Natural Science Forum / Physics / Acoustics / May 2006

It would be best to avoid using any flammable materials as sound insulators.

First thing to check is that you are not violating a basic principle of
ventilation: Move a large air volume slowly, not a small air volume quickly.

In general, smaller fans make more noise than larger fans, so if you can
increase the fan diameter (and probably also the duct size), that would
help.

Next, make sure you don't have line-of-sight down the duct from the fan
to the outlet vent.  Always try and introduce at least one bend in the
duct that will block the line of sight to the fan.

Finally, the inner lining of the duct should be made from a porous
material such as Rockwool or glass fibre (but since these materials can
shed fibres, they should be used in extraction ducts only - you don't
want to blow fibres into the room that people will then have to breath).
 There are various other materials available that will not shed fibres.

So in summary:

 o In order to move a large air volume slowly, make the duct size and
   fan diameter as large as practically possible.
 o Deliberately add dog-legs or bends into your duct to remove the line
   of sight from the fan to the room's outlet.
 o Use a porous material to line the inner walls of the ventilation duct

Chris W

You do not tell us much, but say you want to reduce fan noise in a room
using sound absorbing materials.  You do not say if the fan is in the room
or if the fan noise is being conducted through a duct to the room.  Are you
sure it is fan noise, or is it air flow noise?  Both can be a problem but
can be attacked differently.

A matter of terminology.  Sound absorbing materials are  not usually sound
insulating materials.  They are often thermal insulators.  Sound insulators
are heavy solid materials that block the passage of sound.  Sound absorption
can help improve the insulating ability of a complex wall, but he heavy
surface materials are the sound insulators.

If the problem is flow noise in a duct or at a grille, you can reduce it
very significantly by reducing the flow speed.  This is done by making the
duct or grille larger.  For the same amount of air, the speed through the
duct or grille will be reduced.

If it is fan noise and the noise is being conducting through a duct to the
room, it is most efficient by far to put the sound absorbing material in the
duct.  This is usually done by either lining the duct or using special
sections of duct called silencers that contain absorptive material behind
perforated metal.  The most common material for lining the ducts is
fiberglass duct liner.  It is made with special coated surfaces to eliminate
or at least greatly reduce any chances of loose fibers.  Even if a fiber was
to get loose, it is not small enough to be breathed in like asbestos fibers.
The liner is also treated to resist bacteria growth, but still should be
used with care to avoid excessive moisture in the duct.

You could add sound absorption to the room, but it would take a lot to make
a small change unless the room truly has extremely little absorption to
begin with.  Most rooms have more initial absorption than you might think.

Cardboard is not an efficient sound absorber unless possibly you make some
imaginative construction with it.  I am not familiar with thermocol.