Noise Cancellation in Case Fans

This article points to the possibility
of noise cancelling fans in the future:

Noctua to use Active Noise Cancellation in new Fan

Ed Abbott

Blade Pass Frequency

I've been working on building
a silent computer and also on
finding materials that I can
surround the computer with to
make it more silent.

A concept I find very helpful
is blade pass frequency. Here's
a formula to caluclate blade
pass frequency:

Blade Pass Frequency - BPF

Here's a chart that indicates at
what frequency the noise from a
computer fan might peak:

Acoustics/Noise from Cooling Fans

Scroll down the page in the above
link to see the chart. The sub-heading
for the chart is Noise Generation Mechanisms.

Finally, here's chart that shows noise
loss that occurs when sounds passes
through differing thicknesses of plywood:

Sound Transmission Loss in Plywood

Ed Abbott