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.
Any 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 small gaps can defeat the effectiveness.
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 would also be effective:
______ | | |_______ | | | | / | | | ( . ) | \___/|_____|
Note though that with this layout, the sheet material between fanblade and outlet does need to be very rigid, or it conducts noise from the source to the exit. In practice this is achieved by using thicker material here.
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 and damps the baffles
- 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.