You could get a wall of trees to the North of that and to the south.
Its a pretty easy theory to test out as well.
You just need to get a rotary type with similar frequency output and get it the hover the other side of a tree line. Then take noise readings and see how much it drops the noise level down by over distance. Try different tree types and widths of barriers.
Or you could go over to Scandinavia as most of the regional airports are surrounded by trees. I have been 2km away from one of them that had saab's, ATP's etc operating out of it and you never heard them on the ground just when they took off and cleared the tree tops the noise seem to turn on. But then again that was 1km thick pine forest.
Lets face it with tree's if it doesn't work out you can just chop them down and get the sappers to blow the stumps out. And planting trees can be considered green
And I can't see it costing nearly as much as a concrete shield.