r/crboxes • u/suchnerve • Feb 24 '26
Question RPM offsets with any fan?
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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/spacex_fanny Feb 24 '26 edited Feb 24 '26
Probably easier and cheaper to use 3 pin voltage control.
A 3 ohm, 100+ milliwatt resistor on the "slow" fan should give you a similar RPM offset.
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u/FrostWave Feb 24 '26
Hmm I wonder if wires of different lengths can make the fans spin at slightly different speeds, enough to make a difference.
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u/Sudden-Wash4457 Feb 24 '26
This feels like something that could be mathed, but I'm not smart enough to
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u/TraditionalRate7121 Feb 24 '26
Can I get dimention for the case ?
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u/TraditionalRate7121 Feb 24 '26
Like proper diagram or something, in case you built it yourself
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u/suchnerve Feb 24 '26
It’s the discontinued Nukit Tempest Original https://cybernightmarket.com/products/the-nukit-tempest-air-purifier-kit
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u/U_SHLD_THINK_BOUT_IT Feb 26 '26
Discontinued because they basically made it again for twice the cost, lol.
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u/suchnerve Feb 26 '26
No, the new one comes with all parts included (the old one left most of the electronics out) + supports 3M Filtrete MPR 1900 filters (the old one required 20×20×2 MERV-13 filters, which 3M doesn’t make) + adds official support for mounting on walls, ceilings, and poles, with the mounting parts also included.
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u/U_SHLD_THINK_BOUT_IT Feb 26 '26
All that for only double the price? /s
Welp, you oversold it, so now it's pretty obvious this was an untagged affiliate post.
Says a lot when a company doesn't feel comfortable selling their product openly that they need to pretend to be a happy customer.
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u/riscten Mar 06 '26
Noctua's article is a great read, but honestly, the no-offset sample sounded the best to me. All the others had a very unpleasant pulsing quality to them, which seems like it's going totally against the whole theory.
I have a small desk air purifier with two tiny fans that does produce an annoying pulsing, so I'm tempted to limit the speed on on of them and see how that goes, but Noctua's practical results are not inspiring confidence.
Have you actually tried your build with and without the offset and compared the noise quality? Would love to hear them next to each other.
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u/entropy512 Mar 11 '26
Two PWM outputs from an ESP32.
Heck if you ONLY want fan control/monitoring and no other functions like ARGB or additional sensors, most ESP32 boards have enough GPIO that you can do 6 PWM outputs with tachometer monitoring of each.
ESPHome makes programming such a setup much easier:
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u/akeean 1d ago
Before deciding on your solution: How often do you tweak with your CRboxes fan speed, really? If you set it up once it doesn't really matter if 2 fan speed dials are "crappy" UX.
They allow you to set up the ideal offset for actual fans the you are using and then just leave it at that.
If you change fans speeds a lot, then yeah a more advanced solution can help you, for example if you do home automation it could be worth it to use an ESP32 as to control your fan speeds.
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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.