There's a longer version of this AP report that includes the below quotes. The fact that there were cases of two people in a room where one heard the sound and the other did not was something I had not seen mentioned before. It was a question I had been curious about and would seem to indicate something different going on - rather than an acoustic weapon unexplained by current science that affects some people but not others.

> U.S. officials told the AP that investigators have now determined:

> — The most frequently reported sound patients heard was a high-pitched chirp or grating metal. Fewer recalled a low-pitched noise, like a hum.

> — Some were asleep and awakened by the sound, even as others sleeping in the same bed or room heard nothing.

> — Vibrations sometimes accompanied the sound. Victims told investigators these felt similar to the rapid flutter of air when windows of a car are partially rolled down.

> — Those worst off knew right away something was affecting their bodies. Some developed visual symptoms within 24 hours, including trouble focusing on a computer screen.