A series of "sonic attacks" that sickened U.S. and Canadian government workers in Cuba last year could have been the side-effect of attempts to "eavesdrop" using high frequencies, according to University of Michigan and Chinese engineering researchers who reverse-engineered the attacks in a lab. Beginning in December 2016, at least two dozen U.S. and Canadian personnel in the nations' Havana embassies suffered nausea, ear pain, hearing loss, nosebleeds, vertigo and even trouble walking, according to news reports. Doctors described the injuries as "mild traumatic brain injury" and "a concussion without a concussion."
At the onset of their illnesses, the American and Canadian workers reported hearing concentrated, high-pitched chirping noises. While some experts hypothesized sonic attacks, others pointed to poisoning or a virus.
A pattern the researchers noticed in the acoustic spectral plots in the news video suggested that intermodulation distortion might be at work. They set out to simulate the phenomenon between multiple ultrasonic signals. Using ultrasound, they were able to generate similar "metallic chirping sounds" at 7 kHz with ripples spaced evenly at 180 Hz, mimicking the arrangement in the AP video. Ultrasound emitters are all around us. They can take the form of room occupancy sensors in energy-efficient buildings, rodent repelling devices, burglar alarms, security cameras, HVAC system vibrations. Ultrasound itself hasn't been known to harm humans except with exceptionally extreme intensity, but ultrasound can produce audible byproducts capable of harm. When ultrasonic signals containing multiple tones interfere with each other through a phenomenon called intermodulation distortion, audible sound can result. Intermodulation distortion can down-convert the frequency of ultrasound into the audible range—resulting in high-pitched noises.
The researchers generated ultrasound using two ultrasonic emitters—a 25 kHz tone combined with a 180 Hz tone modulated on a 32 kHz carrier. The method is similar to the concept AM radio, but the researchers used ultrasound instead of radio waves. The team showed how ultrasonic signals—outside the range of human hearing—can combine to produce audible and potentially dangerous tones similar to the undulating, high-pitched chirping that the diplomats described. The experiments were spurred by news video that included sounds and images of acoustic spectral plots.