Not sure how much effort you wanna put into this, but if you wanna go
deeper than rules of thumb (lowering the fan speed or using temperature
control etc.), you might wanna take a look at
http://www.machineryacoustics.fi/LNDandfannoise.pdf (app. 1 Mb).
Hope, it explains the problem statement and issues you are dealing with.
You can skip the first half of the article since it deals with Low noise
design and machinery acoustics and put your effort on the case. To my
knowledge those are almost the only result dealing with comparison of
single and parallel fan operation in this kind of application. Please,
correct if i'm wrong.
And about two fans in series. It depends on how you do it, but basically
you should be able to degrease K. The benefit depends on what happens to
airflow velocity components (swirl?).
BTW, any comments, public or private, on the writing are welcome. But
let's not get into a dB discussion (this is for acoustics group), please:)
PS To my understanding most research done by this kind of fan
manufacturers in last almost 10 years has been concentrating on motor
"I'm glad you asked".
I've been studying and fighting fan (ALL sizes) noise for years.
The "fifth power" rule applies; fan noise increases to the fifth power
of the RPM. Personal tests show a 20 dB rise for a doubling of fan RPM.
If you add a second fan alongside to still get 90 CFM (wow; big fan for
electronics these days), then run two side by side, but at half RPM.
AC fans are difficult to slow down; variable speed drive (VSD) is done
for big building HVAC fans by fabricating lower ac frequencies. But you
can try a dropping resistor, or connecting two ac fans in series. With
DC fans (plenty out there these days, especially in the "Muffin Fan"
series), you can easily run them slow, since the permanent magnet DC
motors run exactly proportional to applied voltage.
So half rpm gives 20 dB less noise. Placing two half speeders side by
side gives 3 dB more noise; net gain is 17 dB. Not too shabby...
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