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/