A variety of nonlethal acoustical weapons have been proposed and evaluated. Some of these are little more than fancy loud-speakers, while others involve more subtle or sophisticated processes and truely deserve the designation of acoustic weapon.
Simple high-intensity sound causes the inner ear to generate nerve impulses that register as sound. Since the inner ear also regulates spatial orientation, saturation of the inner ear by high-intensity sound may cause spatial disorientation. For example, loud music was used by American forces to drive Manual Norriega from the Vatican Embassy in Panama in 1990.
High-intensity low-frequency sound may cause other organs to resonate, causing a number of physiological results, possibly including death. Acoustic weapons pose the hazard of being indiscriminate weapons, potentially imposing the same damage on friendly forces and noncombatants as on enemy combatants or other targets.
A low-frequency sound transducer produces sounds below the audible frequency range [below 20 Hz]. The sound transducer is driven by its own amplifier that uses the output of a low pass filter as its input. Known as infrasound, these vibrations are felt but not heard. The observer needs to be placed on a wooden platform so that the vibrations are transmitted to a suitable area.
Infrasound would be a powerful ultralow frequency (ULF) weapon that could be directional and tunable, penetrating buildings and vehicles. High Intensity infrasound could induce disorientation and reduced sensory motor functions. At higher levels of intensity, experimental have shown that animals may cease breathing temporarily. But this has seemed to be not a very practical weapon, since large banks of speakers were required to provide directionality, and power demands were deemed excessive.
Diference Tones are more sophisticated arrays that project a sound to a specific location. The resulting sound can only be heard at that particular location as the result of interference patterns created by the interaction of sounds transmitted from multiple remote speakers.
The Curdler, a device that emits a high shrieking noise at irregular intervals, was reportedly used by the British as a means of riot control in Northern Ireland. In this case the sound is at levels lower than the pain threshold, and is intended to be annoying rather than inducing disorientation.
The American Technology Corporation (ATCO) has produced a Long Range Acoustic Device (LRAD), a long-range hailing and warning directed acoustic beam device. LRAD was developed to communicate at operational ranges with authority and superior intelligibility in high ambient noise environments. LRAD systems are in operational use or evaluation in maritime, check point, vehicular, airborne, and integrated system applications by the USN, USMC, US Army, and USCG. LRAD is a flat panel, multi-transducer, phase coherent emitter. Designed for highly-directional communication at 300+ meters over land and 500+ meters over water, LRAD can also issue a warning tone.
A more potent weapon under development in Russia since the early 1990s is a high powered very low frequency (VLF) modulator. Operating at frequencies below 20 KHz, the device requires a 1-2 meter dish to project a so-called "acoustic bullet." The device was attractive because the power level is adjustable. At low power, the system would cause physical discomfort, while increasing the power could induce nausea, vomiting and abdominal pains. The highest levels can cause a person's bones to resonate, which can be quite painful.
New systems are being developed and evaluated by the US Army's Picatinny Arseanal. The Aversive Audible Acoustic Device (A3D) is a highly directional device that can be hand-held or vehicle mounted. It directs an acoustic beam, which has tailorable intensity, and is used as a public address system, to focus on a specific individual or to deliver aversive sounds to alter a combatant's behavior.
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