I remember, years ago, a video on TED of a guy presenting working technology for generating sound using collapsing waves from multiple sources, so you can have

I remember, years ago, a video on TED of a guy presenting working technology for generating sound using collapsing waves from multiple sources, so you can have sound limited to desired volume or cancel noise, so you could have silence next to a busy highway, etc. He was boasting high interest from military. His page/name and the video went offline soon after.