In Vietnamese custom, the living are obliged to worship their ancestors, by keeping shrines for them and giving burial rites to the newly dead. One who dies without descendants to keep holy his cult becomes a wandering soul, drifting forever without rest. Folklore tells of Con Tam, killed while still a child by her half-sister Cam, left to wander. When Cam laid down on her hammock after the killing its creaking was so deafening and frightening that Cam cried out in tears to the half-sister she had damned. The creaking of a hammock even in today's Vietnam serves as a reminder to keep one's ancestors holy and avoid death before leaving somebody to keep a shrine.

During the Vietnam War the US Special Forces' Psychological Operations division discovered this custom and sought to employ it in psychological warfare. To this end they recorded a Vietnamese linguist saying phrases like "Put down your weapons, lest you end up like me." They processed the recordings with echo effects and pitch changes, which resulted in an eerie, howling cascade of voices. Tapes were made of the sound, and played by loudspeaker from helicopters above Viet Cong territory. Rather than simply warping facts as normal propaganda does, the Wandering Soul project aimed to prey on religious sensibilities and scare the enemy into abject panic.

Needless to say, the soldiers, especially those from rural areas, were terrified of the recording. It usually caused them to drop whatever they were doing (even if it was defending themselves) and cover their ears, and sometimes even try to defect and escape the country with their families. The recording was so effective that it couldn't be played in areas with any allied Vietnamese soldiers -- being told that it was a recording made it no less frightening to them.

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