Fan noise

[Jim Lux]

There is a huge difference in noise level (many 10s of dB) between various 
models and makes of the actual fans.  Since the noise is in no way related 
to how good the fan is at pushing air (a tiny fraction of the motor power 
goes to making aerodynamic noise, so it's not an efficiency issue), some 
mfrs spend time on making the fan quiet and others don't.  Designing a 
quiet fan is an expensive proposition, largely based on empirical 
techniques (i.e. cut and try), so a low cost fan is less likely to be quiet.

A lot of the noise may actually be from the vents and airflow through the 
chassis, rather than the fan.  Even less money is typically spent on 
looking at noise on a commodity PC (no profit margin there), although some 
more expensive units intended for "up-scale" office use are more quiet 
(again, because the price premium can pay for the  expensive acoustic design).

The easy solution, for a typical PC maker, is to make the fan variable 
speed (since noise is VERY strongly dependent on blade rate and air flow) 
and only run it fast enough to keep the temperature within some (possibly 
reasonable) limit.


Some practical advice:

1) Figure out if it is the fan (or which fan) or flow through 
slots/louvers/etc that is causing the noise.  Run one of the fans out in 
free air.  If it howls, then look for a less noisy fan and replace it.

1a) Fans that have the same number of support struts as blades will be 
noisier, because all the blades pass all the struts at the same time.  You 
want 5 blades and 4 struts, for example.

2) If it is louvers, etc causing the noise, then start looking at how to 
smooth the edges of holes, etc. Be forewarned that this is a tedious process.

3) Put everything in an enclosing box.  You want the walls of the box to be 
highly damped so they don't transmit the noise.  Traditional approaches are 
using thin sheet lead or two thin layers separated by dry sand.  The air 
duct going in and out should have absorbing material on the inside, and 
should have appropriate bends to prevent a straightline noise path.

4) You could also use one big high pressure blower (which you can spend all 
your time making quiet) and some hoses to distribute the cooling air to 
your chassis.

5) (not real practical, but it works).  Do away with the fans and sink all 
the PCs into a liquid coolant bath (mineral oil or fluorinert).




At 12:17 PM 7/8/2002 -0500, Pengcheng Zhang wrote:
>Is there a way to reduce the noise of fans? Our 8-node cluster
>makes me feel like working in an airplane. They are running
>Redhat 7.3. The same problem happens on a PC. When it's under
>WindowsXP, it's really quite; but under Linux it's noisy.
>
>Thanks.
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