 Complete the following phrase:  Acoustic devices have been used for military purposes including to stress enemies, as an aid in interrogation, and to create an infrasonic sound barrier. The British Army used Squawk Boxes to emit ultrasonic frequencies causing various discomforts. 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. Operation Wandering Soul broadcast voices purported to be dead Vietcong. Other examples include the 350 watt HPS-1 Sound System that could be heard 2.5 miles away and was used on the Vatican embassy in Panama where ousted president Manuel Noriega was in refuge. At the 