Attenuation in a long concrete riser with bends

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Tom Harper - 28 Dec 2005 15:10 GMT

Hi All

I have a issue with predicting the noise losses along a concrete riser.
This is for a underground facility that has some huge fans needing to
get air to and from the surface.

The risers are typially 3metre x 4 metres in cross section and vary in
length between 12 metres long to about 100 metres. Each of the risers
has a number of bends where the cross section changes shape. Some of
the risers have large chanmbers in the middle of the tunnel that then
continue on to the surface.

Not much absorption in this sort of tunnel this is the source of my
problem. How to predict the TL along a tunnel of this size?

A friend of mine suggested impedance mismatch could be the key. My
little brain thought back to the days of uni and the study of reactive
silencers and such things and this seams quite sensible but I am a bit
vague on the physics now.

Any help or pointers would be greatly appreciated.

Cheers
Tom

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Salmon Egg - 29 Dec 2005 00:47 GMT

On 12/28/05 7:10 AM, in article
1135782605.987011.326530@g47g2000cwa.googlegroups.com, "Tom Harper"
<gu_tar@yahoo.com.au> wrote:

> Hi All
>
[quoted text clipped - 20 lines]
> Cheers
> Tom

My expertise in acoustics is limited. Nevertheless, because we are talking
wave propagation of some kind, I can guess at the fundamentals of the
problem.

My guess is that you are getting some form of waveguiding. While impedance
concepts can be valid for such a situation, that by itself is probably not
enough. The chances are that your situation is to complicated for a simple
model. I would not be surprised if there are numerical partial differential
equation solvers for you situation, but I cannot help you find them or use
them.

Bill

-- Ferme le Bush

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Angelo Campanella - 30 Dec 2005 06:50 GMT

> The risers are typially 3metre x 4 metres in cross section and vary in
> length between 12 metres long to about 100 metres. Each of the risers
[quoted text clipped - 3 lines]
> Not much absorption in this sort of tunnel this is the source of my
> problem. How to predict the TL along a tunnel of this size?

The first appoximation to attenuation comes from the use of the familar
duct attenuation computation methods common to the HVAC industry. They
have relatively abundant data on sheet metal ducts of a variety of
sizes. The effects of scaling are usually evident in their (The ASHRAE
Guide) tables of empirical information. One would substitut4e the
absorption coeficients for conrete (small but real) vs those for sheet
metal.

> A friend of mine suggested impedance mismatch could be the key.

Such mismatch is modeled approximately in the ASHRAE tables

> My
> little brain thought back to the days of uni and the study of reactive
> silencers and such things and this seems quite sensible but I am a bit
> vague on the physics now.

    The ASHRAE Guide uses approximations that are as often as not based on
physical laws. It's not perfect, but it is useful.

    Good Luck.

        Angelo Campanella

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Natural Science Forum / Physics / Acoustics / December 2005

Attenuation in a long concrete riser with bends