about exaust bibration 2

[molossi74]

hi to averyone !

How can I reduce vibration from exhaust system without reduce torque and power ?

Does anyone know any formulas / materials about resonances of exhuast and pipelines system ?

Many thanks

 

[GregLocock]

"How can I reduce vibration from exhaust system without reduce torque and power ?"

Softer exhaust hangers. or try adding 25 kg weights near each exhaust hanger. Or clad it in lead sheet. Or move the exhaust hanger to a nodal position.

"Does anyone know any formulas / materials about resonances of exhuast and pipelines system ?"

There are no useful general formulas for the structural behaviour of exhaust systems.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[molossi74]

thsnks Greg .

anyway I have a strange vibration (with acoustic boom) on V8 engine : 3000 rpm , 200 Hz forced by 4° engine order , just with by-pass valveopened . But I can't solve the problen closing the valve ....!!!!

by

Molossi74

 

[Rob45]

Check to see if any of the hangers are attached to the chassis at points having a 200 Hz natural frequency. Resonances of the attaching points are far more like to be the problem than are strucutral modes of the exhaust system itself.

You could begin duplicating the vibration with the vehicle stationary - on a chassis dyno if necessary to load the engine - and disconnect the exhaust hangers one by one.

 

[molossi74]

Thanks Rob ,

we have already done what you say .

I have put accelerometres everywhere and I don't think is a structural problem , but an airborne transfer path (air excited by a fluidodinamic exhaust gas around the engine resonance forced chassis panels and cause noise and vibration ...)We think to put a resonator in order tu "kill" 200 Hz !

 

[Rob45]

OK, put a microphone near the exhaust outlet and compare the outlet noise spectrum with the interior noise spectrum.
If you have the euqipment to do so, simultaneously record exh outlet noise and driver's ear noise, compare spectra and transfer function from one to the other, also looking at the phase relationship between the two.

Then (or instead) make up a remote exhaust or an add-on muffler to reduce outlet noise, and see if that has an effect on the interior noise. If so, then a prop[erly located Helmholtz tuner should solve the problem.
The best tuner location of course would be internal to the muffler.
Remember also that the product of the tuner volume (in cubic inches) and the tuning frequency should be at a minimum = 18000 ("Hz-cubic inches")to get an effective tuner. In your case, that would be 90 cubic inches, or about 1.5L.

The nice thing about a Helmholtz tuner, if it solves the problem, is that there's no appreciable performance loss.

Regards,
Robert

 

[kirkkse]

Hello all, I'm new to this group so forgive me if I ask newbie questions.
I understand the basic physics of 1/4 wave tuners etc. (from electical and RF work) but now I have the problem of reducing the noise from a blower fan and need some practical guidance.
Problem is a noisy "leaf blower" fan with constant speed.
Fan will be in a isolated box and I have about 1 meter of duct where i can put tuners. Rob45 makes it sound simple by merely using a "T" juntion (approriate length etc.). Is that all I need to worry about? any special tricks at the joints etc? Or waht about using varying diameter pipe to make a notch filter, which is better?

Thanks in advance this thread has already given me some good ideas.
PS I'm doing this work in Vietnam and I don't have good access to technical books so any web references would be helpful, thanks!

Kirk

 

[GregLocock]

You'll find it much easier to tune things if you make them cylindrical and use a psiton to adjust lengths.

Find the position of maximum pressure in the duct for the frequency you are trying to cancel with a mic, that is where you need to puth the side branch.

Quarter wave tuners are easier to get right than Helmholtz, if you have the space, and don't need broadband attenuation (that's a debatable point, but we seem to end up using both). 1/4 wave tubes do not have to be straight. Good luck, please post pictures of the setup and your results if you can. The more info you give us the more we can help. If you don't have a spectrum analyser use your computer's soundcard and any number of free or cheap signal analysis programs.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kirkkse]

Thanks Greg!
I like the piston idea easier than cutting and capping for accurate tuning. Any advice on finding the positino of maximum pressure? I have pretty limited resources here so far I am using a off the shelf microphone (karaoke mic actually), mini amplifier and my o-scope with an FFT function. Can I cut a small slot in my duct and slide the mic down the slot to look for the maximum? Does peak sound (from mic) correspond to peak pressure?

Thanks for the help, I like this forum, lots of good serious advice

Kirk

 

[GregLocock]

That instrumentaion will work fine. You might find it easier to poke the mic down the pipe with a stick, otherwise drill lots of holes and tape over the one you aren't using.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kirkkse]

Thanks again Greg,
One final question I would just like to ask for my project planning. Should I consider active noise control as an option. In principle I know what needs to be done, but in practice active noise control seems to be not so easy. I am working with noise from a duct so much more controllable but should I consider it at all (implied is that I won't do any fancy processing of the signal which seems to be the usual approach. If using a mic, amp, simple filtering and speaker (the most simple setup) isn't plausible then I'll abandon the idea for this low budget project and jsut focus on the passive ideas.

Thanks for your feedback, it is very helpful!
Kirk

 

[GregLocock]

Durable loudspeaker and error correction mics are usually too expensive for practical systems.

Very simple ANC will work at constant speed.The first system I saw was manually tuned, it worked. First thing to automate is the gain, then consider variable phase.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.