YES!! FINALLY, someone who has SOME scientifical knowledge! Just too bad that I don't..... Anyways, since I don't (but I love to hear it discussed) have any fluid dynamics experience or knowledge, tell me what you think of this idea:

Perhaps that metal used, even stainless steel, is too thin? I would think that if it is thin (You wont be able to tell by looking at it. You'd need a micrometer to really tell I would think- o?) then, it would act like a pipe organ.

Not only would the length of the pipe change its sound, but the thinner that the wall of the piping is, whether it be steel or aluminum, (or aluminium for everyone else int he world) would cause the sound to be flat or change sound and resonate badly. Am I close here? Probably not....

I

 

One experience I've had with that sort of thing is a reduction of sound levels after painting a muffler - it adds some additional thickness to the can, (albeit measured in thousandths of an inch,) and if you use a flat paint, if provides a less acoustically reflective surface compared to bare metal. HOWEVER, paint on polished stainless doesn't look nearly as good...

 

okay, if i remember my physics correctly, having the entire pipe heavier (i.e. denser or thicker) will simply change the frequency of the resonation... decreasing the frequency will also decrease the range of propagation (i.e. reduce the resonating)...

but actually when i think about it, the weight might be a better idea... because when you go from a light medium to a heavy one, the wave separates and only a small portion of it travels past the transition while the majority of it will rebound and head back upstream... here's the part i'm not sure of though... would the engine be the source of the resonation or would the exhaust be the source of the resonation (i.e. transient hammer effect)? either way, assuming the pipe can handle the constructive wave interference, this will limit the resonation to the portion of pipe between the source and the weight...

 

I had 2 3 hour drives this weekend on the highway and the noise was really annoying.

I might try that weight thing... but it would have to be heat resistant. I wonder where they would have that.

 

You guys are awesome!

Does it matter where this metal is welded on ( intermediate pipe, axle back pipe, or muffler)?

I could probably take an old speaker of mine and attach it to the exhaust to see if anything changes also. Correct?

 

you'd probably get the best effect from using something solid... and since none of us are 100% sure this will work (reality doesn't always follow theory), i think the best thing to try would be to get some lead tape and wrap a big wad around the pipe...

Now, I can think of 3 possibilities for causes of the noise:

1) If the sound comes from a transient hammer effect (just like shutting your tap suddenly) then you should lead tape it just before the mazdaspeed muffler (i'd say keep it 1.5 times the diameter away from muffler itself to allow for wave development). Then clamp it secure with those metal plumber's straps... this - in theory - should make the resonation wave shoot back into the muffler. NOTE: This assumes that the primary cause of the noise travels through the piping, not the air.

2) If it's actually the engine that causes the resonating, then you need to do the same thing between the first-cat and the exhaust header... -- and same assumption again --

3) If it's not the metal, but actually the air that carries the vibrations that make this thing so damn loud, then the only solution I can think of that won't severely limit power is making a T-connection somewhere in the piping, and installing a surge-tank of sorts...

I'm actually in civil eng, not mech... which is why I'm not 100% sure about any of this stuff! but my mech eng buddy should be back from HK in a couple weeks. I'll ask him and see what he thinks...

 

Good reasoning Chumpman! I wish I were a little better versed in the physics/mechanical engineering side of things, myself. Ok, I’ve contemplated this some more and came up a few irrational thoughts:

“Know Thy Enemy.”
By Dr. Wulf, G.E.D., S.O.B.

First up I considered wavelength. After checking the formulas I found the freaking wavelength of sound is on the order “miles.” Our approximate 10.5’ of exhaust pipe don’t mean diddly. Alrighty. What about the actual frequencies we’re dealing with?

Hum, things get interesting…

If one revolution of the crankshaft produces two exhaust pulses, (Two pistons on the compression/power stroke and two on the exhaust/intake stroke – correct me if I’m wrong, please! I hate to think I’m running on two cylinders per crank rev, but that seems to be the logic of it.), then at a 750 RPM idle, you’d generate a 25 Hertz tone. 2000 RPM would give you 66.7 CPS. And 3500 RPM would produce 116.7 CPS. All low frequency stuff. (If you ride in the backseat while someone rows the gears in city traffic and the sub-woofer is cranked you’ll experience some wonderful acoustic phasing between Saliva and MazdaSpeed. Well, not really - it’ll give you one hell of a headache.)

All said and done, we’re dealing with a suspended piece of pipe. Hit the right frequency and you’ve got a tuning fork, or at the very least a standing wave generated down the length of the beast. The same said frequency will be penetrating the cabin with unpleasant consequences to your ears and possible damage to the gray matter between them.

All of this is why appreciate Chumpman’s input – it’s an excellent explanation of what’s going on and possible solutions to it. I think Nissan was using the lead weight to dampen an exhaust resonance and it would seem that we have a good shot at doing the same thing. It’ll be a few days before I can try any experiments, myself, so keep us posted if you get any results SedanMan and P5Freak!

 

Just a suggestion. Would we get more accurate info if we emailed Mazdaspeed about this? I'm not saying that they will change their design or anything, but at least they might tell us if we're on the right track (adding weight to the exhaust).

Probably they will say that it's not a good idea (liability issues), but no harm asking right?

(And I really hope you guys can really solve this issue, coz i'm dying to upgrade my exhaust )

 

hey BigBadWulf, just out of curiousity, where was the weight on the nissan?

and yeah... that's exactly what i was thinking... cuz if you hit anywhere near the natural harmonic frequency (or any mode thereof) it's gonna be REALLY loud...

okay, now if it IS the engine that's the cause, and there is a mode of the natural harmonic frequency at 3000rpm, then logically the muffler should get really loud around there, quiet down once you're past there, and get really loud again when you hit the next mode... I don't have a mazdaspeed installed but could someone tell me if this is true?

 

As best I remember the weight was about 12" back from the muffler and toward the rear axle. It was attached to a small piece of angle iron that was welded to the side of the exhaust pipe and this was located before the muffler flange. The rear flange was closer to the muffler on the Nissan - you had maybe 6" to the end of the can. I'm guessing the weight was about 10 pounds judging by the size of it.

There were a couple resonance points, though I've taken my MazdaSpeed off for the time being and can't check this. It seemed like there was one in the lower 2k RPM range and another around 3500 RPM. (A.K.A. "The Interstate Banshee.") As I'd get closer to 4K and over, the system quieted down.

 

3400rpm is where things get loud. It drops off again around 4800k rpm.

I don't know if this helps but the back seat does not resonate anywhere near as bad as the abuse the driver takes.

I put a few speakers on the exhaust last night just to experiment with ANY change in noise, it did get a bit deeper but I would say that it did not conclude any relavent information.

A friend of mine said to use header tape and just do a bunch of the exhaust. Whats your opinion on that?

I need to find a metal shop and hope their time won't cost me an arm and a leg to try this experiment.

 

hmm... if it quiets down past 4k, then it would appear that it is a harmonic resonance problem... the speakers probably didn't work because they weren't heavy enough...

our main difficulties in solving this problem are:
1) we don't know the exact souce
2) we don't know the effective length of the vibrating portion
3) I never learned how to calculate wave resonance around bends!!!

the only idea I have right now, is to attach a pretty heavy weight (i.e. lead taping) around the portion that vibrates the most...
in Euler Buckling Theory (not sure if it applies to wave harmonics) if you force an addition nodal point, you can force the 2nd natural mode to come out and which increases the "frequency", reduces the "wavelength" and more importantly reduces the "amplitude"... but once again I don't know if Euler applies to wave harmonics... if you do get that experiment done... plz tell me!

 

This week is jam-packed or I’d be trying some follow-up experiments. What I’ve got in mind to use is a suitable (as close to 2 1/8” as I can find) “U” bolt, or actually two of these. The ones I’ve looked at come with a drilled flat plate used to close off and clamp the open end of the “U” with a couple nuts. With two plates, I can install the first one next to the exhaust pipe and use the second plate as a clamp for the weight. If I can find lead tape I can even wrap this around the second plate and then attach it. The beauty of this arrangement is the flexibility to move the weight up and down the exhaust pipe and see if it helps. If it doesn’t work, then I’m not out that much money. If it does work, then look for the soon to be released “Chumpman & Wulf Do-It-Yourself Exhaust Tuner Kit” at an automotive store near you…

 

maybe its got defective muffler bearings? j/k

this is the most scientific discussion on exhaust resonance i have ever read on the web! we have some real brainiacs here[smile]

 

YES!!! and we'll... we'll... we'll make uh... hUNDreds of... er... BILLIONS of dollars!!!
But honestly if it does work, that'd be sweet then! a low cost, good flow, good sounding axle-back that doesn't make ya go deaf!

hahahha! yeah... sorry, when he asked for a scientific opinion, the geeky side of me along with all that residual crap lying around in my brain from courses long past sorta just SPLURGED out! hahahhaa!