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 Post subject: Using high-CFM fans at low CFM... Noise??
PostPosted: Wed Apr 02, 2008 4:18 pm 
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Joined: Thu Dec 15, 2005 11:06 am
Posts: 2049
Location: Klamath Falls, OR
from an earlier posting, repeated as reference:
Code:
YL SM 1041RPM 560prop 48.2dBA
GW NCB 970RPM 560prop 44.9dBA (NCB sample #2)
SC PWM 910RPM 560prop 48.6dBA Kama PWM 100%
AC PWM 902RPM 560prop 46.1dBA Arctic Cooling PWM 100%
UC12EB 880RPM 560prop 41.2dBA (a sweet-sounding fan)
SilenX 866RPM 560prop 45.4dBA "11dBA" version
SFlexE 852RPM 560prop 42.3dBA (quieter than NCB!)
9blade 686RPM 560prop 39.6dBA Scythe Slipstream

Today we have 3ea 120mm fans all rated 110CFM. The Cooler-Master is manually adjusted by varying a resistance. The Everflow is a PWM fan, and the Scythe Slipstream 3-pin fan.

I've been reading about some folks wanting to use a high-CFM fan, turned to low RPM, for a quiet fan because they want the ability to boost the CFM (cooling) 'way up for various reasons. Herewith, some noise measurements made at (some) equal airflows. In particular, all 3 fans are tested in my standard 120mm constant-airflow fixture at sensor RPMs of 514 and 811 (see the final postings of the earlier Slipstream User Review, made with 560 sensor RPMs).

The first thing you should notice is that, although the 3 fans have CFM ratings of 110.03, 110.03, and 110.31, their maximum airflows are very different, with sensor RPMs of 1610, 1285, and 1242. The Everflow is the high-outlier of the bunch. I started with that fan, because as a PWM fan, SpeedFan does not allow infinitely small RPM adjustments:
Code:
Evr 100% PWM 12.00V 1610Sensor 2460RPM 61.3dBA 5"
     75% PWM 12.00V 1433Sensor 2183RPM 57.6dBA 5"
     45% PWM 12.00V  811Sensor 1316RPM 44.2dBA 5"
     30% PWM 12.00V  514Sensor  858RPM 33.7dBA 5"
      0% PWM 12.00V  440Sensor  756RPM 32.7dBA 5"

C-M full on  12.00V 1285Sensor 2568RPM 56.7dBA 5"
    (adj)    12.00V  811Sensor 1728RPM 47.1dBA 5"
    (adj)    12.00V  514Sensor 1163RPM 43.5dBA 5"
   
Slp full on  12.00V 1242Sensor 1888RPM 51.8dBA 5"
    (adj)     8.99V  811Sensor 1197RPM 38.6dBA 5"
    (adj)     7.77V  514Sensor  785RPM 29.9dBA 5"

Readings taken at 5" (1/8 meter), ambient 27.5dBA
* That 29.9dBA reading would be just over 27 if it were corrected for the ambient noise floor.

Conclusion: At 811 sensor RPM the PWM fan is 5.6dBA noisier than the high-CFM Slipstream and the C-M is 8.5dBA noisier. At 514 sensor RPM the PWM is 3.8dBA noisier (uncorrected) and the C-M is 13.6dBA noisier.

The increased noise level of the C-M at low CFMs is real; you can hear clicks and machinery noises clearly if you hold the fan to your ear.

edit: possible explanation for noise at low RPM removed since it was clearly wrong - it appears the highest RPM occurs at the highest resistance. At least that's the case with the manually adjusted Silverstone 9-blade hi-RPM fan, to be discussed in a later posting in this thread.


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 Post subject:
PostPosted: Fri Apr 04, 2008 4:21 pm 
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Joined: Thu Dec 15, 2005 11:06 am
Posts: 2049
Location: Klamath Falls, OR
I purchased two other high-rpm fans for these tests. One was a Rexflo PWM 2600RPM 96CFM-rated fan; in fact, its maximum RPM proved to be 1871, woefully short of its specification. So far short that any measurements I took on this sample of the fan would not be applicable to a normal Rexflo, so I tossed it.

I also bought a Silverstone manually adjustable fan with exactly the same specifications as the Everflow PWM fan in the first posting on this thread, except that the Silverstone is manually adjusted like the C-M, and is not a PWM fan. Put the two fans side-by-side, and except for the plastic color and the wiring connections, the two fans are identical in appearance. Here's the full-on and ~half-speed data of the two fans:
Code:
Evr 100% PWM 12.00V 1610Sensor 2460RPM 61.3dBA 5"
     45% PWM 12.00V  811Sensor 1316RPM 44.2dBA 5"

SSt full on  12.00V 1526Sensor 2378RPM 59.3dBA 5"
    (adj)    12.00V  811Sensor 1292RPM 42.6dBA 5"

The SSt appears slightly quieter - its RPM adjustment range did not allow checking the 514Sensor setting.

Here's what the Silverstone looks like running on the equal-airflow test fixture:

Image

The white blotch on the sensor propeller (which is not duplicated on the other end of the prop) helps when using a strobe to measure the sensor RPM. The setup was running when this pic was snapped, but the camera "stopped the action" nicely.


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