I notice that a significant amount of the remaining sound in my case is being heard through the front of my case.
......The remaining opportunity is to dampen the lower front of the case to reduce sound coming out the front of the case. The question is, how do I dampen but not overheat the hard drives (ie. without completely restricting airflow).
I'm assuming the typical dampening material will not allow enough airflow through it (and while most have some porosity...not really that much). The material I'm using right now is simple carpet underlay.
Interesting how this thread quickly got away from damping. Before you read on, be aware that I did the entire foam application in one pass so I quantify can't how much the following helped. I present it here as food for thought.
I did (more or less) as I suggested in an earlier post. I lined the lower portion of the cover with 2.2mm thick vinyl(?) barrier material. The reasoning behind that choice was that it wouldn't block airflow and the barrier material should have "some" impact on airborne noise since it's not as hard as plastic. Plus it has the added benefit of damping structural vibrations.
Then I turned my attention towards the case itself. I had previously trimmed out the stamped grill, leaving me with a gapping, square hole. I ended up applying 16mm foam to the left, right and top case walls surrounding the hole, leaving the bottom open for airflow. The 16mm foam doesn't completely fill the gap between the cover and the case so if any air was drawn upwards before foaming, well it still has something of a path to follow. (I really doubt the airflow up the front cover above the lower fan opening amounts to anything.)
Then I noticed that by applying foam to the interior and exterior case surfaces I had in effect created a 41mm foam "tunnel" to the case interior. When I had covered up every apperciable, accessible surface there was still some foam leftover so I stacked more 16mm and 25mm foam around the fan opening resulting in an 82mm foam "tunnel" to the interior.
As I stated earlier, I can't quantify in any way just how effective this exercise was beyond the fact that rapping on the front cover results in a dull muffled thud. Insulating the case has only increased the CPU and MB temps by ~1-2 degrees C. And, FWIW, the HDD was installed in a SmartDrive 2002 enclousure and mounted in the lowest 5.25" bay a week or more before insulating and it's temps _may_ have risen by 1 degree C. (I really should be more anal about noting/recording ambient temps, but I'm not. Spring has sprung and ambient temps have jumped slightly.)
Bottom line on temps: CPU maxs out at 50c with an occasional, brief jump to 51c; a couple weeks ago 49c was the max. MB hit the upper 30's whereas it used to hit mid 30s. HDD remains at 44c max and idles in the upper 30's. Case fan is the same 80mm L1A at ~8.5v that I've been running for about a month. The Antec PSU fan has hit highest RPM's I've seen from it, about 1800 I think; but that's only a jump of ~100. I may change the case fan resistor jumper to a lower value as summer is approaching I'm comfortable with these numbers.
This post has gotten far more long winded than I expected. I hope this helps.