Perforation hole sizes in absorptive mufflers

[BricerRicer]

Hello, I am a student designing a muffler system for a motorcycle engine used in a collegiate racing team. Our team doesn't have access to an industry standard engine simulation software, so determining some design parameters is coming down to making best guesses based on data found in reports by others (experimentally determining them is out of scope).

In a paper outlining an approach to muffler design (

https://www.researchgate.net/public...Design_Development_and_Prototype_Validation),

the authors specify that the diameter of the perforation holes within the muffler should be chosen to attenuate the first 4 modes of the firing frequency of the engine. I understand why the author suggests this, but am unsure of an equation or method to determine the optimal diameter for perforation holes. The authors only provided an equation, D = 1.29/sqrt(Max Engine RPM), which doesn't aid my understanding or really have anything to do with the suggestion made. Can anyone provide literature or a convincing explanation to the authors' suggestion?

I am not able to post in Acoustics forum as I registered as a Student lol... Hoping someone knowledgeable can assist or bump my post to the appropriate group/person.

 

[GregLocock]

I suspect they are suggesting that you use the holes as a Helmholtz absorber, that is, crudely, the mass of the slug of air acts as a draggy piston, bouncing on the elastic volume of the chamber behind it. Sorry can't help you with equations but Helmholtz is widely discussed.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[BricerRicer]

Okay. I did some reading on Helmholtz resonators and can see how these perforation holes may act with the volume of the muffler to be a resonator of such kind. However, I am not sure how to apply it with these boundary conditions...

The equations I found online for sizing the helmholtz resonators all require a positive value for "neck length" which isn't the case for a perforation hole, unless the wall thickness of the pipe is what I'm overlooking. Additionally, I feel it isn't safe to assume the volume of the muffler (minus the volume taken up by the pipe where exhaust gas is travelling) as the effective volume doing work on the pressure wave.

In almost all examples I see of helmholtz sound absorbers, each resonator has a closed cavity in which only one hole is connected to. This isn't the case in any muffler I have seen or read about thus far, leading me to believe that the authors that claim to have created a helmholtz absorbing effect in their mufflers never observed if their calculations/choices proved worthy, instead only being concerned with the overall transmission loss of the muffler which is affected by the other methods of sound absorption present (e.g. insulation material).

Very confusing rabbit hole that I wish I could prototype and experiment my way out of.

 

[GregLocock]

Yes, the wall thickness of the pipe is the (approximate) length of the slug of air. there is no problem with having multiple holes into a volume, it is common in both exhausts and air intakes. I stopped developing exhausts long before there was good simulation software, I believe Ricardo Wave is the usual way to get a first stab at a design but physical development is still needed.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?