Noise abatement for a rack

[David Mathog]

Anybody here ever try noise insulating a rack???

I have to share a room with our new 20 node cluster
and it would be nice to be able to do so without having
to wear earplugs all the time.  The 20 2U single Athlon
systems are mounted in an open frame 4 post rack.  Using a 
Radio Shack sound level meter (catalog #33-2055) set to
dBA, fast weighting the following values were obtained
for (front,left side, back, right side):

room ambient (no power) <50dBA  (below detection limit)
1  node  at 6"  from side, front panel open:    66,59,67,63
1  node  at 6"  from side, front panel closed:  63,58,67,62
1  node  at 48" from side, f.p. closed:         53,- ,- ,55
20 nodes at 6"  from side, f.p. closed:         72,70,76,72
20 nodes at 48" from side, f.p. closed:         66,- ,- ,66

As you'd expect - it's all fan noise.  The left side
is quieter than the right because there are fewer ventilation
holes on that side and the dual internal fans are mounted
closer to the right side of the case.  My goal is to drop
the dBA at 48" down to no more than 53 dBA with all nodes
operating.

Sound measurements next to an Antec SX-630 case (no
sound insulation, just sheet metal) were 65 dBA with
the side off, 57 with the side on.  So adding sides to
the open rack may help a little and shouldn't be much
of a problem for ventilation.  But how to treat the front
and back of the case???  If I close them off with a 
solid sheet of sound absorbing material (lead would work
but something a bit less expensive and toxic
would be better) the system will become really quiet
- because it's going to overheat and die.  Some sort of
sound absorbant coated louvers maybe?  Or for the back,
since it's close to a wall, coat the wall with sound
absorbing tile?

Hopefully somebody has already dealt with this.  But
if the solution is out there on the web I've not found it.
All the racks I saw were just sheet metal and the front/back
doors, if any, were either plastic or metal grillwork - good
for maybe a 3-4 dBA sound reduction.

Regards,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech