Author Topic: Loud hum from variable speed ac fan mptor  (Read 2758 times)


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Loud hum from variable speed ac fan mptor
« on: August 21, 2014, 08:47:54 PM »
I have two centrifugal duct fans (Fanteck FRD 24-14) installed in my condo in two locations which have a loud hum when running. The manufacturer says the sound is what it is. The spec sheet on the motor has  a data chart which says for my operating speed (900 to 1250 RPM)  the (Sound Power re 10(to the -12th) Watts ranges from 82 at (63 Octave Band, HZ) and decreases to 800 at (125 Octave Band HZ) to (76 at 225 OB HZ) to (76 at 255 OB HZ) to (67 at 500 OB HZ) to (62 at 1000 OB HZ)... to 47 at (8000 OB HZ).

The sound unpleasantness is in the lower if not lowest frequency range and emanates both downward from the motor axis and through the duct work. I have some what (about half) reduced the level with "Quiet Barrier HD Sheet material" applied in a double layer to the access panel cover to the fan assembly.
I would like to further reduce that sound direction but need to consider weight.

My most significant and completely unresolved issue is the transmission of the sound through the duct work cavity. Is there a product which could be installed inside the duct work to break up the sound waves or can you suggest another approach?
Thanks for any suggestions.

Randy S

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Re: Loud hum from variable speed ac fan mptor
« Reply #1 on: August 25, 2014, 06:17:03 PM »
yes you can use our SSP foam mat material.
Here is the link to the many sizes we carry.

If you have the room put a couple baffle turns in the duct.

Google " baffle box" and you will see what looks like rat mazes, these are called baffle boxes and you want to mimic this in the duct.

Randy S.