It's time to start soundproofing my basement, and I think I'm going to start with the windows, which are currently single pane + a translucent piece of plastic on the exterior.

Initially, I thought about a thick piece of plastic (acrylic), but that stuff is about half as dense as glass. Glass is denser, but gets tough to find once you exceed 1/2" (and acrylic >1" thick gets quite spendy as well).

One of the glass guys suggested a double glazed vacuum sealed piece. 1/4" glass, 1/2" vacuum gap, 1/4" glass. Has the same mass as 1/2" glass, plus has a decent space in there. How effective will it be in cutting the transmission through the windows? The numbers I've seen suggest around 35 STC (http://irc.nrc-cnrc.gc.ca/cbd/cbd240e.html) but I'm wondering if any of you have experience or insight about double glazed + vacuum sealed windows and sound insultion.

Thanks!