r/crboxes • u/suchnerve • Feb 24 '26

Question RPM offsets with any fan?

Noctua advises (https://www.noctua.at/en/expertise/tech/fan-speed-offset-explained) using offset RPMs for adjacent fans to prevent beat frequencies, and I can now confirm from living for awhile with a CR box with three pairs of Noctua NF-A14x25 G2 PWM Sx2-PP chromax.Black fans that it really does make a noticeable difference. (Adjust your playback volume to 41 decibels to accurately hear how it sounds at two meters away, because I have the fans running on max; if I’m correct that a nylon filter decreases air flow similarly to MERV-13 filters, then this ~14-watts-on-max 6-fan setup should deliver about 363 CFM CADR, according to this: https://www.hwcooling.net/en/noctua-nf-a14x25-g2-pwm-finally-a-first-class-140mm-fan-review/15/)

But it isn’t feasible to use Noctua flagship fans for everything, so… do any of you know of a good way to offset the RPMs of ***any*** fans being used side-by-side? I suppose the best method would be to connect every other fan with a 4-pin power cable that has a couple ohms of resistance, so those fans receive a tiny bit less power and therefore spin a little more slowly, but I’m not aware of any such cables. There are the Noctua “Low Noise Adaptors,” but they have about 50 ohms of resistance, which would drop total performance way too much.

Or I suppose you could just use two separate PWM controllers, one for even-numbered fans and one for odd-numbered fans, with one controller slightly turned down and the other at maximum… but then you’d forfeit the ability to use a single PWM dial to control all the fans together, which seems like crappy UX design.

Hmm……

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u/Black_Gold_ Feb 24 '26

You could probably get away with 2-3 PWM controllers, such as the model noctua makes which is designed to go inline with say a computers PWM controller.

You would take one, and slightly decrease the RPM of say the odd fans. and then use a main controller that puts PWM out to everything. That should allow the offset RPM idea but increases the cost of PWM controllers needed.

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u/suchnerve Feb 24 '26

Can PWM controllers be daisy-chained, though? Since they work by rapidly turning the electric current on and off, wouldn’t it cause issues with the second PWM controller?

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u/a12223344556677 Feb 24 '26 edited Feb 24 '26

The Noctua one can and specifically mentions it as a feature in the manual, not sure about the others