Two fans in canopy generating harmonics...HELP!!

[plankton]

OK About a month ago I setup a 12" tall canopy for my 100 that measures 60"x18"x12" with two 4" Toyo 75 CFS fans on each end. They work great and only produce 30 db for the 75 cubic square feet of air per second.

THE PROBLEM: When the two fans are on they setup a low resonance harmonic in the canopy that can be heard (felt?) from anywhere in the house! If I turn one off then it stops. But, if they are both on which of course is necesary to keep the canopy cool with 3-175 halides in it then there's the harmonics. The back of the canopy is open and the front has two doors. Openning the doors slightly changes the harmonics but not by much.

What are my options!!!

Thankyou for your time.

Scott

[Snook65]

I had the exact same problem when I set up my canopy with 4" fans on either end. Get yourself some rubber washers and place between the wooden canopy and the metal frame of the fan. I had to use two or three per screw to solve my problem. I found mine at my friendly Ace Hardware store. Good luck.

[plankton]

Snook,

Thanks!!

I'm currently using 1/2" nylon spacers and will be hunting for some rubber ones.

Scott

[texreefer]

this may be a silly and obvious question.. but you do have them in push pull configuration correct???

[Wazzel]

If the washers do not work try adding some stiffners. When you change the mass and stiffness of an object you also shift the natural freq. If the problem is bad it may take more than a dampner to solvw the problem.

[plankton]

texreefer,

Actually, both fans blow cool outside non-saltwater laden air into the canopy. The canopy has an open back. How would this change the harmonics as opposed to one fan blowing in and the other out.

Later...

Wassal: What's a 'stiffner'?

Scott

[Snook65]

I tried that when the orginal problem came up. I had one fan blowing in and one out, then both blowing out but still got the same noise. As of now, both fans blow in and no noise with the rubber washers.

[plankton]

Thanks. Yup, the rubber washers definitely helped alot. The annoying low frequency humm is gone.

SCott

[Wazzel]

Plankton

A stiffner would be a brace or gusset. Basically a triangle attached to two perpendicular sides. It could also be a brace that run front to back or side to side. If the washer work I would not go thru the construction hassel.

Mark