Sound Attenuation, and how to obtain it.

[Asa]

Afternoon gents
I have a question that I hope someone can answer me. What, in an exhaust system, conclusively provides the best sound attenuation?

Background: I've joined the Formula SAE team here at my university, and we are well on the way to completing this year's car. Unfortunately, the exhaust is too blasted loud. The largest muffler we've built has been measured at 116 dBA (I can put up the specs for it, if you guys want to do any math,) and ideally we'd like to drop a solid 10 decibels.

You guys are the best option I can think of for some solid math to figure some of this out. Thus far all of the info we've gleaned is going to amount to a lot of small changes (for example, running too rich adds a couple decibels), but I can't help but think that we've ignored something major.

I realize I haven't given a lot of info to go off of, but I want to talk general info before we start talking specifics.

http://students.sae.org/cds/formulaseri ... _rules.pdf
Page 84 of the rules outlines the requirements for the exhaust measurement
Our particular engine is a Suzuki LTR-450, and gets tested at approx 7500RPMs

 

[xctasy]

Pm sent.

 

[Soldmy66]

I am not sure what exactly you are looking for, but if you have not looked into Helmholtz Resonators, it might be worth your while.

 

[80broncoman]

I don't know if this might help at all? if not just ignore it.

I had 83 Honda shadow 500 years ago back in 1991 its (500cc V twin) mufflers rusted out.
we had a motorcycle Salvage year around Daytona (I can't remember where) and I bought a whole exh off a Kaw 1000cc sport bike. I used both of the mufflers off of it on my 500.
It was Quiet as a mouse, no flow problems obviously. (since it only had a K&N filter and jet change)

 

[Asa]

We were able to find a Ducati 900ss' muffler that dropped us down to 99 dBA, so we are going to be able to compete. The system is nothing but baffling, the only packing in it is to prevent vibration between the casing and the inner perforated tube

Deano, I'm hunting down those books and seeing if we can get the software, we've got one or two of the books in the shop. THank you

 

[Ono Nadagin]

Try a supertrapp muffler they make them for everything from a mower to race cars, they are tunable more disks you add to them the louder they get.

http://www.supertrapp.com/

Also if the rules allow you can ask em to sponsor your schools team

 

[Asa]

Dean mentioned a Supertrapp as well. We are looking into them, however we want more tech info and haven't been able to get ahold of anyone that can help (and now that we have a passing muffler, we're concentrating on other aspects than this since we need to complete the build within a couple weeks)

We also got our muffler to work at 107dBA by the addition of an expansion/baffle chamber that causes the flow to take an indirect route to the outlet, while passing through a perforated tube.
I shudder to think of what it's doing to our flow, but it will let us compete.

Summer will be fun with testing.

 

[Asa]

Fun fact:
Tip length is very important. We added an 18" tip to our muffler (just mandrel bent tube) and started to remove it from the end in .5" lengths. We ended up moving back through the nodes and anti-nodes of the resonant frequency. The more we cut off, the quieter the muffler became until we reached a limit of roughly 104 dBA at about 8" of tip removal. From there we kept removing material (weight is bad on a racecar) until we got to about 107 dBA.

After that (based on a tip from a former FSAE member on another team) we took the tip inside and made a copy with some other mandrel bent tube we had, then we squished the tip to an oval shape. This knocked down the sound to about 105 dBA.
From what I understand, doing this disrupts how easily the sound waves travel out of the tube, effectively shredding the frequencies at the tip, while not disrupting flow too much.

 

[ludwig]

I would have to second your discovery, although my experience was more empirical. My exhaust pipe stopped just inside the rear bumper. I mounted an 8" turn-down tip to get the smoke out behind the bumper. The reduction in cabin sound was immediately noticeable.

 

[Asa]

I'd be willing to bet that yours was more about pointing the exhaust away from the body of the car, than a significant reduction of sound. But without testing, who knows?

 

[ludwig]

That and exiting AFTER a cup-shaped steel object connected to the frame.