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Noise SPL Measurements
Sound pressure level (SPL) measurements are done from a distance of one meter using our usual
high sensitivity sound level meter: A Bruel & Kjaer (B&K) model 2203.
This professional caliber SLM is >20 years old, weighs over 10 pounds,
and is completely analog in design. It has a dynamic range that spans 140
dB. The unit can measure accurately down to about 16 dBA. A quiet environment
is a prerequisite to low noise testing; the lab has been measured down to
~17 dBA at night.
B&K model 2203 sound level meter measures down to ~16 dBA.
Good as our SLM is, it is not our most important tool for evaluating noise.
While it does a good job of sorting out the coarse differences between fans,
the human ear is a much more sensitive instrument for judging fine nuances.
This is because what we perceive as quiet is much more than simply
low volume. Our recent article called
What is a "Silent" Computer? describes in detail what we mean
by this, but in a nutshell the quality of noise can be just as important
as the amount.
Don't believe us? Most of the fans that we test measure below the ambient
noise in the constant airflow test. Nevertheless, we still consider some of
them too loud for use in a quiet system. This is not because they are producing
a lot of noise; it's because they're producing the wrong type. The noise of some
fans blend into the background much more easily than others, and these are
the ones that we're looking for.
Audio Recordings
But don't just take our word for it. Try downloading these two sound files:
Smooth Sample, Ticking
Sample. Each sample contain five samples of ambient noise followed
by five seconds of a fan blowing 10 CFM. Both samples measured less than 18
dBA@1m — the ambient noise at the time of recording. The recordings were
made from a distance of one meter and mirror closely what we heard from that
distance.
The two recordings illustrate exactly why sound quality matters: The Smooth
Sample is completely inaudible; you can't tell that there's a fan on. If you
listen very closely, you might hear a slight pop in the middle of the file
that marks where the recording of the fan was joined to the recording of the
ambient noise, but the subsequent noise just sounds like... more ambient.
The Ticking Sample, on the other hand, leaves no ambiguity that there is something
being recorded. You notice when the transition between ambient and
fan noise occurs because there is an audible difference. There's no doubt
that the Smooth Sample is quieter, even though their measured SPL is the same.
Because we test fans at a constant airflow, noise measurements are not that
useful because keeping airflow constant means that most fans measure very
similarly. Instead, we use our ears to determine which fans are quietest...
and we encourage you to use your ears as well.
Every fan that we test is recorded four times, according to our
standard Audio Recording techniques. The four recordings are as follows:
- Alternating ambient noise and the fan running at 5V, 7V, 9V, and 12V,
recorded at a distance of one meter.
- Alternating ambient noise and the fan running at 5V, 7V, 9V, and 12V,
recorded at a distance of one foot (30 cm).
- Five seconds of ambient noise, followed by the fan running in the constant
airflow test, recorded at a distance of one meter.
- Five seconds of ambient noise, followed by the fan running in the constant
airflow test, recorded at a distance of one foot (30 cm).
As always, we recommend that you listen and compare the recordings in a specific
way. The green box below describes how we make our recordings and what you're
supposed to do with them.
HOW TO LISTEN & COMPARE
These recordings were
made with a high resolution, studio quality, digital recording system,
then converted to LAME 128kbps encoded MP3s. We've listened long and
hard to ensure there is no audible degradation from the original WAV
files to these MP3s. They represent a quick snapshot of what we heard
during the review. Two recordings of each noise level were made, one
from a distance of one meter, and another from one
foot away.
The one meter recording
is intended to give you an idea of how the subject of this review
sound in actual use — one meter is a reasonable typical distance
between a computer or computer component and your ear. The recording
contains stretches of ambient noise that you can use to judge the
relative loudness of the subject. For best results, set your volume
control so that the ambient noise is just barely audible. Be aware
that very quiet subjects may not be audible — if we couldn't
hear it from one meter, chances are we couldn't record it either!
The one foot recording
is designed to bring out the fine details of the noise. Use this recording
with caution! Although more detailed, it may not represent how the
subject sounds in actual use. It is best to listen to this recording
after you have listened to the one meter recording.
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Vibrations
Just as with hard drives, the direct airborne noise is not the only component of a fan's acoustic signature. Fans also vibrate, and those vibrations can cause audible noise. The balance and precision of the bearings, the way the the blades interact with the air, the composition and structure of the frame — all of these can affect the vibrations exhibited by a fan.
To examine fan vibrations, we've adopted the technique we developed originally for hard drive testing. We gauge the effects of vibration by placing the fan on an aluminum electronics project box (43cm x 25cm x 10cm). The box acts as a sounding board and resonating chamber for the fan
vibrations. The box exaggerates the vibration-induced noise to make it easier for
us to hear and evaluate. The air inside the box resonates along with the
thin aluminum panels at the primary RPM frequency and
its harmonics, and any resonant frequencies intrinsic to the material and structure. Some plastics have a pronounced resonance at certain frequencies that are easily excited at specific RPM speeds, but not at others.
The aluminum box amplifies vibrations so we can hear and assess them more easily.
Because of the time and effort required, we will limit vibration testing to those fans whose airborne acoustics are already judged to be good. It's not yet clear whether we can use the same 1-10 scale used for the hard drives. The quality of the sounds are quite different from that of hard drives due to the much lower mass and rotational speed of the fans, which makes comparison between drives and fans difficult. (Because the spin speed of quiet fans is many times slower than the 5400 or 7200 RPM of hard drives, the fundamental audio frequencies are much lower.) Whether we use the same scale or develop a new one just for fans, we will be commenting on vibration, particularly at the speeds that each fan is most likely to be run by PC silencers.
CONCLUSION
With this methodology in place, SPCR can finally begin to seriously undertake
the project we began more than two years ago. There are plenty of fan round-ups
on the web, but we think you'll agree that none of them covers noise as extensively
or as realistically as we do. We've worked hard to make sure that the fruits
of our labor are as accurate and as useful as possible. We are confident that
we have taken a big step towards filling a sizable gap in the knowledge available
on SPCR, but, as always we are sure there are things we have missed. If you
have suggestions or improvements for SPCR's Fan Methodology v2.0, don't hesitate
to tell
us about them in our forums!
[Editor's Note: Some readers will be asking, "But what about fan blade geometry and design?!" Well, this is a fascinating topic, but it's unrealistic for us to try and tackle it. For one thing, it is extremely difficult to correlate a fan blade design with airflow or noise performance. Secondly, we don't really care how a fan manages to be quiet, but that it is quiet. Certainly we'll touch upon unusual aspects of a fan's design, including its blade geometry; no attempt will be made to address the performance consequences of those design aspects, however. ]
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