You have to be REAL careful when doing ANY type of absorption. Due to most "absorbers" not absorbing the entire frequency range in a linear fashion, they can actually do more harm than good.
Egg crates, egg crate foam, drapes, etc... will all skew the tonal balance due to the fact that they will do little to nothing for low frequencies while playing BIG games with mids and treble. As such, this can be beneficial for an overtly "hot" or "bright" system but can now swing the tonal balance too far towards the "warmth" side. This is not only true for "homebrew" designs, but also of "commercial" designs.
For more info on absorption rates and how it can affect tonal balance, take a look at some of the frequency response charts that Audio Advisor has for some of the Pro-foam and ASC stuff.
For well thought out and tested "homebrew" absorbers and bass traps, try looking at Jon Risch's site. He is both an EE and works with acoustics on a daily basis. Sean
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http://members.nbci.com/Jon_Risch/index.htm