Windows... Glass as opposed to.....

What are the disadvantages of using something other than tempered glass as your studio windows (such as plexiglass or clear acrylic sheets)?

Loss of mass/density and a real big acoustic hole in the wall that sound will go directly through. Plexiglass or acrylic would have to be very think to be of any use, and still effective, like the bullet-proof panels you may have studied at banks...assuming you still have a bank account:) But it still would not compare to the same thickness in a laminated glass.