The noises made by fans...
Posted: Sun Jan 13, 2013 5:14 am
I am totally surprised, that even in 2013, one MAJOR source of noise in computers has not been properly addressed, yet is so elementary
FAN NOISE!
EVERY fan used in computers, regardless of blade shap, or rotation speed, size, bearings have one thing in common:
The blades are equally spaced around the circle! Choose any fan in your system, or on your bench, count the number of blades and divide 360 by the blade count. You will have the number of degrees between the same spot on every blade
Why is this important?
Air flowing thru a running fan has a little pressure wave in the air exiting the fan housing, little, tiny, bareley audible, whu, whu, whu, whu, in time with a blade passing any given spot in space. Like helicopter blades going Whup, Whup, Whup, you know the sound
now, fan blades are a tad smaller than choppa blades, but the pulses are of a higher frequency, and that frequency gets up into (human) audible range...
So, hold that thought...
Now theres another noise maker action at work, and that is where the arflow goes
Back in the 70's, EMD-GM locomotive company did some testing and research on locomotive radiator cooling fans, and they found that size, speed and HUB MOTOR MOUNTING were all crucial factors, so they came up with Q-Fans.
This photo is an 8-blade fan with the blades unevenly spaced.
A 9-blade fan has 3 sets of 3 blades, and in each set of blades, the middle blade is closer to the one (in the photo below) to the left.
The 48" fans generally run slower where possible.
This means that the pressure pulses are now no longer regular, and if you didnt already realise it, sound is a steady stream of pressure pulses
EMD also worked out that the bars that the motor is mounted on should be on the intake side of the fan, having the mounting bars on the exhaust side creates a siren effect. Your fan grilles can also contribute to the same kind of siren effect
FAN NOISE!
EVERY fan used in computers, regardless of blade shap, or rotation speed, size, bearings have one thing in common:
The blades are equally spaced around the circle! Choose any fan in your system, or on your bench, count the number of blades and divide 360 by the blade count. You will have the number of degrees between the same spot on every blade
Why is this important?
Air flowing thru a running fan has a little pressure wave in the air exiting the fan housing, little, tiny, bareley audible, whu, whu, whu, whu, in time with a blade passing any given spot in space. Like helicopter blades going Whup, Whup, Whup, you know the sound
now, fan blades are a tad smaller than choppa blades, but the pulses are of a higher frequency, and that frequency gets up into (human) audible range...
So, hold that thought...
Now theres another noise maker action at work, and that is where the arflow goes
Back in the 70's, EMD-GM locomotive company did some testing and research on locomotive radiator cooling fans, and they found that size, speed and HUB MOTOR MOUNTING were all crucial factors, so they came up with Q-Fans.
This photo is an 8-blade fan with the blades unevenly spaced.
A 9-blade fan has 3 sets of 3 blades, and in each set of blades, the middle blade is closer to the one (in the photo below) to the left.
The 48" fans generally run slower where possible.
This means that the pressure pulses are now no longer regular, and if you didnt already realise it, sound is a steady stream of pressure pulses
EMD also worked out that the bars that the motor is mounted on should be on the intake side of the fan, having the mounting bars on the exhaust side creates a siren effect. Your fan grilles can also contribute to the same kind of siren effect