I'm interested introducing sound design (which could wind up being noise masking) into urban spaces with the intent to improve brain health. There is research out there on tinnitus and stress as related to sound/noise. I'm wondering if anyone has run across information (books, articles, websites, etc) that deal with the connection between sound and brain health in general. The simplest way I can ask this is "What impact, positive or negative, does sound have on brain health?"
Are you sure it's "brain health" that you're trying to cure?
This sounds very reminiscent of the many attempts to bring sound to public spaces, so I would start researching the cases where music has been bought to public spaces in order to create comfort and "increase well-being" (or to manipulate people like they do in shopping malls).
Now if you put "sound design" in that then I guess you're just approaching it from a bit more freeform angle musically.
You might also wish to look into the concept of binaural beats.
textareawikipedia article on binaural beatstextarea
It's an extremely difficult read and not directly related to what you want to study but i recommend this book to every person i meet who is interested in sound and music.