Apologies for being late to the party, but if you want some interesting reading on sound reduction, check this link:

http://downloads.bbc.co.uk/rd/pubs/reports/1991-09.pdf

The BBC have put a lot of time into investigating lightweight sound reduction ideas. They're looking at it from the perspective of recording studios rather than workshops but, if you don't mind it getting a bit technical, the PDF linked might give you a better idea of some of the considerations involved.