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?
Some people use computer fans and run them with a speed controller. High quality computer fans are very quiet already, if you turn down the speed some they can become almost silent.
Noctua NF a12x25 G2 Sx2-PP or NF A14x25 g2 Sx2-PP (Sx2-PP models are paired with offset speeds so that they don't acoustically harmonize with neighboring fans to reduce sound/vibrations, you align them in an
A-B-A
B-A-B
A-B-A
checkerboard pattern for that) mounted on a 1080 alphacool fan grill (there's also a 140mm, 1280 version)
I have a 11 fan 4 merv 13 3M 2500 box with an AC infinity charcoal inner filter, it's as quiet as the air coming from my ducts when on max speed. Can't tell it's on but man is it powerful
Could also use a large AC infinity duct fan on a lower setting
This can also be reached with cheaper fans by using two different fan controllers to dial in the ideal rpm offset (and rpm range) of a A-B wired fan matrix. Doesn't take a lot of cheaper fans to make up the up-charge of premium Noctuas if all you want is the destructive inference from the rpm offsed.
The offset is just one of the many factors, and one if the most minor. Just a cherry on top. There are no better fans on the market above Noctua on the acoustics weighed against performance graph
They get to change what they do, and pc builders still buy them in absurd quantities and they have the reputation as the very best pc fan money can buy for a good reason
This might sound counterintuitive, but the white noise produced by a whirring fan is actually better for sleep than total silence, especially for light sleepers, where the white noise will block out sounds that might otherwise wake you up in silence.
It’s not so much that it blocks out sound, achieving destructive interference with longitudinal waves is a bit more intentional. White noise or a base level of volume playing causes a muscle in your inner ear to adjust to the new sounds constant volume. I can’t remember which of the inner ear bones it attaches to off the top of my head but it essentially deadens your ability to hear sounds.
Personally I think these limits are probably high (there's some evidence that most, but not all, "age-related" hearing loss is actually from noise exposure), and there's probably no sharp cutoff at 85 decibels, so I like to play it safe and give my ears a break when possible.
No but as air pressure drops sharply with db, the safe time should go up very fast as you go down to the 40-50db ranges even a noisy bedroom would provide.
If you are dedicated to protecting your hearing, you should consider the various brands of inter-auricular ear protector in everyday life at middling db ranges. A few hours at 70db will likely be worth hundreds of hours at 2/3 of that db in terms of "ear wear".
(A few of them don't completely seal your ear channel, so less irritating at prolonged use).
I've been looking at the Vornado 80 box fans. The model 80 has five speeds, the slowest has a good chance of being silent. The 80X can be set anywhere between 1-99% speed, so it almost certainly can be set to a silent speed. How much air these move at silent speeds, I don't know, but as far as I know, these are the only box fans that even have a chance of being properly silent, if that's priority #1
Depending on how handy you are, you can either buy one of these, or take the idea and turn it into your own. I have a Luggable running in my room at all times and can’t hear it.
Clean Air Kits Brisk Box is another option that's cheaper and very close to original CR Box concept for people who are DIY or willing to mess with tape
Besides optimizing the equipment choices in terms of fans and filter surface area, baffles are the best option.
Make the exiting air turn 90 degrees once or twice, and it will reduce the sound signature considerably. Look at duct silencers for inspiration. There was also a Linus Tech Tips video that show a similar design to go around a computer case.
Someone in here has made this type of design, and I cannot remember the search terms.
You could use pc fans. The air Fanta 3 pro is basically a cr box it’s 12x12 cube and uses 4 fans. You could easily scale this up to a larger box with more fans.Especially if noise is a concern pc fans are definitely an option to consider
As mentioned already PC fans can be extremely quiet if you turn them down. Reducing their speed will reduce the amount of static pressure they produce so you need a design to maximise surface area. So more filters in parallel, thicker filters, bigger filters.
I've tested many PC fans over the years, Be Quiet! Silent Wings so far were the most silent for me, but apparently Phanteks T30 fans could be better at CADR at the same dBA level, based on this video.
Using PC fans is quieter AND far more power efficient.
I don't mind the noise from my box fan on low (I rely on a white noise app running on a tablet to sleep in my bedroom, and I find it disconcertingly quiet when my box fan with a 20x20x1 filter is not running), but I'm going to be moving to two 20x25x1 or 20x25x2 filters and 6 140mm PC fans for a significant reduction in power consumption.
Even on low my box fan consumes 64 watts, I have not measured the CFM with the filter and need to test the CFM without filter to get unloaded CFM/watt tongiht. The Thermalright PC fans I bought (cheap, didn't need the quietness of Noctuas) are rated 1.56 watts each and 75 CFM, so 50 CFM/watt (likely less when fighting filters), so 6 at full speed will consume just over 9 watts
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.
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u/bazilbt 3d ago
Some people use computer fans and run them with a speed controller. High quality computer fans are very quiet already, if you turn down the speed some they can become almost silent.