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I have a machine with a loud fan, its specified and measured as 73dB of noise 1m away from the machine. I have been asked to reduce it bellow 55dB. There is a cover that can be lowered that covers the fan 85%, but is otherwise partially open to the environment and needs to be for air flow reasons.

20dB sound reduction when i research it is a lot, seems to be mostly applicable for fully enclosed rooms. And achieving that by putting few mm of sound absorbing material inside the cover that encloses the fan, but not fully, seems like the only solution i have but given that most of the sound is at low frequencies ~200 Hz i am not sure this is realistic to even try and test out at all.

Can someone provide insight or a sanity check here. Is this a fools errand?

Some additional information. The peaking of the sound is at 200 Hz, bandwidth is 130 to 400Hz approximately.

The fan is floating on a fixture mounted to a steel structure / electrical cabinet.

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  • Very difficult to judge if we don't know if there are also mechanical sounds or have a spectral representation of the sound profile.. 200Hz is low but is this sound coming solely from the fan or is it resonating with something bigger becoming a speaker thus resulting in the 200Hz? I would start by reassuring that the fan is "floating" on rubber to take mechanical resonation out of the equation, then treat whatever travels through the air... Also, fans need lubrication, etc, so first service the fan so it works as well as it gets, an old fan will make noise..
    – frcake
    Apr 4 at 12:07
  • Is this an actual work thing or a school project? I’m not sure it’s feasible. Can you relocate the noise source at all? And you can’t add to walls of the room at all? You have to make the sound in the same room as the fan 20 dB quieter? Apr 4 at 12:34
  • This is work related. No big changes to mechanics, etc are within scope of this. The machine is placed outdoors. It should measure bellow 55dB 1 meter from the machine where it is placed outdoors. It is part of a mechanical frame. The fan, which is actually a oil cooler is "floating" on a fixture attached to a electrical cabinet. The peak of the sound is at 200Hz, but the lobe has some energy down to 130 and 400 hz. So the bandwidth is 130 to 400 hz with most peaking at 200Hz
    – Stonie
    Apr 4 at 13:09
  • The datasheet for the fan / oilcooler states the noise as 71dB, we measure 73dB so the source itself is definitely this loud with only minor additions from mechanical resonation i presume.
    – Stonie
    Apr 4 at 13:16
  • yup, you're gonna need mass to attenuate this, higher frequencies can be redirected/bounced off of surfaces but lower rumble noise needs mass to be contained. Maybe a thick tunnel (cement/heavy wood) to another fan that can be located somewhere else? A photo/drawing of the situation would help to be honest...
    – frcake
    Apr 5 at 10:55

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