Academic Question on Aeroacoustics

[NVHNovice]

Hello Again,
I would like to know if someone has any idea of how to predict the sound pressure level for a case where sound is being emmited from three rectangular slits at the end of a long tunnel like a ventilation duct. The slits are very small compared to the dimensions of the tunnel and are placed at approximately the centre line of the tunnels end face. The slits open into atmosphere.The tunnel has a fluid flow, most commonly air at a certain speed which causes the noise at the slits.
What theory/equations could be best used? Can the the slits be modelled as Quadrupole sources?
I would be really thankful if anyone sends pdfs or links on similar measurements to correlate the model that i am trying to build.
Thanks in advance.

 

[hacksaw]

like a vent or a long tunnel, the guys on this forum (not me by the way) can help you a lot more if you are more specific

state what you are dealing with, pressures, flow (or velocity), current noise levels, etc

 

[NVHNovice]

Here is the problem in more detail. We are trying to predict the SPL or SWL coming out from a ventilation grill which opens in to an open space such as a room. The SPL/SWL determination is to be done just at the exit of the grill so we can model it as a noise source. This grill is at the end of a long ventilation duct which obviously has a certain air flow.
Now the question is, given the velocity of the air in the duct and the duct and grill cross section areas, how does one calculate the SPL/SWL coming out of the grill?
Any help will be GREATLY appreciated
Thanks in advance.

 

[GregLocock]

To be honest I'd measure the sound power spectrum produced by a similar grill with a similar airflow. Architectural acoustics or ventilation specialists may have some ballpark figures or rules of thumb, but for some reason we don't have many of either round here.

Once you know the sound power spectrum at the grill the attenuation in the duct/room is easy to estimate, any textbook for architectural acoustics will have appropriate equations.

I strongly recommend you read a general acoustics book such as Beranek.





Cheers

Greg Locock

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

 

[NVHNovice]

Thanks for the reply Greg, but we dont have a physical prototype of the system. This is an academic project where we would like to just build up a model and verify it analytically. Aim is to be able to predict the sound generation mechanism in a ventilation duct with airflow under a given air velocity given the duct and aperture(grill) geomteries.
Yes i have already looked into Beranek but it didnt help as much i had expected it to.
Thanks a lot for the reply again.

 

[Rob45]

How do you "verify" a model with no physical testing whatsoever?
Doesn't that come perilously close to circular logic?