sound art in public space

I am interested in examples of sound(ing) art in public spaces. That is, not so much in temporary sound(ing) art works but in permanent sonic interventions meant to improve the sonic quality of a public space. Anybody around who knows of some concrete examples? Any literature on this topic? Any “best practices” by which I mean concrete collaborations between a sound artist and architects, urban planners, engineers, city governments, etc.?