http://www.cyro.com/Internet/SiteContent.nsf/paraglas2004!OpenPage
(this is for the clear sound barrier option, best company I have found so far. Just about to connect with them for contractors in Southern California USA)
http://www.fhwa.dot.gov/environment/noise/5.htm (positive/negatives of different materials, via USA gov study.)
http://www.fhwa.dot.gov/environment/noise/3.htm#3( See 3.4 for sound barrier defraction info, very good, bottom line, high frequencies, when deflected off a wall, continue up, low, bend down. High = wind noise in wheels of cars, Low = rumble of trucks)
http://www.fhwa.dot.gov/environment/keepdown.htm (great USA gov article about basics of sound barriers. See "Type of material" for 20kg density info, tho entire article is really helpful)
http://www.simetric.co.uk/si_wood.htm (info for wood density, good against the 20kg info)
