Another one for perusal....


A '''baffle box''' is an effective way to reduce the noise of enclosed
fans.

Each time a sound wave hits a baffle it changes propagation direction,
and with each such event the sound level reduces. Hence a sound
reduction box can be made with mutiple baffles, by making the sound
take a serpentine path. This method was used to make car exhaust
silencers for many decades.

_____________________
| |
| |
| | | | --- air flow
| |
________|_______|____

Simple 2 stage baffle system


Unlike many sound reduction methods, this approach allows free passage
of air, making it the method of choice for boxed fans.


A baffled air path offers very slight air resistance, and this is one
reason why car exhaust silencers have moved on to other methods.
However this very slight resistance isn't enough to cause a probem in
the usual DIY applications. Baffle boxes made with 2 or 3 stages work
fine with axial fans. Where very great noise reduction is needed and
more baffle stages are used, a centrifugal fan can be used to maintain
full airflow.


Slight movement of the baffle box during use should be minimised, as
any movement causes direct transmission of sound through the box. This
is achieved by:
* Firm mounting of the box so it won't move
* 3 or 4 fixing points to help avoid the box bending
* Use of stiff rigid sheet material for the box & baffles
* Damping of the sheet material to kill any resonance

Another point to watch for in construction is to ensure all gaps are
sealed. Even tiny gaps can defeat the effectiveness.


==Layout==
The air path can be laid out linearly as in the diagram above, but
there is no need to stick to any particular shape of layout. As long
as the air path turns corners and any passage along the line of sight
is blocked, it should work ok. Hence the layout below is also
effective:

______
| |
|_______ |
| | |
/ | | |
( . ) |
\___/|_____|



==Applications==
Baffle boxes are used wherever free airflow needs to be maintained.
Hence they're mostly fitted to extractor fans to silence them.


Additional noise reduction can be achieved by
* lining (partly or fully) the box interior with long pile carpet
increases the sound reduction at each reflection
* Mounting the fan on rubber washers to prevent it passing vibration
to the wall
* Adding damping material to the fan casing to prevent resonance and
consequent transmission of sound. Fire resistant material is
preferable.


==See Also==
* [[Special:Allpages|Wiki Contents]]
* [[Special:Categories|Wiki Subject Categories]]



[[Category:Noise]]
[[Category:Cooling]]
[[Category:Bathrooms]]
[[Category:Kitchens]]


NT