Strange 'sonic attacks' on U.S. Embassy personnel that took place between late 2016 and August of 2017 could have been an unintentional side effect of ultrasound listening devices, according to a recent study from the University of Michigan. The attacks affected dozens of embassy workers, leaving some with lasting injuries and brain abnormalities.

The study, titled On Cuba, Diplomats, Ultrasound, and Intermodulation Distortion, concluded that "if ultrasound played a role in harming diplomats in Cuba, then a plausible cause is intermodulation distortion between ultrasonic signals that unintentionally synthesize audible tones. In other words, acoustic interference without malicious intent to cause harm could have led to the audible sensations in Cuba."

The University of Michigan's Kevin Fu, an associate professor of computer science and engineering, performed the study in partnership with visiting professor Wenyuan Xu and doctoral student Chen Yan of China's Zhejiang University Department of Systems Science and Engineering.

"We've demonstrated a scenario in which the harm might have been unintentional, a byproduct of a poorly engineered ultrasonic transmitter that was meant to be covert," Fu said. "A malfunctioning device that was supposed to inaudibly steal information or eavesdrop on conversation with ultrasonic transmission seems more plausible than a sonic weapon. That said, our results do not rule out other potential causes."

Ultrasound itself isn't normally harmful, but it 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," Fu said.

In the case of the 'sonic attacks' in Cuba, this likely would have been produced when two sources of ultrasound, such as listening devices, were placed too close together--this could have generated the necessary interference to create the intense noise reported by victims.

James Cason, a former top U.S. diplomat in Havana, believes this could explain the phenomenon.

“This is a variation of what I have always thought,” Cason said. “It explains the sonic part, that no one was spotted planting new devices inside the homes and doing it from the outside would require something huge.”

According to Cason, U.S. diplomats are aware that the houses provided to them by the Cuban government contain listening devices, and have lived in them for years without incident. A malfunction in the devices could explain why they occurred only in specific homes and at two Havana hotels, but not at the embassy.

“That cannot happen at the embassy in Havana because Cuban personnel are forbidden to enter higher floors, where many diplomats have their offices," Cason said.

It is still unclear if damage caused by ultrasound could result in the same symptoms from which the victims suffer.

Doctors from the University of Pennsylvania were able to rule out certain causes--such as poisoning, viruses, or mass hysteria--but were unable to definitively state the origin of the victims' concussion-like symptoms.

In addition, an article published in the Journal of the American Medical Association stated that there is no evidence that the symptoms were caused by audible sounds.

“This data is currently under peer review by a high-impact journal,” Lisa Worley, a spokesperson at UM’s Miami Miller School of Medicine, said in a statement.. “As the primary acute care providers in this case, we believe our work represents a high level of comprehensive detail that has not yet been reported. We look forward in the very near future to sharing our findings.”

The study out of the University of Michigan acknowledged that there is yet no consensus among researchers regarding damage caused by ultrasound, but argued that enough is known about the harmful effects of audible sound to posit a connection between ultrasound and the symptoms.

"We find little consensus on the risks of human exposure to air-borne ultrasound. Airborne ultrasonic waves on their own are not necessarily harmful, but it may become harmful at large intensity or when in direct contact exposure to a vibrating source," the study stated.

"Although little is known about how audible sound waves can cause neurological damage rather than merely be correlated with neurological damage, the safety community has studied how certain audible sounds can cause pain and hearing damage," it continued. "Thus, ultrasonic intermodulation distortion could produce harmful, audible byproducts."