Muffler design

[reng]

Hey all, this is about the only science forum I was able to find - after reading some of the posts I determined this is a good source for help and sarcasm if I do something stupid I have a tl1000r (motorcycle) that I want to convert the two exhaust mufflers to undertail sound absorption rather than trying to cancel it out. (There are no aftermarket exhaust systems for this conversion.) I will draw up the design when I get home, this is my first post and wanted to introduce myself. My idea was to perforate sheetmetal and use about 1/4" thick sheet of some decent inexpensive sound absorbing material (obviously with a high heat tolerance because the headers and exhaust pipes are heat wrapped) and then insulate that with some sort of glass coating wrapped finally with more metal. I need it to be a relatively small muffler because I need to fit two of them undertail. (Photos of the bike and what I want to do when I get home). Any input would be appreciated, I have no experience designing this kind of thing.

[Externet]

Hi. Not much that I can help with, but your question touches a point that has always been spinning in my head...

 

Imagine that a SINGLE pipe collects all the cylinders exhausts; if that pipe is split in two equal branches downstream; they both carry in-phase pressure waves along their lenghts... The peaks and valleys of the noise waveform coincide equally along both branches.

 

By the law of physics, if both equal branches are joined together later downstream, the peaks and valleys would add and the noise result would be again the same. But if one branch is made a certain amount longer than the other, in such way the longer branch yields 180 degrees out of phase noise, joining them again downstream would cause the peaks to be cancelled by the valleys, attenuating the noise to a theoretically great degree.

 

Or am I missing something ? How could this cancellation of waves principle be achieved?

 

Miguel

[Cap'n Refsmmat]

Hi. Not much that I can help with' date=' but your question touches a point that has always been spinning in my head...

 

Imagine that a SINGLE pipe collects all the cylinders exhausts; if that pipe is split in two equal branches downstream; they both carry in-phase pressure waves along their lenghts... The peaks and valleys of the noise waveform coincide equally along both branches.

 

By the law of physics, if both equal branches are joined together later downstream, the peaks and valleys would add and the noise result would be again the same. But if one branch is made a certain amount longer than the other, in such way the longer branch yields 180 degrees out of phase noise, joining them again downstream would cause the peaks to be cancelled by the valleys, attenuating the noise to a theoretically great degree.

 

Or am I missing something ? How could this cancellation of waves principle be achieved?

 

Miguel[/quote']

That would work if all of the noise was of the same wavelength (the length difference of the sides depends on the wavelength) but not if it was a range.

[reng]

that and if you seperate the pipes from each cylinder they are already firing out of phase, and you have to remember that a 4 stroke 2 cylinder motor will while one is firing the other one is not firing, it is preparing to fire so it will not produce the same noise.. the only way i see is if you can match the pipes for cylinder fire,

[lightwave]

That's nicely thought out Externet and should work to reduce the noise level somewhat.

 

Lengthening each pipe and placing heat absorbing material inside will reduce the final gas temperature and reduce the noise as a result.

[Externet]

Hi Cap'n

 

...."That would work if all of the noise was of the same wavelength "... Can you please explain, I want to understand your thinking. ¿Is it related to the distance travelled by a valley and a peak of a high frequency is much shorter than at a low frequency and that would determine the needed lenghtening of one branch?? I see something there, but unclear.

 

The noise in EACH branch of the pipe would be EQUAL at all moments. One branch would have to be delayed properly... I think. Even if the bang, bang, bang, source noise changed to bing, bang, bung, will still be the same in each branch, the point is how to achieve cancellation.

And the speed of sound propagation is a constant.

 

 

reng, For a nice initial question, you do not know what you are saying on you last reply. It is to cancel the noise, not the bursts timing:-(

 

Miguel

[Cap'n Refsmmat]

Hi Cap'n

 

...."That would work if all of the noise was of the same wavelength "... Can you please explain' date=' I want to understand your thinking. ¿Is it related to the distance travelled by a valley and a peak of a high frequency is much shorter than at a low frequency and that would determine the needed lenghtening of one branch?? I see something there, but unclear.

 

The noise in EACH branch of the pipe would be EQUAL at all moments. One branch would have to be delayed properly... I think. Even if the bang, bang, bang, source noise changed to bing, bang, bung, will still be the same in each branch, the point is how to achieve cancellation.

And the speed of sound propagation is a constant.[/quote']

That's true, but you have to take into account that the delay would have to be different for different wavelengths of sound. Cancellation will only occur if the waves are offset at exactly 1/2 of a wavelength, but the wavelength of the exhaust varies greatly.

[reng]

reng' date=' For a nice initial question, you do not know what you are saying on you last reply. It is to cancel the noise, not the bursts timing:-(

 

Miguel[/quote']

i was thinking from the headers back perspective..i was thinking that by seperating it there it might be easier, but you would have to match cylinders that are firing simultaneously. i am not really sure what i am talking about - i really don't much about this type of thing; just throwing it out there trying to help out. it was meant to be more of a question but it came off wrong.

[chilehed]

reng,

 

I know a little bit about exhaust systems. You're proposing a very ambitious project. If you want to learn stuff, knock yourself out. If you want a system that performs better than stock, it'd probably be better to buy aftermarket.

[reng]

well, i am waiting on the caliper (sp?) to get here so i can start measuring... i can get pipes bent for free so no worries there.

 

the only aftermarket i want is termi and sato..neither of them make exhausts for my bike (:

[gcol]

Are we confusing two things here? Noise reduction tuning and performance tuning do not go hand in hand. you tune for one or the other. That is why racing machinery shatters your eardrums. Uninformed tinkering with an already tuned system can seriously degrade engine performance.

[chilehed]

reng,

 

The fabrication is the easy part, it's the design issues that'll trip you up. Remember that the exhaust is just a part of a total system, and when you change it you affect the intake side as well. So tuning the system is important, it's really easy to end up with resonant interference between the ntake and exhaust sides and a loss of total airflow. Depending on how the fuel system is designed you can run lean at some engine speeds and rich at others, not the hot ticket for performance or durability.

 

Other things being equal, for a given power output there's a minimum size for the muffler chambers in order to acheive a desired amount of noise reduction, so if you want to reduce the size of the mufflers you'll end up sacrificing either noise or power. The stock mufflers look kind'a big, so my guess is that if you use a single unit and squeeze it under the seat it'll take a bit of finesse to avoid losing power.

 

At the very least, remember that when you bolt in the new system you'll have to look very carefully at your fuel and spark calibration to make sure how much you need to change them. It would really stink to end up with a melted piston because you ended up lean and oversparked.

 

Also, don't forget the heat shielding.