Acoustic weapons, also known as sonic or ultrasonic weapons, are devices that distract, harass, injure, or even kill a target using sound. Obviously, sound is not just a medium for speech or music. It is energy in the air, and its energy density can be measured in watts. At enough decibels, it can rupture the eardrums and cause extreme pain. At lower — but still high — decibels, acoustic weapons can be extremely annoying and distracting. The human threshold of pain for sound is 120 – 140 dB.
Acoustic weapons have a history similar to particle or laser beam weapons. Tons of speculation and science fiction, but few working devices. Research into acoustic weapons has been ongoing for a number of decades but practical applications have only truly rolled out since 1995 or so. Leading the pack is the long range acoustic device (LRAD), developed by the American Technology Corporation. The LRAD is basically a small array of high-powered speakers which projects a tweeting noise in the direction of the offending individual or group. It was originally invented to be used by US Navy vessels to warn other ships encroaching without permission, but its use has been extended to the ground (in Iraq) and by police.
The LRAD is an acoustic weapon that only weighs 45 lb (20 kg) but projects a sound beam as intense as 146 dB (1000 W/m²) at 1 meter, 90 dB at 300 meters. At close range, this is enough to permanently damage hearing, an aspect of some concern, especially in a domestic use context. Its manufacturer states that the acoustic weapon is only meant to be used for short bursts of a few seconds. The LRAD is also used by cruise ships to deter pirates in hot spots like the Caribbean.
Other more experimental or speculative acoustic weapons have been proposed and some are in various stages of development. Some may have already been developed but are kept secret. These include phased arrays of infrasonic emitters which can penetrate armor and concrete walls, demonstrated by the US Department of Defense; a Vortex Ring Gun (weak versions demonstrated by hobbyists, and sonic bullets in development for anti-hijacking airplane kits. There has been unconfirmed speculation about the use of ultrasound beams used by Navy ships to communicate with submarines having a dual-use function as anti-frogman weapons.
Even more hypothetical or experimental acoustic weapons concepts include an infrasound beam that can cause a building to resonate harmonically and collapse, an ultrasound beam powerful enough to liquefy living tissue, and a beam of focused sound like focused light in lasers. Disintegration of solids in liquids using ultrasound has long been known in industry, but large-scale acoustic weapons applications have been elusive if not non-existent.