Silencing a register/air duct

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Didier A. Depireux - 22 Dec 2003 17:20 GMT

I am moving my lab to another building, and among other things I am
inheriting a former cold room. They want to install an air feed to it, but I
am concerned that in the process of installing a register (air exhaust) I
will wind up with a major source of noise, as is the case in my current lab.
Is there a simple way to make sure the register will be silent? Is there a
good place on the web to educate myself?

Thanks, as always,

Didier

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Didier A Depireux ddepi001@umaryland.edu didier@isr.umd.edu
685 W.Baltimore Str http://neurobiology.umaryland.edu/depireux.htm
Anatomy and Neurobiology Phone: 410-706-1272 (off)
University of Maryland -1273 (lab)
Baltimore MD 21201 USA Fax: 1-410-706-2512

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Angelo Campanella - 22 Dec 2003 17:39 GMT

> I am moving my lab to another building, and among other things I am
> inheriting a former cold room. They want to install an air feed to it, but I
> am concerned that in the process of installing a register (air exhaust) I
> will wind up with a major source of noise, as is the case in my current lab.

Quite possible. This problem is solved in audiometric test booths by
placing the fan on the outside surface of, or remote from, the booth.
The air is routed on a path through the interior of a wall section
(running parallel to the studs) where sound absorption is already
present to be a de facto "lined duct". This path is made long enough to
provide suitable quiet moving air.

> Is there a simple way to make sure the register will be silent? Is there a
> good place on the web to educate myself?

An open hole is quietest, NO diffuser. The larger the final opening
crossection, the quieter..

Angelo Campanella

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Noral D. Stewart - 23 Dec 2003 12:43 GMT

You have three possible sources sound. First is the fan or other devices in
the system. You have to provide adequate silencing between these devices
and the air grille. Next would be flow induced sound in the duct system
between the fan and grille. This is handled with an appropriate combination
of duct liner and/or larger ducts so the velocity is low. Finally, you
have the sound generated at the grille, and often by a damper just behind
the grille. This noise is strongly related to the velocity of the airflow.
Sound increases about 20 dB for each doubling of air speed. Thus, a larger
opening with few constraints allows the slowest air speed and the least
noise. As you reduce the size of the opening with grille bars or by closing
a damper, the air still tries to get through, but has to speed up to get
through the smaller opening, thus more sound.

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Noral Stewart
www.sacnc.com

> I am moving my lab to another building, and among other things I am
> inheriting a former cold room. They want to install an air feed to it, but I
[quoted text clipped - 6 lines]
>
> Didier

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Natural Science Forum / Physics / Acoustics / December 2003

Silencing a register/air duct