Push/Pull Fan Resonance?

I recent installed 2 Cooler Master (R4-BMBS-20PK-R0) fans in a push/pull config on an H60 radiator.

Each fan individually is very quiet even when spun all the way up, however when run together off a 4-pin splitter, they create a fairly nasty resonance at/above 65% duty cycle.

I'm not sure if this is solely an aerodynamics issue, or if its a phase issue between the 2 fans rpm speed(s).

I'm going to experiment with mounting pads, etc.

Any other suggestions?

Thanks

K

I seem to remember reading of at least one other person posting on here about getting a resonance problem on one of those LCLC units like your H60, but I don't remember what he did to solve the issue. Or even if he was able to solve it. I personally haven't run into that problem with the H50, H70 or H80 when I tested them. You could try changing the fans to separate fan headers and see if that helps.

Hmmm, not a bad idea. There is another 4-pin available on the mobo. I'll give it a try. Thanks

Yeah, if you have another 4 pin header open, you can also vary the rpm just a tad between the 2 fans and that might help.

I played with the configuration for a while last night, with some interesting results.

I switched one of the fans to an alternate 4-pin header, did some load testing using AG's Intel Burn-in Test, and played with the speed variance between the 2 fans. I'm able to change the frequency somewhat by varying the speeds on the fans separately, though the effect on db was minimal if the fans were withing 250rpm's of each other.

For giggles, I disconnected 1 fan, and ran some more load testing to see how 1 of the CM fans would do by itself. Very much to my surprise, the max temp under high load dropped 2*C. Ambient remained the same throughout.

I'm wondering if the flow characteristics of these particular fans dont work well used in pairs, or if he radiator itself is lending to the flow issue. The drop in temp would suggest some sort of reversion/inversion occurring at the initial intake fan. If so, the result would be a light vacuum being created at the radiator by the 2nd fan.

For the moment, back on one radiator fan, at least its working well, lol.

You record your ambient temps? 2C can account for a lower room temp in the evening.

Conumdrum said:

You record your ambient temps? 2C can account for a lower room temp in the evening.

Click to expand...

Yes. Its necessary to know the ambient temp in order to establish a proper base-line. I have a digital thermometer in the room, its proven to be quite accurate.

In any case, the load test were done less than an hour apart, the room temp had not changed.

Wow, well you got that covered. I think your correct about the fans not working well together.

It's true about push pull. The whistling was driving me nuts so I took the rear fan off my megahalems and the noise became much more tolerable. The temp difference doesn't matter because I can run it faster and it still sounds better. It went from sounding like a tea kettle to just sounding like a fan spinning fast. Much better.

I wish mine had been as simple as a 'whistle'. The resonance was a deeper harmonic, loud and damned annoying. Sounded like a small turbo-prop aircraft taxiing out to the runway.

My Tt water 2.0 Pro (double-thick radiator, push/pull fans) had some resonance. The solution for mine was to run the fans a bit lower or to run the pull fan a bit lower.

The Megahalems is famous for this. I've I've taken to calling mine a wind instrument.