http://motherboard.vice.com/read/a-hist ... s-a-weapon
The actual volume of sound frequency isn’t the only way sound has been used in war. In his 2009 book Sonic Warfare, a key body of research in the understanding of contemporary sonic thought, Goodman included a chapter titled "Project Jericho," which explored the US PSYOPS campaigns during the Vietnam War.
Goodman described a particular campaign known as Operation Wandering Soul. The Curdler, a helicopter-mounted sonic device, produced the “voodoo effects of Wandering Soul, in which haunting sounds said to represent the souls of the dead were played in order to perturb the superstitious snipers, who, while recognizing the artificial source of the wailing noises, could not help but dread what they were hearing was a premonition of their own postdeath dislocated soul.”