Hypersonic sound broadcasts audio in a focused beam, so that only a person standing directly in its path hears the message. The big brotherness of what I am about to write feels creepy, but it may be that law departments of the future will have hypersonic reminders to lawyers to review their backlog of bills, for example.
Instead of e-mail reminders (how 20th century!) or announcements on the wiki intranet, the general counsel might turn on hypersonic reminders about the upcoming legal management meeting or the deadline for submitting evaluations of staff. These futuristic musings played in my mind after reading Wired, April 2008 at 60.