Kevin Fu and Wenyuan Xu, researchers with labs at the University of Michigan and Zhejiang University in China have done a fair amount of research into the use of ultrasonic waves that may have been the possible cause of recent ailments of diplomats living in Cuba.
You may recall the same researchers reported in 2017 of being able to cause cell phones with voice recognition to react and trigger specific actions through the use of inaudible ultrasonic sounds- voice commands that were converted to ultrasonic frequencies. The voices could still be picked up by the cell phone microphone even though human hearing could not detect it.
Causes of the symptoms of the diplomats and their families have not yet been confirmed, but one of the possible causes offered was the use of some sort of sonic “weapon”. Hearing about this caused Fu and Xu to wonder if ultrasound could be the culprit. Their research does not definitively explain the ailments, but tries to explain possible ways the effects that were described could have been created.
In the process they also discuss possible ultrasonic eavesdropping and ultrasonic microphone jamming.
In this article in the IEEE Spectrum magazine, “How We Reverse Engineered the Cuban ‘Sonic Weapon’ Attack” they report on the research they performed.
The Cuban ultrasonic mystery was too close to our research to ignore.
One thing we knew going into this investigation is that acoustic interference can occur where you least expect it. Several years ago, Fu became annoyed by an ear-piercing sound coming from a lightbulb in his apartment. He took spectral measurements and noticed that the lightbulb tended to shriek when the air conditioner turned on. He eventually concluded that the compressor was pumping coolant through its pipes at the same resonant frequency of the filament in the bulb. Normally, this wouldn’t be a problem. But in this case, the coolant pipes ran through the ceiling and mechanically coupled to the ceiling joist supporting the lightbulb. The superintendent opened up the ceiling and separated the joist from the pipe with a piece of duct tape, to dampen the unwanted coupling. The sound stopped.
In their experiments they also were able to create an ultrasonic eavesdropping microphone that was able to transmit audio through the air while being completely inaudible to the human ear.
Using this method of eavesdropping makes more sense when considered in terms of exfiltrating information or data that may be air-gapped, where no electronic connection was possible.
While the equipment we used in our Cuban re-creation is relatively bulky, ultrasonic emitters can be quite tiny, no larger than a piece of Rolo candy. 
Online, we found a manufacturer in Russia that sells a fashionable leather clutch that conceals an ultrasonic emitter, presumably to jam recording devices at cocktail parties. We also found electronics stores that carry high-power ultrasonic jammers that cause microphones to malfunction.
One advertised jammer emits 120-dB ultrasonic interference at a distance of 1 meter. That’s like standing next to a chainsaw. If a signal from that caliber jammer were to combine with a second ultrasonic source, audible by-products could result.
Sounds heard by the ailing diplomats indicate that the source was not just ultrasonic (which would be inaudible), but possibly a result of a combination of both ultrasonic and other waveforms.
While the math leads us to believe that intermodulation distortion is a likely culprit in the Cuban case, we haven’t ruled out other null hypotheses that may account for the discomfort that diplomats felt. For example, maybe the tones people heard didn’t cause their symptoms but were just another symptom, a clue to the real cause. Or maybe the sounds had some sort of nonauditory effect on people’s hearing and physiology, through bone conduction or some other known phenomenon. Microwave radiation is another theory. One positive outcome from all this would be if more computer scientists were to master embedded security, signal processing, and systems engineering.
Read more in Fu and Xu’s article: The IEEE Spectrum
Their paper with full technical analysis: “On Cuba, Diplomats, Ultrasound, and Intermodulation Distortion”
Other articles:
NY Times: Diplomats in Cuba Suffered Brain Injuries. Experts Still Don’t Know Why.
Engadget: Cuba’s ‘sonic attacks’ may have been a side-effect of spying
American Teens Hear Strange High-Pitched Sound at Havana Airport