US and Canadian diplomats in Cuba have mysterious ailments that were blamed on a high-frequency sonic weapon. However, ultrasonic frequencies—from 20 to 200 kilohertz—don’t propagate well in air and don’t cause the ear pain, headache, dizziness, and other symptoms reported in Cuba. IEEE Spectrum reported on work described on Youtube.

Kevin Fu, a computer scientist at the University of Michigan and his colleague Wenyuan Xu, a professor at Zhejiang University, in Hangzhou, China, and her Ph.D. student Chen Yan have determined that it was intermodulation distortion occurs when two signals having different frequencies combine to produce synthetic signals at the difference, sum, or multiples of the original frequencies.

Two ultrasonic signals (ultrasonic jammer and an ultrasonic transmitter) accidentally interfered with each other, creating an audible side effect.