Acoustic devices have been used for military purposes including to stress enemies, as an aid in interrogation, and to create "an infrasonic sound barrier".[8] The British Army used "Squawk Boxes" to emit ultrasonic frequencies causing various discomforts.[8] Audio harassment was also used by the U.S. military in the Vietnam War and was famously depicted in the fictional movie Apocalypse Now as helicopters descend on the enemy with loud speakers.