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HOW WE MEASURE FANS
An SPCR fan test starts with a web search of the fan's model number. If we can't find
a record of it on the web, the fan is set aside — a fan that can't be found
online is not as worthy of a review. Chances are you won't be able to buy one.
DC axial fans on the retail market used to come with minimal packaging. They were mostly surplus stock from large OEM orders that ended up in retailers' inventory for use as replacement parts. Some of the fans we test still fall in this category, and they sometimes come supplied only with bare wire leads. In the past five years or so, the retail market for computer fans must have grown a lot, because so many fans are supplied in fancy packaging, complete with screws or other mounting devices, multiple cable terminations, and even variable speed controllers. All of these added features and details are covered in our reviews.
Fan Specifications
Assuming we do find a record of it, we take note of its official specifications,
taken from the original manufacturer's documents if possible. The specifications
are included as a guideline for what to expect, and give us some idea of how
the fans are supposed to perform. Not all specifications are accurate,
however. It is not uncommon to find multiple specifications that conflict
with each other, especially if they are published at different times or by
different companies.
Retail brands that buy their fans from large OEM companies
occasionally quote different specifications than those of the
original manufacturer. This is especially true for noise specifications;
there are numerous known cases of companies publishing fraudulent
or incomplete noise specifications. Even "honest" looking numbers
don't always mean much, since there is no universally agreed upon standard
for measuring fan noise.
SPCR tracks the following specifications:
- Model Number
- Bearing Type
- Power Rating
- Airflow Rating
- Rotation Speed
- Noise Rating
- Header Type
Bearing Type
Most of these are self explanatory, but bearing type deserves some explanation.
There are countless bearing designs out there, but most fall into one of two
categories: Ball bearing or sleeve bearing. It is generally agreed that ball
bearings tend to be more durable and last longer than sleeve bearings, especially
in high heat situations. For this reason, most CPU heatsinks and power supplies
use ball bearing fans. In addition, ball bearings are less likely to fail
without warning — worn ball bearings generally start to grind when they
wear, giving an audible warning that something is wrong before they fail entirely.
Sleeve bearings, on the other hand, simply seize up when they fail, which
can be catastrophic if not noticed for some time.
That's not to say that sleeve bearings are always worse. There is some evidence
that sleeve bearings tend to sound smoother than ball bearings, especially
after the fan has seen some wear and tear. They are also slightly cheaper
to make, although that difference doesn't seem to mean much on the retail
market. Lastly, sleeve bearings are more shock resistant. Drop a ball bearing
fan, and the pressure point between the ball and the center shaft is likely
to cause damage — one of the balls may develop a flat spot, a chip, etc.
Sleeve bearings don't suffer from this problem, as the center shaft is evenly
supported by the sleeve across its whole length — there are no pressure
points to cause damage.
There are also several unusual bearing types, such as Hypro, FDB, or Maglev
bearings, and countless unnamed variations on ball and sleeve bearings. Bearing
type can significantly affect reliability and how a fan sounds as it ages,
which is why we report it, but keep in mind that the quality of the
bearing can have as much impact as the underlying technology. Unfortunately,
things like reliability and bearing quality are almost impossible to determine
without looking at hundreds if not thousands of samples. This kind of testing
is far beyond our capabilities, so we leave you with bearing type, and let
you draw your own conclusions.
[Editor's Note: If you undervolt the fans the way most quiet PC enthusiasts do, the wear and tear on them is dramatically reduced. It's not unrealistic to expect a good fan to last a PC's useful life. A few fans in some of SPCR's lab systems have been spinning slowly 24/7 for as long as the site has been live. )
Our Tests
After jotting down all of the relevant specifications, the actual testing
begins. Our two main tests are for noise and airflow — the two parameters
necessary to determine the fan with the best airflow to noise ratio. We also
measure rotation speed and power consumption, and we make an attempt to determine
the lowest voltage at which the fan will start reliably.
Each test is duplicated five times, each with the fan running at a different
speed. Four of the tests run the fan at a constant voltage — 5V, 7V,
9V, and 12V — yielding a good idea of how noise and airflow scale with
speed. The fifth test is run at a constant airflow: 10 CFM for 80mm
fans, 15 CFM for 92mm fans, and 25 CFM for 120mm fans.
A custom-built four channel fan controller is used to produce exactly the
right test voltages.
The constant airflow test is important: It gives a common point of comparison
for every fan tested, making it easy to rank the fans based on their airflow
to noise ratio. Whichever fan is quietest in the constant airflow test will
be crowned the quietest fan overall. The extremely low level of airflow that
was chosen for this test reflects our bias: Most fans will be very quiet at
this level while still providing enough airflow to cool a moderately powerful
system. Some fans, the best fans, are completely inaudible from one meter
at this level. Others never quite disappear entirely but are still tolerable,
while the worst fans exhibit a clearly audible noise signature.
The Fan Harness
All measurements are done with the fan mounted in a specially made harness
that ensures that every test is done with the fan in the same orientation
in the same surroundings. The harness is made out of soft foam that almost
eliminates vibration resonance as a source of noise. This is an ideal mounting
system; any real system will transmit some vibration to whatever the fan is
mounted to, potentially causing resonance. In fact, an earlier version of
the harness built with closed cell foam produced an audible hum when used
with some high vibration fans. Did you know that foam could produce noise?
We sure didn't!
A Foam Harness decouples the fan and reduces resonance noise.
The harness is supported by thick closed-cell foam and a plywood base —
an earlier version had a tendency to collapse under the weight of the fan during
measurement.
Sample Variance
Sample variance in fans can be quite dramatic, and we try and test multiple
samples whenever possible. It is usually not difficult to gather two or three
identical models, but this is hardly an appropriate sample size for true variance
testing. Given the time and expense of collecting (and testing)
even five or ten samples of every fan, we have not made an effort to measure
sample variance.
Even so, we collect quiet fans. Odds are good that if a fan is quiet, we've
received samples from multiple sources and can make some assessment of sample
variance. For this reason our policy is: If we have multiple samples, we will
examine them all and report any variances we find.
[Editor's Note: It is my opinion that damage due to shock during transit or handling is the uncontrollable variable — a dice-roll factor — for small fans. It helps to explain why different shipments of samples from what appears to be the same production batch can sound significantly different as groups. Bearing damage can cause more ticking and buzzing, increased broandband noise, and imbalances that cause subtle axial wobble, which translates into increased vibration. These effects may be imperceptible to a casual user or listener, and I can't imagine any supplier who would accept such samples back as defective or damaged goods, but the differences can be heard and are significant if you seek the lowest noise. I believe that shipping/handling damage explains a big part of sample variance, and some of the differences in experience and judgement among careful listeners/users of fans. This seems especially true for ball-bearing fans. There is no way to prove this without a fancy, large scale study; it's my hypothesis based on some 6~7 years of messing around with a whole lot of small DC axial fans.]
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