Resonant Frequency of Screw Holes

[CornishPasty]

Hi all,

I have been asssigned to work on investigating the acoustic characteristics of large rotors with numerous screw holes tapped into it. The holes are M42 of total length 80mm. At the minute I am in the process of assesing the realtive contribution these holes have to the overall noise signature of the rotor.

I started using the Helmholtz principle where the resonant frequency is equal to 55*sqrt(Open Area/(effective length * chamber volume)) to pinpoint the frequency the holes would excite. I have read a lot of literature on the effective length of resonator boxes, and keep coming across the effective length being equal to neck length + 0.3*open diameter. However, this seems to rely on the chamber being large in cross section when compared to the neck. In the case of a screw hole, there is no real change in X sectional area hence my question is does this theory still hold?

Also how would you account for a screw thread in the calculation?

Any thoughts would be appreciated

Kinds Regards

Graeme

 

[IRstuff]

How about:

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/clocol.html#c2

TTFN

FAQ731-376

 

[GregLocock]

They won't behave like Helmholtz resonators. They might behave a bit like quarter wave tuners - similar to organ pipes. That is lambda=L/4, and you will need to correct L for end effects and the diameter.

BUT

I wouldn't bet on it. Much more likely is that they'll act to trip the flow, helping to generate blade passing frequency and its harmonics.


The solution is obvious. Insert a foam plug into the screw hole to fair it in to the surrounding surface.


Cheers

Greg Locock

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

 

[CornishPasty]

Thanks for the thoughts...

Greg:

Ive looked into quarter lambda tubes and did analogise it to my application. The link from IRStuff gave some further information.

From the point of view of it creating passing frequencies. We have done numerous tests blocking the holes with some interesting results in the frequency range of my preliminary claculations for the holes as opposed to operating frequency. Now trying to understand.

Thanks again

 

[Twoballcane]

Did not know holes vibrated..

Tobalcane
"If you avoid failure, you also avoid success."

 

[Tmoose]

Can test holes be played like this?

http://chapuis.zasu.us/photos/ya-bottle1.gif

=================================================


Warning - off topic and possibly inappropriate bottle image link below

http://byfiles.storage.msn.com/y1pL...5HFnNbncKvkMaveE0fdVQDqoGyp1J-mHAffz-lkBFRWfQ

 

[IRstuff]

Twoballcane, some holes even have mass:

http://en.wikipedia.org/wiki/Effective_mass

TTFN

FAQ731-376