Natural Science Forum / Physics / Acoustics / July 2005

My experience with ducted fan systems, makes me wanna ask why do you
think your friend's problem is a duct resonance? Is the fan attenuated
some how? Fan and possible dust collector could well cause narrow band
noise generation and acoustic resonances as well. The same applies with
the inflow and probably (since i don't know the system) flow in the
ducts. And all these can interact.

The insulation obviously helps if most of the noise comes through the
duct but if it comes from open end(s), well...

But "killing" of the resonances could be done using structures described
(at least) for standardized fan measurements. Basically they are cones
outside ducts with absorption and "holes" into the duct.

Changing the duct geometry, possibly combined with attenuators, could
also help. Possibly chance in fan speed does it.

Probably some one else can help with cleaning, but why should it be a
problem with dry dust?

Unfortunate it's hard to help to more without knowing basically anything
about the system and it's response. So seeing the actual case and narrow
band (and otherwise applied) acoustic intensity measurements would most
probably be fruitful (remembering the earlier conversation on acoustic
intensity:).

Kind Regards,

ari

This is where you use mylar faced absorption materials, protected by
perforated metal to provide erosion protection and to prevent the dust
penetrating into the absorption materials, should be used.

I have seen splitter attenuator panels (10 feet metres by 10 feet 8"
thick) removed from coal mine fan exhausts with 12" nose cones of built
up dust and 1" thick layers of dust "growing back" 3 to 4 feet from the
leading edges. Beyound the built up layer the panels were still fine
with no dust penetration into the absorption material and only a light
dusting of dust across the remaining surfaces.

If the material is "dry" and duct velocities sufficient (5000 fpm or
greater) cleaning should be unnecessary for many years , if at all.