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Discussion Starter #1
I need to pick the brain of someone knowledgeable on the subject.

In short I want to 'tune' the acoustics via internal chambers to damp the most egregious sound at the lower rpms, say...off idle to 4000rpm, and just let some minimal packing take care of business above that. I figure at the speeds I'll be traveling it won't matter, what I want to try to do is keep it civil around the neighborhood. Once I'm moving at speed (away from home) or off bashing some pass a little extra noise isn't a huge deal.

So there you have it.....
 

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killing off he low frequency stuff is the more difficult to do. packing needs to be rather excessive otherwise it passes right through. Packaging/style will be your limiting factors as you'll need an expansion chamber type configuration to kill the low frequency waves. to calculate it you'll need to know/have a good guess at things like exhaust gas temperture
 

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Do a search on Helmholtz resonators.
Absorption seems to work best for high frequencies but your right about reflected wave form for low frequencies.
You'll need to set up interference ranges
I don't know anything about acoustics :D
The frequencies shouldn't be too difficult to work out, may not have the effect you want at higher rpm though (maybe get one of the electronic exhaust cut out flapper things?)
 

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Not too much but there's a ton of design info out there. Walt Strader of Strader exhausts and later hooker headers fame was a smart guy and he has a patent on a multi tube muffler that's worth looking at. Several others have applied similar principles including Dunstall Decibels with the tubes stuffed in the end.

The challenge is to get enough volume in a neat package. That's why bikes like the CB400 Hawk ended up with a muffler box under the motor where it wouldn't be noticed and a pair of "megaphones". Two into one into two also allows each cylinder to breathe out of both pipes which in theory should make it slightly less noisy.

I think the thing to keep in mind is that sound waves and gas don't do the same thing. Sound waves bounce around and cancel each other out - think noise cancelling headphones - and gas just pushes on out. I was doing some calculations last night on a two stroke exhaust and realized that a Jemco pipe on a GT750 has a volume to around 18 times the volume of the cylinder that feeds it. Ignoring expansion of the gas with temperature, that means that it takes several revolutions to push that gas out. It's not as if the one pulse of gas goes all the way out for each cycle. but the noise does. Noise travels at the speed of sound, gas down not. We tend to forget about things like that.

Not sure if any of that helps or if I'm just rambling again.
 

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quote:Originally posted by Teazer


I was doing some calculations last night on a two stroke exhaust and realized that a Jemco pipe on a GT750 has a volume to around 18 times the volume of the cylinder that feeds it. Ignoring expansion of the gas with temperature, that means that it takes several revolutions to push that gas out. It's not as if the one pulse of gas goes all the way out for each cycle. but the noise does.
If you don't get one out for each one in, how much does the pipe weigh after 20 minutes of run time? For 20 minutes you've been stuffing 18 intake cycles in and only getting one out... there must be a lot of exhaust gas stuck in there somewhere. [}:)] (I'm playing devil's advocate here...)

Mass flow out of the pipe equals mass flow into the pipe or you have issues. Assuming you aren't building a compressor. Conservation of mass, people. It's not just a good idea...

Rule of thumb is for a quiet system you want to have ten times engine displacement in the tuning elements (mufflers). You aren't trying to build a quiet system so that doesn't really hold true here but you are trying to tune low frequencies, and they're a bitch without a lot of volume.

Off idle to 4000rpm.. I'm guessing it's a four cylinder? Four stroke? You're trying to control the fundamental frequency. so assume an idle of 800rpm and the frequency range you're looking at is roughly 25 - 135 hz.

You will not be able to fit a helmholtz tuner on the bike that will effectively tune 25 hz . That doesn't really matter though, because even if you could it wouldn't tune the entire range from 25 to 135hz. Helmholtz tuners are rather narrow band filters used to address problem frequencies. If, say, at 2800 rpm the exhaust note got louder and then dropped off again, a Helmholtz could be your friend. But if you put something in there tuned at, say, 75 hz you could well see the exhaust get quieter at 2250 rpm but get louder at 2150 and 2350.

For broadband tuning of low frequencies your best bet is to use smaller tubes. Smaller tubes attenuate lower frequencies. You should also make the mufflers as big as you can since more volume (cubic) typically means lower sound levels. You could also stick a converter in there, they work well as broadband tuners. ;)

There are ways to get a helmholtz tuner to work over a broader band at the expense of max attenuation at the target frequency, but you're still not going to get 100hz of range out of it.

As for packing in the muffer, in the case of typical aftermarket muffler the packing is more or less transparent at frequencies below something like 300hz depending on the type of packing and a a few other things. Certainly below 150hz anyway. Though having said that, I have seen cases where packing in the muffler actually helps low frequency attenuation too. Not by an amazing amount but measurable.

So basically use the smallest tubes you can, the biggest muffler you can, and don't worry about getting fancy because without testing or some good modeling you're likely not going to get what you want anyway. Oh yeah, remember that exhaust gas gets less viscous as it cools so the outlet doesn't have to be as big as the inlet if it's back pressure you're worried about (granted the temp doesn't drop much through a bike system, but still)


P.S. I designed OEM automotive mufflers for about 20 years and motorcycle mufflers for about 5 minutes... if you really do want to get fancy I have a couple ideas but for the work involved you might not be happy with the results.
 

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Eh? Are we being a little obtuse here?

I didn't say that it continues to fill up for ever. :) The point was that the slug of gas that leaves the port on one cycle doesn't reach the end of the pipe on that same cycle. Think of it as a series of donuts being pushed in at one end and pushing out the other end some few revolutions later.

I thought that I made it pretty clear, but I guess not.
 

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Discussion Starter #8
Expansion volume is certainly in question as this IS a bike muffler and there just isn't room for a huge one. I think this will be one of those long term 'mess with it when i have time' projects. Too much to do at present to pursue, was just hoping for a bone.
 

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DD, there isn't a constant flow from cylinder so 3:1 isn't a problem unless it's way too small for single cyl
The latest David Vizard book has some information on exhaust design (it's automotive but the principles are the same)
 
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