Determining noise levels?

[CCR5600Design]

I love working outside my area of expertise. I have you guys to bother when I come across something I am with which I am unfamiliar...

Do any of you have experience in determining noise levels for equipment? Specifically, an enclosure I am working on requires approximately 2700 CFM of airflow for cooling purposes. Our customer has asked that we achieve this airflow through the use of multiple fans. The reason for the multiple fans is in the event of a fan failure, airflow will continue, albeit at a reduced rate. The current configuration utilizes 12 fans.

Now comes the challenge... I have been asked to provide a cooling fan arrangement that meets GR-63-CORE requirements with regards to noise levels. I have researched the document and found the standards by which the fan arrangement will be tested, but I am not sure as to how to determine the overall noise level of a collection of fans. If one fan is rated at 50dB @ 27C (81F), how will I determine what the projected noise level of 12 combined fans will be? Is it a cumulative number? Is there a formula to figure this out?

Thanks in advance,

Ron

[Marky]

I love working outside my area of expertise. I have you guys to bother when I come across something I am with which I am unfamiliar...

Do any of you have experience in determining noise levels for equipment? Specifically, an enclosure I am working on requires approximately 2700 CFM of airflow for cooling purposes. Our customer has asked that we achieve this airflow through the use of multiple fans. The reason for the multiple fans is in the event of a fan failure, airflow will continue, albeit at a reduced rate. The current configuration utilizes 12 fans.

Now comes the challenge... I have been asked to provide a cooling fan arrangement that meets GR-63-CORE requirements with regards to noise levels. I have researched the document and found the standards by which the fan arrangement will be tested, but I am not sure as to how to determine the overall noise level of a collection of fans. If one fan is rated at 50dB @ 27C (81F), how will I determine what the projected noise level of 12 combined fans will be? Is it a cumulative number? Is there a formula to figure this out?

Thanks in advance,

Ron

Ron.....It is all fans combined. A Decibel or Sound Level meter will help...try Extech. Here's the OSHA link http://www.osha.gov/pls/oshaweb/owad...ards&p_id=9735

I typically use 85db (for 8hrs) measured where the operator of the machine is.

Hope this helps

[CCR5600Design]

Marky,

Yes, that helped but I still have a question...

the GR-63-CORE document states that the maximum noise level per enclosure for fan noise is not to exceed 78 dB @ 27C. If the dB values of all 12 fans are combined, then the noise level is 600 dB? I don't think that is right as 600 dB would be equivalent to a small atomic blast...

I am trying to choose a cooling fan that will fit the criteria as we have to send this unit off for NEBS testing and I would like to hit the nail on the head on the first whack.

Thanks again,

Ron

[Marky]

No it's not 600 dB....that's why you'll need a meter. Are you just starting the design? If so....you'll need to make a mock up of the cabinet with working fans in it.

You can run the fans on the benchtop....but thats not realistic.

To give you an idea.... an copier machine runs around 70-75 db.

Are all of the fans venting to the outside....if I looked at the cabinet...would I see 12 fans facing me?

[zeke]

The formula for adding n incoherent noise sources is

10*log(n)+ N0

where
N0= noise from one fan
n number of identical fans
for n=10
you get
N0+10log(10)=N0+10
for n=12
you get
N0+10log(12)=(N0+10.79 )db

So, for example the noise from 1 fan is say 80 db, then 12 fans will get you
80+10.79=90.79db

If the noise is different from each fan then the formula is

10 Log(10^(N1/10)+10^(N2/10) +....Nn)^(Nn/10)

For example say we have one fan @ 80db and one at 85 db, then

I get 86.19 for both

The formulae assume the fans are in close proxiimity

[CCR5600Design]

Zeke, you da man!

That is EXACTLY the info I needed. I really appreciate it. Now, based on this info, I can order sample fans that will be in the ballpark for testing. I have found many fans that are in the 55-60 dB range that meet the minimum airflow requirements, but I wasn't sure if they would meet the noise limits. Now I can plug this info into the formulae and see what happens. Hopefully, I can find several fans that will qualify for the end user so lead times can be kept to a minumum.

I owe you a cup o' coffee, Zeke.
Thanks again,

Ron

[machineengineering]

Ron,
Just another 2 cents... I had a similar problem once. Most pancake fans come with a safety grille rather close to the impeller which is responsible for generating the vast majority of the noise due to its proximity to the blades. If you coud safely remove the grille (add ducting to prohibit curious fingers from reaching the blades) it might help.

Regards, Jim

[Marky]

This laser that I had worked on many years ago had 6 10" fans in the back......you couldn't put it too close to the wall because the air flow out of the fans would hit the wall it would roll the laser across the room.

We used to have to tell nurses and docs to lock the wheels.