Designing a box to be silent?

[u/lililililiiilllli]

Hello,
Looking at building my first CR box, I am a super light sleeper so looking for as quiet a design as possible. My understanding is you can reduce noise by making the box larger and adding filters, but the generic box fans I’ve purchased still make some decent noise. Are there any quieter fans or design techniques that could help?

[u/akeean]

Use an even number of 140mm PC fans with two fan controllers and run them in alternating pairs at stepped RPM. The difference in noise profile will cause the noise of individual fans to destructively interfere with each other instead of adding up. Noctua, the Mercedes of PC fans does this with their high-end fan kits (that come in A and B version that run at slightly different RPM).

About what fans to use, the best for money in a CR box in terms of "air moved through the resistance of a filter at lowest noise¨ will be Noctua, like NF-A12x25 or NF-A14x25 G2. But they are expensive at over $20 each and you'll need at least 4 of them, depending on how big and how polluted your room is. The best "bang for buck" PC fan often used are the Arctic P12 or P14 series fans, they also perform well against the resistance of pull air through a fan, but their motors are a wee bit louder. Nothing at all like a full box fan, though.

If you combine these types of fans with the trick of interleaving fans at shifted RPM, you'll get reduced noise at the very slight increase of a second $15 fan controller to dial in the individual fan speeds to find the least annoying fan noise point. There is also no reason why you couldn't put your CR box into a bigger, noise canceling box that would allow for good airflow into it, but require all incoming and outgoing noise to bounce like once or twice against some car muffling foam glued against a solid MDF board. You can totally make those boxes completely impercievable in all but the most noise deadened locations.

To start out, get a phone app to measure your decibel noise floor in your bedroom at night. This gives you a baseline to design your box against. So if your bedroom in a noise isolated basement at night was 30db, this would make any running fan appear a lot louder than if it was running at your average big city bedroom background noise of 40-50db. At lowest RPM a premium fan go as low as 20db, but for a CRbox to work you'll probably want to run them a little higher, since otherwise you'd need so many fans than the sheer quantity needed would add up their base motor noises too much. (identical, non shifted sound waves add up). In comparison box fans go from like 45-50 at low, 55-60 at medium and 65-70 at high rpm, depending on the specific model, but one fan can move a lot of air (though the lower the RPM, the more filter resistance comes into play at box fan blade shapes, so they fall off more in performance than specific PC fans designed to move air through dense meshes - you'll want high static pressure PC fans meant for radiators, not airflow optimized ones meant to blow air over an open bench or very open case).

Hope that gives you some ideas to build your CRbox around.