Tuning 70Hz & 140Hz noise with 3 cyl 4S engine

[jeyaselvan]

thread384-207749

Dear All

I was referred a noise problem with 3cyl engine driven (Deuz)compressor. On analysing the spectrum, the engine firing frequency and its harmonics are contributing to the overall levels, as one would expect. The engine has a muffler , which is sourced outside. I am looking as to how to minimise the 70Hz and 140Hz frequeny contents using additional resonators. 140 Hz is a little easier with the muffler designs. Is is possible to reflect the 75Hz as well with the muffler? the wavelength (~8.5m) appears to be too large for a 1/4 wave resonator or expansion chamber.

Kindly for your suggestions

Thanks
Jeyaselvan

 

[GregLocock]

I assume this thing is running at 2800 rpm?

Low frequencies like 70 hz are primarily attenuated by the impedance mismatch between the end of the duct, and the open atmosphere. As such there isn't much you can do about that.

Could you extend the pipe so that the overall length is half a wavelength?

You may find you are exciting a structural mode of the exhaust rather than simple acoustics.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Tmoose]

What's the intake like ?

 

[GregLocock]

Looking at the responses from the different mics I am not really convinced that you have a simple problem with noise coming out of the tailpipe, for both frequencies.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[jeyaselvan]

Thanks for your feedback

This is for the test speed of 2800 rpm which the continous operating speed where noise is declared.

As Greg pointed out, there are other excitation sources as well from the compressors, but are at different frequencies with lesser amplitudes. I tried to check this by measuring near field sound press measurements.(attached). I will be able to meet the target (overall 70dBA @ 7m) if I could contain the 70Hz and 140Hz.

I was thinking whether Helmholtz resonator (70Hz) could be used for this, since these low freqs could not be met with other means. I will definitely give a try to adjust the total length to 2.2m ( @650degC exhaust temp) half wavelength for 140Hz.

I was not very sucessful with Hemholtz in my earlier works. Kindly for your suggestions or references.

Tmoose : In the intake, I do have cylinder firing freqs, but not really dominant. i tried to route the intake away and checked this. Even if we contain the intake pulsation, don't we still have those in the exhaust after the engine? Kindly correct me, if I am wrong.

Jeyaselvan

 

[Tmoose]

Unsilenced car and motorcycle (and recip air compressor) intakes can sure make a lot of noise at full throttle. It's Part of the sales appeal and even the "performance" gains of a system like this.

http://cartoka.com/catalog/images/intake_h102.jpg

I think Intake frequency = firing frequency, slightly out of phase, but with a softer "suction" pulse, so probably fewer harmonics too.