Wait...you want to sound-proof an OUTSIDE wall? What have you done for the inside walls? Cheap solution? Egg cartons! I mean, cover your wall with egg cartons, this will DEFINITELY work. They both disperse and absorb sound, so you're good to go!
I think to maintain the personal privacy or your home sound proof, you need to have a foam insulation or can install some acoustical panels on the wall. You can also install resilient channels between the drywall and studs.
To sound proof it you have to go to the inside wall of the area, put thick insulation ment for sound proofing then put carpet on the inside wall. the carpet sounds really weird but it truly works really well.
For outside? you could put some thick trees along it, they help block sound. that's why they put them along major highways that cut through residential areas. If you dont care how it looks tho you could use egg cartons, but i think that may better for sound proofing a room from the inside