Music
Although it seems to be both more widespread and older, the calculated use of music in “detainee interrogations” is less easy to trace than the use of sound as a weapon.
“Acoustic weapons” have been in development by Department of Defense contractors since at least the 1997 creation of the Joint Non-Lethal Weapons Task Force, accounting for 1/3 of the Task Force’s budget in 1998-99.
For the past year or so, I've been finding myself fascinated with popular music. It seems to me that one of the most important artistic revolutions in our century has been the evolution of popular songs.