Between December 2016 and August 2017, at least 24 employees of the U.S Embassy in Cuba heard high-pitched sounds and suffered injuries thought to be related to the noise. Many speculated that the high-pitched sounds were some high-frequency sonic weapon.

When Computing Community Consortium (CCC) Council member, Kevin Fu from the University of Michigan, looked at the spectral plot of the clip he saw some unusual ripples. Fu worked with his collaborator, Wenyuan Xu, a professor at Zhejiang University, in Hangzhou, China, and her Ph.D. student Chen Yan, and through a series of simulations, saw that an effect known as intermodulation distortion could have produced the sound. Intermodulation distortion occurs when two signals having different frequencies combine to produce synthetic signals at the difference, sum, or multiples of the original frequencies.

As Fu explained in the IEEE Spectrum article, “As acoustic waves containing multiple frequencies travel through a nonlinear system, you can get these bizarre ripples in the spectrum of the signal…At the same time, intermodulation distortion can produce lower-frequency signals than the original signals. In other words, inaudible ultrasonic waves going through air can produce audible by-⁠products. If ultrasound is to blame, then a likely cause was two ultrasonic signals that accidentally interfered with each other, creating an audible side effect.”

Fu and Xu can not be certain that this was the cause but from everything that they can tell, this just seems to be a case of bad engineering and not a sonic weapon. So far no evidence or direct observation has arisen to contradict their findings and the FBI has yet to announce the results of its investigation.

See a longer narrative by Fu, Xu, and Yan that just came out in IEEE Spectrum last week.