Tuning 70Hz & 140Hz noise with 3 cyl 4S engine
Tuning 70Hz & 140Hz noise with 3 cyl 4S engine
(OP)
thread384-207749: Expansion chamber help
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
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





RE: Tuning 70Hz & 140Hz noise with 3 cyl 4S engine
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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Tuning 70Hz & 140Hz noise with 3 cyl 4S engine
RE: Tuning 70Hz & 140Hz noise with 3 cyl 4S engine
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Tuning 70Hz & 140Hz noise with 3 cyl 4S engine
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
RE: Tuning 70Hz & 140Hz noise with 3 cyl 4S engine
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.