And I prefer to run my 8k RPM fans at the speed of "smashed by hammer"
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Hum this is getting interesting. We need to get some empirical data on Nexus current draw vs. voltages. I'm to lazy/busy. Looking forward to other posts though.Solid Snake wrote: Assuming a 6v fan pulls half the current of a 12v fan:
Running the 2 fans at 6v probably pulls half the power of one fan at 12v. Doesn't it seem strange that 2 6v fans each moving 20cfm consume half the wattage of a single 12v fan moving 40cfm? The catch is that you're now running at half the static pressure which will translate to a much lower CFM once the fans are installed than if the 12v fan was installed
When you say "something is lost", I would say "something is saved", basically you save energy because you get roughly the same airflow and consume less electricity.Solid Snake wrote:Another thing to note is the current that these fans draw at 12 and 6 volts. My guess is that the fan will draw half the current at 6v (you can easily test this). This amounts to 25% power and although RPM and CFM remain at 50% (I was wrong on the RPM of this fan) something is lost.
I don't agree. In a computer case, mounting 2 fans in series will not double the static pressure.Solid Snake wrote:Just as in an electrical system where volts are useless without amps or in a mechanical system where rpm is useless without torque, with fans, CFM is useless without the pressure to back it up. Since we're just blowing air through a case, we don't need extreme pressures, but cases that use ducted CPU exhausts or other restrictive passages need more pressure than others.
I'm sure you know that when you put two fans in parellel their CFM is (roughly) doubled.
When you couple fans, their static pressure is doubled.
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Have a look at this article:Solid Snake wrote:Just as you noted, the CFM of a fan goes up somewhat linearly with the voltage. The same is true with static pressure. A fan's air moving ability is somewhat a product of these two numbers.
You can't control both airflow and pressure indepandently.Solid Snake wrote:Assuming a 6v fan pulls half the current of a 12v fan:
Running the 2 fans at 6v probably pulls half the power of one fan at 12v. Doesn't it seem strange that 2 6v fans each moving 20cfm consume half the wattage of a single 12v fan moving 40cfm? The catch is that you're now running at half the static pressure which will translate to a much lower CFM once the fans are installed than if the 12v fan was installed.
My guess is that if you want the same air moving ability (CFM and pressure) as a 12v Nexus you'll need 4 fans running at 6v in a series/parellel configuration. Two side by side with the other two stacked on top. This will also probably give you the same power consumption as a single 12v Nexus.
On that same graph there is an efficiency curve. You can see how it varies... it is near 0 in the "free flow" case, at the far right of the graphic. Installing fans in parallel will help get closer to optimum efficiency, so it is not surprising that the power consumption decreases.Solid Snake wrote:
Check the current draws of the Nexus fan at 12v and 6v. If they're different, that's a red flag that the air moving ability of 2x6v fans will probably not match the performance of a 1x12v fan in anything but a free air environment.
As I already mentioned above, it was because of the A weighting that the 3 dB increase did not show up.hofffam wrote: MikeC's SLM didn't show it - but that doesn't mean the 3db increase didn't happen.
Not true. No such effects heard nor measured. The SLM was moved around. Not the same as tweeters at all, which typically operate >2kHz. Much fan noise is not that directional, especially at slow speed where it's mostly ~<300Hz noise that dominates.It is much more likely to be an issue of acoustic cancellation. Two fans measured by a single microphone, especially very close to the fans, will be subject to narrow bands of cancellation. The fans also generate sound in multiple directions. Mike's SLM picked up primarily the sound from only one side of the fans. The sound from the other side presumably went "away from" the mic and eventually was absorbed in the room... multiple tweeters ...
Assuming these are all identical 120mm fans, 2 @ 5V.is it better for me to use 1 fan @ 7v (yl d12sl) or 2 fans @ 5v (in terms of low dba).
Maybe the fan laws can shed some light on this?my subjective experience is that the reduction in noise between a fan at 6V compared to one at 12V is more like 10dBA than 3dBA.
Thanks for reply JaRoD,JaRoD wrote:I've done a few calculations and it does seem that a computer has very low pressure drop.
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If you want double the airflow though it's not as easy since you now have to have 4 times the pressure.
But for cooling a case, pressure is no problem even when undervolted.
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Since having running a fan at 5 volts gives a noise reduction of 18 db adding more fans quickly increases the airflow but not the db.
Since I already have 3 fans that are inaudiable at 5v inside the case. Adding 3 more should still be inaudiable but provide alot more airflow.
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My "quest" is to have a very silent computer that's cool even when overclocked. Anything over 50 and I start getting worried...
note though that I used an online calculator and there's one value that
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Guess that's about it for my ranting this time...
You might want to try AeroCool Silverlightning 140mm or AeroCool's Streamliner 140mm. They're relatively quiet.Joytoy wrote: I want to try in the Tunnel, a good 14cm fans, because theoricallyI think 14cm fan, is better than 12cm fan.
For my taste, I prefer sleeve bearings fans.
Can someone advise me about a very good 14cm very low dB fan ?
I believe this rule applies only wtih pure resistance circuits and might not be the case here.Solid Snake wrote:I don't think there's any way in hell this is true. CFM does not follow voltage, it follows WATTAGE. Anyone here familiar with Ohm's law knows that P = V^2/R.The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!
I don't know what the impedance of these nexus fans are but it doesn't really matter. For this or any fan, running it at 6v will only consume (6^2/12^2) 1/4 of the power and probably run at 1/4th of the speed.
So what do we have here? 1 fan running at 100% speed vs 2 fans running at 25% speed. 3dB is the difference between doubling (or halving) sound output. If we can assume that RPM and sound output are related, then your "discovery" is far from ground breaking.
Let's say this Nexus fan runs at 1600RPM @ 12v. At 6v, this same fan would run closer to 400RPM NOT 800RPM like many would think. This is because of ohm's law. When you reduce voltage by 50%, you reduce power, noise and airflow by 75%. You've basically proven that two fans, together producing half the airflow are half as loud as one fan at full speed.
The 3dB reduction in sound you've discovered is right in line with the 50% airflow penalty.
I think MikeC's results are more applicable to our daily computing environment (Home office/Bedroom/Living Room), and I am more interested in results with realistic ambient noise. What this means is that I should probably put that top fan back on my P180 and undervolt those exhausts moreGreen Shoes wrote:Yeah.....what we really need here is an anechoic chamber. That's the only way to get true measurements of these things.
I wonder what happens if it were 2 fans at 12V, would it also be less noise?MikeC wrote:
The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!