don't like the idea of having to fly anything electric you need to look at a fourstroke engines like it or not.At our field we don't have anyone close to us and since i like shotgun pipes on my saito fourstrokes the louder the better for now.Can't beat that sound friends and cheers

RE: mufflers
 

Pe,
Our AMA was suggesting 20 feet, 96 db over sod and 98 db over a hard surface in their documents.
Now there are some proposals for competition to request a change to the noise rules measurements to make the measurement at 3 meters (approx 12 feet). But I don't know if anything ever came of it. They also proposed to lower the DB's by about 2 too.

Ref page 12 <cite>www.modelaircraft.org/files/memanual.pdf


Some other noise reduction ideas that the AMA suggested was to use a three bladed prop, as the three blade prop is a smaller diameter which results in a slower tip speed. They suggest using as large as possible canister muffler too. Using a soft mount for the engine reduces the airframe noise from the engine vibration too.



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RE: mufflers
 

Thanks Earl,
I assumed the 3m to be the norm (9.8 ft), because it is so often used for quick checks
Europe has 7 meters and 80dB(A) as general norm (23 feet ), but I prefer to take the readings at 14 meters and then add 6dB to the measured value. The larger the distance, the smaller distance errors and plane dimension influence becomes. Not all clubs use the 80dB limit though.

RE: mufflers
 

You still don't address why people find two stroke engines so disageeable to listen to,does'nt matter how good the aeroplane looks as it goes past they sound annoying at best.The local night club plays their music louder than that:)

RE: mufflers
 

A 2 stroke engine, by definition and practice, fires every stroke. The sound you hear is at the given rpm the engine is running. If that's 7000 rpm, the sound eminating is at 7 KHz. It just happens to be in a range that most human ears find annoying.

A 4 stroke turning 7000 rpm, on the other hand, fires every other stroke, or the equivalent of a lower frequency (3.5 KHz). Most human ears perceive that lower frequency as much less annoying.

To the OP, canisters suppress sound a little. Soft mounted engines suppress sound quite a bit. Three bladed props suppress sound some more. Only using one of these elements doesn't buy that much. Using all three buys a whole bunch...

One of these days I'll post a video of my piped, soft mounted, 3 bladed DLE55. You won't believe it's a 55 in the crate. Electric like quiet. I have posted a bunch of photos in several other threads in the Pattern Forum

RE: mufflers
 

7 Khz is 7000 cycles per second. 7000 rpm is 7000 revolutions per minute. ????

RE: mufflers
 

Yes true...thanks

RE: mufflers
 

so? what happens to noise frequency now? Or should it be tertz band spectrum analisys?

RE: mufflers
 

Frequency annoyance is no less true...that's my point. Drop the freq and annoyance factor reduces. That's your answer to "SO?"

RE: mufflers
 

Most aftermarket motorcycle mufflers come with a removable baffle when purchased. I have run mine with and without the baffle and it does considerably quiet the exhaust down. The reason I mention it is because the baffle appears to me nothing more than a straight through metal cylinder with a couple of steps in the diameter and a decreasing diameter thereafter. On the dyno the baffles don't affect performance at all on most bikes and very little on others. Might be a viable option if done properly. The baffle is mounted into the exit of most pipes so installation would be very easy.

RE: mufflers
 

I happen to know the answer.
It is not so much frequency of firing pulses as the difference in dB between adjacent tertz bands of the single pulse frequency analisys. More than 3dB difference and the noise becomes unpleasant.
Good mufflers can compensate that (not so) easily.

RE: mufflers
 

Lets not forget the sounds that come from the intake to the carburetor.  That can add to the total noise generated from an engine.

Simply running a venturi from the carb inlet to a place inside the fuselage can dramatically reduce engine noise.

Another thing is the speed of the prop tips.  Keep the engine running at speeds that don't allow the prop tips to reach the sound barrier (yes they do!) and that ripping sound will disappear.

RE: mufflers
 

In regards to dB, yes, the pressure is what's measured at least in the A scale. Firing frequency contributes to annoyance...that's my point. Reduce the firing frequency, reduce the annoyance

To the OP, But even when firing at 120X per sec (7200 rpm for a 2 stroke), that noise can be mellowed with a good, soft rubber isolation mount for your engine. Yeah, I know some pundits will say never to rubber isolate a gas engine because..........
Hogwash! My set-ups prove rubber isolation works great!

RE: mufflers
 

With measuring distance adjusted to obtain the same dB readings, A four stroke (no muffler) at 12000 rpm will be very much less annoying than a two stroke (no muffler) at 6000 rpm. Same number of firing pulses.

RE: mufflers
 

Same displacement? Same compression? Same prop? Same mount? Same airframe or bench? Same fuel? Same type of ignition? Show me... I'd like to see and hear that one. Maybe some technique could be developed to further quiet the engines

dB readings would be interesting to measure. Some one's ears may be a better judge tho

RE: mufflers
 

I suggest you do some work yourself and learn about noise management.

RE: mufflers
 

+1

RE: mufflers
 

I'd bet dollars to donuts my set-ups are much quieter than yours. I know practical stuff and care less about theory

You didn't answer the questions I posed to you

RE: mufflers
 

I am glad you had good results. It can be done using sound reasoning. (pun intended)
Without theory, (=knowledge, amongst others) you are groping in the dark and results are lucky strikes at best.
With theory (and practical knowledge of the subject), you know where you come from and you know where you are heading. and you have a much better hunch of what is next.

RE: mufflers
 

Just briefly skimmed through this thread and noticed some references to different frequencies (KHZ) and how they are perceived by the human ear. I have been using audio spectrum analyzers for years and have worked with and tuned all types and sizes of sound/PA systems for longer than I care to admit. The most irritating audio frequencies and the area in the overall sound spectrum that sounds(perceived to be) the loudest and does the most damage to the human ear are the frequencies in the 2,000 to 4,000 K range with 3000 to 3500 cycles being the most irritating, cutting and painful frequency range that basically replicates glass breaking on a concrete surface at high intensities. Get rid of (dampen/cut) those frequencies and everything becomes much more pleasant and less fatiguing. Try it with your home stereo or vehicle system(sparingly) if you have the capabilities. You may like it;)

RE: mufflers
 

That frequency is the human yell as well, especially girls and children.
Have you worked with waterfall tertz band frequency/time analisys as well? A lot of nuisances then show very clearly.
Single harmonic tones (sine shaped) seldom are annoying. It's where the higher harmonics and dissonant frequencies are mixed in (non-sine waves) that your toes start to curl up, especially when frequency shift occurs in time with certain tertzes.