You might have read about the mysterious injuries that American and Canadian diplomats stationed in Havana, Cuba were suffering a while back. The symptoms were consistent with a concussion (memory problems, an inability to concentrate, mood problems, headaches and fatigue) and although the results of several studies by the FBI and independent engineers have been inconclusive, that still led many to believe that the diplomates where being bombarded with some sort of high frequency sonic weapon.
The sound was finally recorded and had a center frequency of 7kHz with harmonics evenly spaced every 180 Hz (check out the video below to hear for yourself). Of course it’s audible, but also directional, and many diplomats reported that they wouldn’t hear it if they just moved a few feet in any direction. This is totally odd for a weapon, which isn’t all that successful if you can just move out of the way to render it ineffective.
Still the sound continued at various diplomatic residences and hotels in Havana and the U.S. government pulled many of its diplomats from the country in retaliation, even though the Cuban government was very cooperative with an FBI investigation. They claimed they had no idea where the sound was coming from either.
Now two researchers from the University of Michigan–Ann Arbor, and at Zhejiang University in China who were working on research pertaining to ultrasonic cybersecurity claim to have reverse-engineered the sound to come up with the likely culprit.
In a fascinating in-depth article by Kevin Fu, Wenyuan Xu and Chen Yan, the scientists determined that the sound was actually intermodulation distortion coming from mixing two high frequencies of 25kHz and 32kHz. This gives you the difference signal of 7kHz, and another one summing both at 57kHz, which wasn’t a problem. These sounds can come from a variety of sources but one is the ultrasonic sensors commonly used to control lighting, which emit 25kHz sound waves inaudible to humans. A change in the reflected sound indicates someone is in the room and as a result, the lights stay on. Other sources include ultrasonic pest controllers and even modern automobiles.
So the conclusion of the researchers? Bad engineering design and the bad luck of the diplomats to be near a confluence of ultrasonic frequencies, rather than a secret Cuban sonic weapon.