"AJ" wrote in
:

Have you tried reversing one at the mount, using one fan to direct the
air in one direction and the other one directing air the other
direction? S.A. having them both direct air physically away from the
heat sink or have one direct air towards the heat sink and the other
removing air from the same area. Sounds like you are getting a
vibration doppler effect due to the physical operations of both fans
in a limited area. "N Cook" wrote in message
...
John Tserkezis wrote in message
u...
N Cook wrote:

2 of the same fan blowing air in over the heatsink but there is a
noise
like
a variable direction wind or slightly gusty wind as though one fan
has a
bearing problem.
It would seem one fan is fighting the other to some extent,
significant
? or
just one of those things ? .
I was thinking of putting a dividing baffle between the 2.

Disconnecting each in turn and they run perfectly normally with no
change of
sound. I may dig out a strobe if i can remember where it is and
see if
the
blade speed does vary in the double active case.

Beat oscillation. Basically, the normal fan physical noise pitch
of
the
two fans are _almost_ the same. What you're hearing is the
difference
between
the two. (it may be less than 1Hz, because the fans will probably be
quite
closely matched in noise and speed).

It doesn't cause any harm other than you getting fed up with the
noise
and
smashing it to bits.

To "fix" the problem, you could try to physically isolate the two
fans
from
each other (or the common chassis) by using some sponge or rubber or
some such, to help prevent fan vibration getting into the chassis or
other fan
(if
you have them physically stuck together).

--
Linux Registered User # 302622
http://counter.li.org

I should have said these are 3inch, 12V dc fans rather than shaded
pole motors so unlikely to match speed that closely. Both are mounted
rigidly, no
grommets, to steel chassis and separated by 1.2 inch , the nearest
parts of
the heatsink fins are only 3/8 inch from the fan surrounds.
Placing a piece of card between the 2 fans seems to stop the
interference but not desirable in the case of failure of one fan
perhaps.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/





sounds like a case of the heterodyning fans!

--
Jim Yanik
jyanik
at
kua.net