Fan noise reduction

From DIYWiki

Fan noise can be a persistant minor irritant. The good news is that noise reduction is fairly simple to install, and can greatly reduce noise levels.

Noise reduction methods

Buying quiet

Domestic extractor fans cover the noise level range from 30dB to over 50dB. Choosing a low noise fan at purchase time can reduce noise by up to 20dB.

The technical details behind the figures manufacturers quote are not always equivalent, and you need to compare equivalent measurements to get a useful comparison.

As a guide, look for a slower fan with more blades. This will move as much air as a faster fan with fewer blades, but more quietly.

Flexible mounting

Fans can be mounted on rubber washers (or other flexible mounts) to prevent transmission of vibration to the wall or joist.

To be effective, screws should be done up until there is no slack movement, but no further. Overtightening would eliminate the flexibility of the mounting. Once mounted the fan should be able to be moved very slightly, but with no slack.

Sometimes it's practical to use flexible mounting both between case & wall, and again between motor & case.

      ___
     |_ _|     <--- screw head
    === ===    <--- penny washer
    === ===    <--- 3/4" tap washer
 ====== ====== <--- fan case
    === ===    <--- 3/4" tap washer
 ////// ////// <--- wall
 ////// //////

Simple but effective flexible mounting

For large fans mounted by threaded steel rod, the rod can sit in a wider hole lined with soft plastic pipe, with rubber washers top and bottom to allow a tiny bit of movement.


          __||__           <--- threaded rod protruding
         |      |          <--- locking nut
         |______|
         |      |          <--- locking nut
         |______|
       ____________        <--- penny washer
       ____________
______|__        __|______ <--- 3/4" rubber tap washer
         | |  | |
         | |  | |
         | |  |<----- threaded rod
         | |  | |
         | |  | |<--- soft plastic pipe
         | |  | |
 Joist   | |  | |     Joist
         | |  | |
_________| |  | |___________
      |____________|      <--- 3/4" tap washer
       ____________       <--- penny washer
         |      |
         |______|         <--- locking nut
            ||
            ||            <--- threaded rod
            ||
            ||

Various sizes of soft (rubbery) plastic pipe can be had from car accessory dealers etc.

Damping

Attaching damping material to the fan case interior can reduce noise to some degree. Of course the damping material should not get close to the fan blades, nor block motor cooling airflow. A fireproof or self extinguishing material is to be recommended, as fans do catch fire at times. Mineral wool insulation is one example.

The amount of noise reduction this provides is limited, but it's simple to do, and nothing is visible when the work is finished.


Speed control

Reducing fan speed at times when full extraction isn't needed reduces noise a fair amount. It does also heavily reduce flow rate, but in some cases this is acceptable.

Permanent speed reduction can achieve quite a bit of noise reduction with some fans. This is most simply done with a dropper or transformer for small amounts of reduction, or an electronic controller for larger amounts.

In the few cases where motor noise is a large component of total noise, reducing speed is not very effective. However such fans are in the minority.

Remote mounting (eg attic)

Remote mounting of fans reduces noise transmission to the house interior. This is often done with bathroom extractor fans, an inline fan is mounted in the loft.

Bigger fan

A bigger fan running slower produces less noise when all other things are equal. Naturally all other things are rarely equal, but looking at fans a size up run at reduced speed can often get you a lower noise figure. Running at reduced speed also increases fan longevity.

The downside is that purchase price goes up rapidly as fan size increases.

Balancing & levelling blades

With ceiling fans, balancing and levelling the blades can make quite a difference.

This sometimes helps with other types of fan if the metal blades have become bent out of alignment - though some seem to work quite happily with blades pointing all over the place.

Blade alignment and balance primarily addresses shaking, thumping, flapping and other low frequency noises. It does also have some effect on the high frequency air movement noise, but not as much.


Duct choice

Corrugated flexible ducting has considerable resistance to airflow, necessitating the use of a more powerful fan to get sufficient flow. Rigid duct enables use of a quieter fan.

Increasing duct size also much reduces air resistance, enabling less fan power or speed. Hence sheet material boxing performs much better than minimum size plastic duct.

Baffle box

A well baffled air path can produce major noise reduction. This measure can produce much more result than the others, and is used for serious noise problems. It can be used not only to reduce fan & motor noise, but also to filter out almost all the noise from outdoors when moving air in or out.

When installing a remote fan in the attic, this effect can be incorporated to a limited but still useful extent simply by snaking the air tube. This is not nearly as effective as a proper baffle box, but is easy to do and helps.

Single baffle

The diminutive form of the baffle box is an empty box lined with carpet (or other material) with an outlet hole at 90 degrees to the inlet. These give modest noise reduction, but are very easy and quick to construct.

These boxes were mainly used to quieten portable appliances such as computers, projectors, lab equipment etc.

New Fan Mechanism

Sometimes fitting a completely new fan mechanism will help. A network video recorder had an internal fan so noisy the machine was banished to inside a built-in cupboard, which was awkward for maintenance. The built-in 40 mm fan was removed and replaced by a "quiet" drop-in replacement, and the equipment became almost inaudible. It could then be installed in a better location, and the cupboard space freed up for normal storage.

See Also