Helmholtz resonator insertion loss

[8911101]

I design a Helmholtz resonator to attenuate engine induction system noise. The resonator frequency is 400Hz. The resonator was connected to the duct, and insertion loss was measured. It was found that the attenuation at 400Hz was very good. However, there were noise amplifications at 280Hz and 480Hz due to the resonator effect. How does this occur? Do I need design more resonators to reduce side band amplifications? Thanks.

L.L.

 

[hacksaw]

The emergence of other peaks has to do with the characteristics of the system after the resonator is installed.

You can detune you resonator sharpess by increasing the opening size and reducing the length of the neck.

Some times, baffles(partial) in the main flow path are enough to break up the parasitic resonances.

 

[8911101]

Thanks a lot, Hacksaw. I am wondering if the resonator changed the standing wave setup in the duct. Do the parasitic resonances always occur in this type of application?

Thanks,

L.L.

 

[hacksaw]

Yes on both counts.

You can never achieve perfect cancelation without creating a new issue to pin down.

It is still an art.

Good luck,

 

[Hatch]

Try filling the resonator with some absorption material (fiberglass) to damp out the resonances. Otherwise add more resonators at the other 2 frequecies and see if you can them them without adding more. C. Hugh

 

[Rob45]

Unfortunately, a Helmholtz tuner always produces sidebands, but one way to reduce those is by designing in some "leakage" into the tuner if it's not purely a side-branch design.
If it is a side-branch, you can also change the angle of the tuner vs the flowstream. The more the flow impinges directly on the resonator throat, the greater the attneuation but also the greater is the sidebranch effect.
So you could try pointing the inlet downstream in the flow.