Neat Invention Alert: the Hypersonic Sound System broadcasts sound in a tight narrow beam that only those in the path of that beam can hear. And no one else. Think of it as a speaker that acts like headphones. This is heady stuff and really reminds me of the personalized advertisements in Minority Report.
The article discusses some of the applications including military uses to deafen the enemy. I could also see this enabling custom settings in movie viewing and just about any other use where headphones are currently used.