Building a rack for servers to lessen fan noise. Material to put inbeween walls?

[Fett]

I'm going to be building a rack to hold servers, my main source of noise is from the high RPM fans in them.

I'm going to have an inner 1/4" plywood wall, about 2" of space, then an outer 3/4" plywood wall.

What would be the best material to put in the empty space between walls to lessen the fan noise?

I'm going to be using this design as a general template:

http://imgur.com/a/uh82C

[Randy S]

we offer a line of inexpensive cotton fiber panels 2" thick class A fire rating NRC 1.02

Give us a call and we can send data sheet and pricing.

Randy S.

760-752-3030