Muffler Design Rules 1

[mapponfish]

I hope someone can offer some pointers on simple design rules for mufflers. I have not come across many good books on the subject - those that I have found tend to be either in-depth acoustic theory or on very 'broad-brush' noise control.

I wish to attenuate combustion noise in small burners and the muffler needs to be specifically designed to handle exhaust condensate. For this reason I would prefer a straight-through muffler with a perforated tube. I need some simple rules to size the muffler based on this design.

I appreciate any help on this subject.

 

[jomandar]

typically combustion or ignition freq are identified are in the range of 100 to 500 Hz so u need to work on that if it is engine.
regards
jo'

 

[GregLocock]

Rough calculations will get you in the ball park. So will fancy computer models. The reality is that unless you can afford an over-the-top solution then you will have to do some practical experimentation. Knowledge of the exact spectrum you wish to attenuate, and by how much, is helpful.

A straight through exhaust can be either an expansion chamber, or a Helmholtz resonator. I suggest you search for those terms to get the equations, but bear in mind that the frequency tuning will be approximate, and the amplitude estimates will be very rough.

Cheers

Greg Locock

 

[unclesyd]

I don't know if they have the information your seeking but they might lead you to it.

http://www.auto-ware.com/shoptalkbody_index.htm

 

[Tmoose]

I think a 3rd type of "straight through" exhaust might be described as "absorptive." A silencer can be a combination of more than one types.

The ASHRAE handbooks had very healthy sections on noise and vibration control. I think there were sections covering duct silencers pretty thoroughly in a general kind of way.

A general rule is "bigger is better." Also the discharge area of an expansion chamber should be smaller than the inlet.

Might your budget include active noise cancellation? It's mighty impressive on lower frequencies if you have some duct lengtht to play with.

 

[Rob45]

You wrote:"... those that I have found tend to be either in-depth acoustic theory or on very 'broad-brush' noise control."

And that pretty well sums it up: the people that know muffler design are pretty hesitant to tell their secrets.

For any muffler, you need to know two very basic things: 1) what are the inputs - that is, if it's an internal combustion engine, what is its' layout, and what is the operating RPM range? and 2) what is the exhaust system layout - pipe lengths and intended muffler placement.
Examining the various component pipe lengths in the system, you determine their first few acoustic modes, and considering the engine, you determine which of those operating modes will be driven. Then you design the muffler to incorporate Helmholtz-tuning elements to attenuate those frequencies, considering of course the variance of the speed of sound with gas temperature.

For your case, the study should be much simplified: you should have a constant input from the combustion noise. I'd guess it to be a broad-band noise that you could best attenuate with a straight-through absorbtive silencer with perforated inner pipe. For maximum performance and durability, the perforated inner pipe would be wrapped with stainless steel (409 or 439) rope or "yarn", and then the space between the inner and outer pipes would be packed with basalt-wool (rock wool) or E-glass.
The greater the difference between inner and outer pipe diameters, the greater will be the attenaution, while the length of the muffler will be the quarter-wave of the frequency attenuated most strongly.

I know this is just another "broad-brush" answer, but it should get you started, and as Greg said, you might want to take some data.
Merry Christmas,
Robert

 

[mapponfish]

Thanks to the above respondents. I think it confirms my suspicions that the only sensible route to solving this problem is the traditional one !

 

[5205205460]

Muffler characteristics Test!
I am preparing a report on Automobile Muffler , It will include :
1 Muffler design & Optimization
2 Muffler characteristics estimated in theory and in experimental means (methods, facility);
but I have few reference ,so i will need help for it !
Would you please give me some reference on it .or can you tell the Websitd where I can find reference !
Thank you !

 

[Rob45]

To 5205etc.:
To repeat what I said before: I don't believe there are any good references that discuss practical methods of designing real-life exhaust systems.[br]
There are bits of useful information here and there in a hundred different SAE papers, but nobody I know of has put it all together in one place in a usable form.
For fairly obvious reasons: those that use this information on a daily basis are in direct competition with numerous other people, and are unwilling to commit their secrets to paper. While the theory is pretty straightforward, the application of the theory involves quite a number of little tricks that come close to putting this field into the "black magic" area. There is in fact an SAE paper by Charles Shepherd of Arvin Industries titled something like "Muffler Design - Science or Black Magic?"
I gave a general outline for a theoretical/developmental method for designing and developing an exhaust system, above.
Regards,
Robert

 

[Artsi]

In a non-related topic a forum member pointed another member to seek info on carburators at this site:

http://naca.larc.nasa.gov/

That site has a vast info piled up on muffler design as well. Use search. Here's one sample on muffler research on that site:

http://naca.larc.nasa.gov/reports/1954/naca-report-1192/naca-report-1192.pdf