John Sayers' Recording Studio Design Forum Archive

Plenum chambers: how do they work, could they be used for a cpu iso box?

I read about them in the great website knightfly recommended (http://www.domesticsoundproofing.co.uk/ventilation.htm
). But I can't understand how or why they work. Can somebody explain me?
Also, using them to make an iso box for a computer is a good idea?
Thanks in advance. Cheers, Andrés

Andrés,

A plenum works as a low-pass filter. High frequency sound waves in the duct are reflected from the opening to the plenum back towards the fan, while low frequencies pass through. The reason this works has to do with the difference in acoustic impedance between the duct and the plenum. The narrower duct has a higher acoustic impedance than the wider plenum (a narrow straw is more difficult to blow through than a wide straw). This is acoustically analogous to light waves traveling between two different media. For example, when light tries to pass from glass to air, much of the light is reflected from the interface back into the glass. In the acoustic case it works out such that only the high frequencies are reflected from the interface between the duct and the plenum. Lining the plenum with foam or fiberglass will further reduce the high frequency transmission by simply absorbing more of the sound that still gets through the interface.

A plenum should work reasonably well for a computer fan, since the noise tends to be high frequency. Another effective technique that would work well in conjunction with a plenum is to simply reduce the fan speed. You can do this very easily and cheaply by putting several diodes in series with the fan.

Thomas

thanks for the explanation. I don't know enough to understand all of it but at least I get that it's a hi-pass filter. It sounds counterlogically to me, since those filters let the hi frequency sounds pass (like computer noise) and reject low frequency noise. But I guess you're saying that the device let the high sounds pass to some place where it's reduced, and not to a place where it's heard.
Do you think you could use such a device, kind of like an "exhaust pipe" to the isolation box? And btw, do you know any good literature about computer isolation boxes? Do you think that an isolation technique like "the room in a room " (here "box in a box")?
Thanks, and sorry if I don't get you the first time (this english is a little to tech/specific for me).
Cheers, Andrés

Sorry, "high-pass" was a typographical error (i.e. brain fart ). I meant to write low-pass. A plenum acts as a Low-Pass filter. I corrected it in my post above.

A box within a box is a good idea considering the fan noise comes out more than just the back of the case, as well as the drive noises.

Thomas