One of an earlier usage of sound as a weapon was the HPS-1 sound system.
During the Vietnam War (1955 – 1975), the US army wanted to be able to communicate and influence enemy combatants, without risking any enemy fire, so they needed a sound system that can send audio and intelligible speech, at long distances, hence the HPS-1 sound system was born.
The HPS-1 sound system was used by the British army in northern island, and later on, it found its way to other usages, for example it was used by the Coast guard and the police.
The HPS-1 sound system was created by Applied Electro Mechanics, which after its dissolution, a group of its employees, formed in 1997, a company called Power Sonix, that specializes, in speech projection, for air, land, and sea.
HPS-1 is a 350 watt, battery powered system, which output can be easily switched from the front panel, between a siren, a voice projection, and a Curdler.
It can be mounted on a vehicle or a helicopter, and its optional accessories include, a playback tape recorder, and a remote-control unit.
Voice projection is directional, and intelligible sound can be heard over a distance of 4 km.
The Curdler unit, can emit a piercing, penetrating, shill shrieking, blatting sound, equal to 120DBM at 9m, comparable to standing behind a jet fighter during takeoff. As such, it is capable of breaking collective activities, such as riots, or slogan shouting and clapping, by forcing protestors to cover their ears.
HPS-1 has multiple usages, so it can be used as part of a PSYOP, as to harass the enemy with annoying sounds, place pressure on a fugitive, propaganda. It can also be used by the police for crowd control. It can be used for emergency response, such as in the case of a disaster, where communication is lost
One of an earlier usage of HPS-1, was chronicled in the New Scientist magazine, issued the 27 of September 1973.
Robert Rodwell, the article author, described how the HPS-1 audio system was demonstrated to some journalists, who deemed it to be safe for usage, and their conclusion was: "The not so secret weapon", which can be bought from over the counter, for £2000.