The number one thing you need to do is to make sure that you are controlling the loudest noise source and making it quieter. If you don't do that, you risk doing nothing at all in the way of noise control. If one noise source is very loud, then a relatively quieter noise source can seem as if it isn't even there at all. Industrial noise control must recognize this dynamic, which leads me to the fundamental tenet of noise control design: "Treat the loudest source (or source group) first." Noise control is very much a top-down process.