Fan box too noisy

[engmechs]

To all,

I have an acoustic problem encountered in engineering dept recently, & it is getting hotter each day.

A add on designed sheet metal box was developed for telecom industry, which hosted 2 fan units inside. The fans were used for cooling equipments inside the enclosure. The box size is about 1.5' x 1.5' x4" (W x L x D).

The fan did its job ok in terms of cooling whole enclosure's temperature but there was a sound problem for the enclosure. Requirement stated that the whole enclosure can't emitt sound level more than 45 db at 1.5m away from it. Right now, the best reading achieved was about 48 to 49 db.

I am not a sound expert, & I would like to ask for some logical directions and suggestions in handling this particular problem.

My thoughts so far include:
#1 Get the fan frequency of cooling fan (rpm), & found appropriate sound absorbant foam material for the range. A good foam material will definitely be one important factor here.

#2 Box layout is critical in bringing down noise level, as an indirect path design can dissipate more sound wave energy, &/or counter-eliminate waves each other. Our original layout is fan is at the top corner of fan box, & air inlet is at diagonal corner. A non-straight air path was constructed between air inlet & fan unit.

#3 Keep a limited air space around fan unit, which keep the sound waves at a limited spot, without excessive chance of spreading sound vibrations to the rest of the structure.

Please help out here if you have any thoughts. Any suggestions are welcome.

Regards

engmechs

 

[GregLocock]

More, slower, bigger, fans.

4 dB is no big deal, usually. Try #2 and some adhesive bitumen sheet for the casing.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[engmechs]

Thanks for your advice, Greg.
I will look into it.

engmechs

 

[Rob45]

See also that the fans are isolated from the housing.

 

[kacstic]

You should also check the spec again and see if they want an A-weighted decibel level of 45dB. If you are measuring the noise with flat-weighting, then another measurement with A-weighting would like bring the noise level down under the required spec, especially if the noise is mostly low to mid-low frequencies.

Whenever they publish manufacturer specs for noise emission levels, they usually report noise levels that A-weighted but this weighting more accurately reflects how humans perceive sound. A-weighting basically includes a correction the lowers noise levels in the low frequency range.

 

[engmechs]

Thanks Rob45, & kacstic, for your comments. I will check on the spec.

engmechs