Fan noise
level is measured in accordance with CNS 8753 standard
(which is similar to DIN 45635) ISO 3744 in a sound-proof
chamber where the background sound level is no more
than 15dBA (COOLTRON uses a dBA rating system to be
in compliance with the Occupational Safety and Health
Act). Fastened vertically at all four corners using
strings, a microphone is then placed one meter away
from the center of the fan from the air intake side.
The decibel readings taken from the sound level meter
are plotted as Sound Power Level against the octave
frequency bands. Each band has an upper limit approximately
two times that of the lower limit.
Calculation
of noise is as follows:
SPL =
20 log10 P / Pref
SWL = 10 log10 W / Wref
Where:
SPL: sound
pressure level
SWL:
sound power level
P: pressure
Pref:
a reference pressure
Wref:
an acoustic reference power
Origin of Fan Noise
The derivation
of noise can be from the air moving device (AMD) itself
or it may even emerge from system disturbances.
System disturbances are the biggest cause of
fan noise. When a fan is designed for low noise operation,
it is sensitive to inlet and outlet disturbances caused
by such items as, brackets, capacitors, transformers,
cables, finger guards, filter assemblies, walls or panels,
etc.;
Other contributions to fan noise include the shedding
of the vortex, turbulence, system load, and construction
of the AMD. Improper pitch angle can lead to shedding
of the Vortex. Vortex
shedding is a broadband noise created by air separation
from the blade surface and the trailing edge. Speed
is another major contributor to fan noise. If the speed of a fan is reduced by 20%, the
decibel level will reduce by 5 dB.
The effects of a change in speed can be seen in the
following:
dB1 = dB2 + 50log10
(rpm1/ rpm2)
Turbulence
occurs in the air stream and is part of the cause of
broad band noise. Disturbances to the inlet or outlet, due to
real sharp edges and bends, will likely increase turbulence;
therefore, increasing noise. As for system load, fans that run at peak efficiency
tend to be quieter, but variations in the system load will also cause noise to change. Last but not least, poor construction of the
fan will often times lead to structure vibration. For instance, bad design or mounting of the
bearings will cause vibration, which in turn causes
noise to increase.
How to Lower Fan Noise?
System
Impedance should
be reduced to the lowest level possible so that the
noise to airflow ratio remains low. The inlet and outlet
ports of a cabinet can be between 60% and 80% of the
total system impedance.
This is too high for low-noise results.
The impedance of the inlet and outlet could occupy
a large fraction of the airflow potential.
As a result, a faster and noisier fan will be
required to provide the necessary cooling.
A
rise in temperature may also lead to a reduction in
noise. Since airflow has an inverse relationship with
temperature, a rise in temperature brings a proportionate
decrease in airflow.
In any instance where even a small amount of
the temperature limit can be slackened may aid in the
reduction of noise.
Even without temperature changes, selecting a
fan size will help reduce noise. Large fans are quieter than smaller fans that
yield the same airflow.
One
final approach to reduce noise is to isolate the fan
from the cabinet. Because fans operate at a low frequency, and
are light in weight, vibration isolators must be soft
and flexible. This solution is favored in cases where the
airflow of the AMD is at less than 20CFM and noise is
predominantly from the cabinet.
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