Noise attenuation due to elbows in ducts 1

[aesoton]

Hello all,
I have been trying to find this information everywhere but without much luck. I want to know the formula for calculating the noise attenuation in a HVAC systme due to the inclusion of a duct. I am aware that for a curved elbow the attenuation is 0dB, but what about a rectangular bend? All I have found is a set of tables from ASHRAE applications 199 Chap46. I would like to know how to calculate the attenuation so that I can tailor the resulta to my own systems.
Any help appreciated.
Andrew299

 

[obg]

I don't think you will get a simple answer to this one. Mitre(rectangular) bends are frequently equiped with internal turning vanes. The number and size of these will determine the airflow resistance of the bend - which will inturn change the amount of re-generated noise. Most people will just take the centre path dimension and allow for distance attenuation only.

 

[Hatch]

The answer to your question should be covered in the paper:

"Explicit formulas for the calculation of regenerated noise in ducts" by Tim M Marks

The entire paper is available online. Do a google search and you should find the downloadable paper. The link is below but I find that long links tend not to work.

http://www.marshallday.com/download...calculation_of_regenerated_noise_in_ducts.pdf

You can also do a google search on "ashrae noise control" to find it. The search details are:
The 33 International Congress and Exposition on Noise Control ...File Format: PDF/Adobe Acrobat - View as HTML
ASHRAE: Systems and Applications Handbook. 1987; [6]. PA Holmes BSRIA 1975; [7]. SRL: Noise control in Mechanical Services . ...

 

[aesoton]

Thanks very much for your replies.
Hatch that was exactly what I was looking for - have a star.
Andrew299

 

[DrRTU]

The Tim Marks paper is a good source for regenerated noise. I believe your question was on attenuation of existing noise and attenuation. Trane has booklet that gives the designer suggestions on minimizing regen and attenuation of noise generated by the prime mover.

http://www.trane.com/Commercial/DocPage.aspx?Item=478