Step aside, sticky foam. Move over, tear gas. This may be the first case of a technologically advanced non-lethal crowd control weapon in active use. No longer science fiction...
Sound weapons are a reality.
We're talking about a directional sound wave generator. The depelopers in the US Government won't comment, so details are sketchy, but the device most likely operates by generating low-frequency sound pulses and sends them toward a target.
Some earlier sound weapons simply caused pain from high decibel noises (like Mariah Carey), or generated severe nausea using subsonics (subsonic or infrasonic noise is defined as any sound whose frequency is less than 20 Hz, like Barry White's voice).
The human body does not react well to extremely low frequency noises: Certain low frequencies can cause sickness, balance problems and discomfort and pain to soft tissues and organs. At higher energy levels, a subsonic shock wave is powerful enough to do damage.
Previous limitations on this technology included massive amounts of power needed to run a weapon of this type, and the physical size of the device itself (subsonic sound weapons fire such low-frequency sound waves, the gun aperture itself has to be large in order to generate the "noise").
For more information, see this brief history of sound weapon technology.