Silverstone Raven Two

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TESTING

Measurement and Analysis Tools

FAN MEASUREMENTS

All of the case fans were removed from the chassis for some basic testing. The 120mm fan was simple to removed. The 180mm fan removal was slightly more complex, but also not difficult.


Remove the dust filter, undo two screws and slide the 180mm fan out. The slide mechanism fits tightly enough that there's no rattling when the fan is mounted in place. Shown here without the protective cover.


The big fan has seven blades and its mounted on a tray that's also a frame to hold the dust filter show behind. The high-low speed switch also had to be removed from the chassis for the fan to come out.


The bare fans, side by side: Nine blades on the 120mm fan, whose strut geometry is not as good as the 180mm fan. The struts on the 120mm fan are nearly parallel to the blades, which can increase tonality. In contrast, the blades and struts on the 180mm fan intersect at nearly perpendicular angles.

The big fans were tested only at 12V, with the speed governed by the two switches. The rationale was simple: It is the way these fans will be used by 95% of all who use this case. The 120mm fan was tested at 12V, 9V and 7V; the speed fell too low at 5V for it to have any real use as a case cooling fan.

120mm Fan

Measurements of 120mm fan from RV02
120x25mm, 12VDC, 0.18A
Voltage
SPL at 1m
RPM
12V
18 dBA
950
9V
13 dBA
750
7V
11 dBA
570

The 120mm fan had a touch of tonality but was quite smooth sounding at full speed. At 9V, the tonality all but disappeared, and the noise dropped to a level where it would be inaudible in 99% of applications. The noise dropped further at 7V, but the reduced airflow may not make it worthwhile to run it so slow. This is much quieter than the 120mm fan in the original Raven.

180mm Fans

All three fans were tested. The results were very close; SPL measurements were within about 2 dBA. These fans have twice the blade area of a 120mm fan and push that much more air at the same speed.

Measurements of 180mm fans from RV02
180x25mm, 12VDC, 0.3A
Speed
SPL at 1m
RPM
H
33~35dBA
1000~1050
L
19~20 dBA
660~700

180mm Fan Mystery

The 180mm fans were surprisingly noisy at full speed, and they exhibited an annoying and prominent tonal quality. The overall level dropped dramatically at the Low speed, but some of the tonality still remained. A large component of the noise was mostly random broadband noise, as it should be with such a large fan.

The bare 180mm fan test results conflicted strongly with an earlier casual listen of the fans inside the case; they had seemed smooth and quiet. What made them sound so different in free air? This called for a closer investigation.

A close listen and measurements of each fan was done...

  • at different distances and angles
  • from intake and exhaust side
  • with the fans blowing up, down or sideways

...all with the same annoyingly noisy results. Admitting defeat, I proceeded to remount the filter frame and protective grill on each fan to put them all back into the case.

Eureka!

On a hunch, I tried listening to one of the fans after the honeycomb protective grill was mounted back in place. Lo and behold! The sound that now emerged was almost entirely without tonality, and it actually measured 3 dBA lower at one meter. This was a eureka moment — and it recalled a comment by Devon Cooke in the SilverStone FT01 case review:

"On a whim, we pulled out the front filter to see if its impedance was causing the problem, but this did nothing but bump up the SPL by 2 dBA and introduce an annoying warbling into the noise character."


A protective honeycomb grill is screwed to the top of the fan. Along with the dust filter on the other side, it has a marked damping effect on the noise.

Extensive listening and SPL measurements were done again, this time with the honeycomb grill on and off each of the fans, and the dust filter as well. The end results are summarized for one of the 180mm fans.


Here's a spectrum analysis of one of the 18cm fans. The blue line represents the bare fan at full speed, with total SPL of 34.5 dBA. The pink line shows the same fan With the filter and grill on, the SPL dropped over 4 dBA. Note the dramatic reduction of highly audible midband peaks (400~2,000 Hz), many of them over 10 dBA, and the overall level reduction above ~800 Hz.

SPL @1m, 180mm fan from RV02
w/ and w/o honeycomb grill, dust filer
Speed
Honeycomb grill
Dust filter
SPL
Air Velocity
(fpm)
H
off
off
34
620
on
off
31
510
off
on
31
480
on
on
30
380
L
off
off
20
380
on
off
17
310
off
on
17
290
on
on
16~17
240

The dust filter and the honeycomb grill had similar effects on the fan noise. Both dramatically reduced the amount of tonaility, and actually effected a SPL drop of ~3 dBA@1m. The dust filer may have had a slightly greater effect, but this subjective impression could not be consistently confirmed by audio measurements. The dust filter and the honeycomb cover together effected an overall SPL drop of about 4 dBA@1m.

The reduction in noise comes at the price of reduced airflow. The air velocity was measured and it is shown in the table above, in Feet per Minute. (This is not the same as Cubic Feet Per Minute, which is the widely used fan airflow parameter; the relative air velocity is what's important here.) Both the grill and the dust filer reduce air velocity, by 37~38% when both are on. The RPM of the fan was unaffected. Whether this airflow reduction causes any cooling issues will be covered in the thermal system testing later in this review.

Here's a recording of the effect, at 1m distance. It's an MP3 sound file that begins with 7~8 seconds of the ambient in the anechoic chamber, then 8 seconds of the bare 180mm fan running at full speed, followed by 8 seconds each of the same fan with the grill on, with the filter, and finally, with both filter and grill on.

MP3 Sound File of Dust Filter and Honeycomb Grill effects on 180mm SilverStone Fan
(right click and download)
For best results, set your sound level so that the starting ambient sound is just audible, then turn it down to make it just inaudble, and don't touch the volume control again while you listen to the recording.

The second sound file below is of a 18cm fan set on Low speed, first bare, then with the honeycomb grill. The measured SPLs on this one were 17 and 20 dBA@1m.

Recording of Honeycomb Grill effect on 180mm SilverStone Fan on Low
(right click and download)
For best results, set your sound level so that the starting ambient sound is just audible, then turn it down to make it just inaudble, and don't touch the volume control again while you listen to the recording.

When informed and queried about these effects, Tony from SilverStone admitted that all this was news to them. Their prototype did not even employ the honeycomb grills, just ordinary wire frame grills. We'll call it serendipitous.



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