Early reports blamed a sound weapon for the attacks — but based on what we know, it seems unlikely that a sound weapon was used. Nauert told reporters that people including US government employees began experiencing "some kind of symptoms" starting in December 2016. The most recent reported incident, she said, was in late August. Nauert has not confirmed what the specific symptoms are, or said more about the "incidents" that caused them, beyond calling the situation "dangerous."
News reports are a little more detailed: some of these incidents might have been attacks with a covert sound weapon that "operated outside the range of audible sound and had been deployed either inside or outside their residences," the Associated Press claimed, based on the statements of unnamed US officials. Other attacks may have produced a loud buzzing or scraping noise, anonymous government sources told CNN. On Thursday, the AP added that these incidents may have happened at night, when victims felt vibrations or heard ringing noises near their beds at home, and, reportedly, at a hotel. However, others who experienced symptoms don't remember hearing or feeling anything out of the ordinary, the AP says.
Silence would be unusual for a weapon that uses sound energy to disorient, incapacitate, or deter people. In theory, a sonic weapon could do this by causing ear pain, by making a person dizzy, or by vibrating a person's insides at a frequency that could "stun them, nauseate them, 'or even liquefy their bowels and reduce them to quivering diarrheic messes,'" as one journalist wrote in the 1990s.
In reality, sonic weapons are much less sophisticated than the hype, and they involve significantly less diarrhea. They work by being noisy and obnoxious — which means that the crudest sonic weapon is loud music. In 1989, for instance, US forces unleashed a barrage of Black Sabbath and Guns N' Roses to drive Panamanian dictator Manuel Noriega out of hiding. In 1993, the FBI tried the same trick — blasting recordings of dying rabbits and Tibetan prayer chants in an effort to end a standoff with the Texas-based religious group, the Branch Davidians. (The attempt didn't work.) Loud music has also been used to torture detainees at Guantanamo.
The Department of Defense's list of non-lethal weapons includes ones specifically designed to use sound — but these work by loudly startling their targets, not by subtly causing major health complications. The flash bang grenade, for example, uses a bright flash of light and a loud bang to stun its victims.
The list also includes so-called acoustic hailing devices that can project both verbal commands and earsplitting beams of noise through the air (or underwater). One such device is called the Long Range Acoustic Device or LRAD, a sound cannon that can "send messages, warnings, and harmful, pain-inducing tones over longer distances than normal loudspeakers." If fired too close to its targets, the LRAD can cause ear pain and long term hearing loss, according to the 2009 book 'Non-Lethal' Weapons. The New York Police Department faced a lawsuit for unleashing the LRAD on protesters and journalists in 2014.
Of course, the potential to cause deafness is a major design flaw for a sound weapon. Why? "[I]t can be expected that the weapon will cease to work if the victim quickly becomes permanently deaf from exposure to high intensity sound," according to a report prepared for the Space and Naval Warfare Systems Center.