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 Post subject: fans tests with controlled airflow impedance
PostPosted: Wed Nov 07, 2007 12:22 pm 
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Location: Klamath Falls, OR
OK, I have my simple fixture to compare fan noise at equal airflow working. My next set of measurements will have the SLM tripod-mounted, not hand-held. Here's the fixture, in case you haven't seen it before:

Image

Image

This fixture compares fans under free-air conditions, the same as in an acoustic testing facility. So far, there are no facilities to test (or compare) fans under controlled airflow impedances (that I know of). So, I'm gonna make one.

All that has to be done is to add a box on the front of the fan shown in picture 2 (the red sensor propellor is at the back of the fan). The air inlet on the front of the box will have a hole which is smaller than the swept area of the fan blades. If the fan blades are 110mm wide, and the hub is 1.6" in diameter, then the fan blades' swept area is about 12.72 sq inches.

An XP-90 heatsink is about 82% open, based on fin thickness and spacing. Plus, the air coming into the case often has to execute one or more right turns. It seems 70% (~8.9 sq in) would be a reasonable air inlet size on the box for initial testing.

This would result in some noise measurements under known airflow and a known airflow impedance. I have not heard of such measurements being made previously, but this is an obvious extension of the simple fixture pictured above.

Vary the hole size to provide different impedances? I hadn't thought of that! :D

It'll be interesting how the Scythe Slipstream fans match up to low-pitch fans such as the Yate Loons (which have even lower pitch than the GW NCB!) under controlled airflow impedance conditions.


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PostPosted: Wed Nov 07, 2007 4:53 pm 
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Ooh, I like the sound of this... If possible, could you include NF-S12 and NF-P12 [if you can get them] fans in the test?

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PostPosted: Wed Nov 07, 2007 7:27 pm 
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sheninat0r wrote:
could you include NF-S12 and NF-P12 fans in the test?

Not without buying them, and if I liked the Noctua, I'd already have one. What I will do, once the design has stabilized, is publish details on the fixtures sufficient that anyone interested can duplicate it... or improve on it. I don't do requests - costs too much money and time for something that doesn't interest me. That's why I still don't own a Nexus 120mm fan. :)


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PostPosted: Sat Nov 10, 2007 11:32 pm 
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The first pic shows the original test fixture on the right, and the add-on for controlled impedance testing on the left. Upside down, so you can see the foam that encloses the fan under test. This fixture's top and two sides are made of transparent .06" polycarbonate from McMasters; the rest is misc styrene from towerhobbies:

Image

The second (poorly lit) pic shows the add-on mounted on the original fixture and fan. The basic add-on has a 3.6" square hole. Plates can be added that have smaller holes. The added plates are in two halves so the impedance can be changed on the test fixture without disturbing either the fan under test or the sound level meter, whose microphone protrudes into the square opening to within a eighth of an inch of the fan. The microphone shaft is a half-inch in diameter; that should be subtracted from the square hole area. This photo shows the 3.1" square impedance plate, half in position for test:

Image

Here's a side view. It's a good thing the entire fixture is in two parts, because otherwise it would be awfully hard to change the fan!

Image

Night is a good time to take flash pics, but a very bad time to cut styrene to size (can't see the scoring on the white plastic). So tomorrow I'll make two more impedance plates and then do some testing. I'll post the test results in the User Review forum, probably Sunday night or Monday.


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PostPosted: Sun Nov 11, 2007 3:42 am 
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This looks interesting, but I really need to see the executive summary of the whole thing :)


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PostPosted: Sun Nov 11, 2007 10:45 am 
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To those who, like me, were interested in this experiment with controlled airflow impedance: we have a situation here.

Simply placing the default box with its 3.6" square opening on a GW NCB fan raises the noise level 8dBA. OK, I can sort of explain that away by saying noise in the box - instead of dissipating - echos around. BTW, the ambient noise level is actually a little lower than normal daytime because it's Sunday morning; not much traffic.

But the killer is that just placing the box in front of the fan just kills the sensor propellor's RPM! Instead of 560RPM, it drops below 250 and at that point it's hard to measure with a strobe in daytime. Also, I can feel with my fingers that the airflow drasticly dropped - the sensor prop ain't lying!

I can't explain what's going on. I'd swear a 3.6" square hole is pretty much equal to the swept fan blade area of a 120mm fan. Yes, I tried another fan - actually I tried the SFlexE first, but when the results made no sense I switched to my old favorite 120mm, the GW NCB.

I can't see anything at all that would cause the drastic drop in airflow I'm seeing. I'm flummoxed. Anybody got any ideas, besides the fan-gods may be angry?

Surely a PC case doesn't have the same effect on airflow that my little fixture is having? And did I ever mention that sometimes experiments have unexpected results? :(


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PostPosted: Mon Nov 12, 2007 2:03 am 
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The increase in noise, and drastic decrease in airflow, occasioned by the box are both real (I did some playing around yesterday). Which is really interesting.

Because it doesn't seem to me that there's all that much difference between that box and an ATX PSU with a 120mm fan. Next: noise measurements on fans in PSUs with the PSUs not connected or operating, just the fan.

edit: changed "noiseflow" to "airflow"


Last edited by Felger Carbon on Mon Nov 12, 2007 9:11 am, edited 1 time in total.

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PostPosted: Mon Nov 12, 2007 4:43 am 
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Do you mean 'decrease in airflow', not in 'noiseflow'?
If your '3.6" square' means 9 x 9 cm = 81 cm2, then its' pretty close to the blade area of an 120 mm fan indeed.
I tend to use 85-90 cm2 in my case concepts, but as I said, 81 is pretty close, shouldn't generate such a dramatic airflow decrease.
Since the opening is placed at some distance from the fan, the extra impedance introduced shouldn't be significant.
May I suggest to build a differently shaped box, like a top-cut pyramid, that has the same 9 x 9 cm2 opening, and compare with the first one?


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PostPosted: Mon Nov 12, 2007 9:20 am 
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Tzupy wrote:
Do you mean 'decrease in airflow', not in 'noiseflow'?

If your '3.6" square' means 9 x 9 cm = 81 cm2, then its' pretty close to the blade area of an 120 mm fan indeed.

I tend to use 85-90 cm2 in my case concepts, but as I said, 81 is pretty close, shouldn't generate such a dramatic airflow decrease.

Since the opening is placed at some distance from the fan, the extra impedance introduced shouldn't be significant.

Yes, thank you (corrected).

Yes.

I agree, it shouldn't - but it did. :(

I agree, it shouldn't. The results suggest we're both wrong.

Experiments are performed to confirm theory, or to prove the theory is wrong. The theory that impedance raises noise and decreases airflow is hereby confirmed IMHO. :D

But geez, Louise, I got a heck of a lot more confirmation than I expected or wanted! Help! :shock:


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 Post subject:
PostPosted: Mon Nov 12, 2007 11:43 am 
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Quote:
the rest is misc styrene from towerhobbies:


try taking away all the styrene,leaving just the polycarbonate, then add the styrene pieces one at a time, see if there is one particular bit which causes the airflow to halve. be interesting to see if the airflow goes back to 'normal' w/ just the polycarbonate. some smoke flow visualisations may shed more light on why airflow is dropping so much (can get smoke generators/pellets on ebay or maybe local hardware store?).


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PostPosted: Mon Nov 12, 2007 12:26 pm 
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Location: Klamath Falls, OR
jaganath wrote:
some smoke flow visualisations may shed more light on why airflow is dropping so much (can get smoke generators/pellets on ebay or maybe local hardware store?).

My smoke detector is very loud and very paranoiac. :roll:


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