The device was so sophisticated, it was outside the range of audible sound, the official said. And it was so damaging, the source said, that one US diplomat now needs to use a hearing aid.
What we don't know is what kind of device may have been used, where exactly it was placed, and who put it there.
But for years, sound has been used as a nonlethal, yet potentially harmful, weapon. Around the world, different types of sonic devices are used for crowd control, to protect areas of attack and to incapacitate soldiers or workers.
Long-range acoustic devices
Since the early 1990s, long-range acoustic devices -- also known as LRADs -- have helped authorities control crowds of people, especially protesters. They emit a loud, painful sound over a long distance and make people run away.
In the US, police used such devices during demonstrations at the 2009 G20 Summit meeting in Pittsburgh
. They were also used more recently, during the 2014 protests in Ferguson, Missouri
, following the fatal shooting of Michael Brown.
The popularity of the devices among law enforcement has risen now that the LRAD Corporation, the company that originally created the device, now sells them to more than 70 countries, the company said in a recent financial report.
The so-called "mosquito" produces a very high-pitched sound that can be perceived by teenagers, but not to adults. It's used in several countries to prevent people from loitering.
"It's not audible to adults because your hearing fades as you get older," said James Parker, an expert in sound and law at the University of Melbourne in Australia.
Those who can hear it usually feel discomfort and nausea.
Infrasound doesn't need to be heard for it to work.
Since the 1990s, the US military and private companies researched infrasonic devices that could cause behavior changes at frequencies too low to be audible.
Parker says infrasound is probably the hardest to weaponize.
"There are (sonic) devices that are used as a weapons, but none that I know of use infrasound," Parker said.
Like infrasound, ultrasound is not audible. With ultrasound, the frequency is higher than what humans can hear.
Jürgen Altmann, a physics professor at Technischen Universität Dortmund in Berlin, said it's not clear what type of device -- if any -- may have been used in Cuba.
"The story is mysterious indeed," he said.
But he said ultrasound devices can be challenging to use, especially if they're placed at a distance.
"One may speculate whether ultrasound (that is inaudible) was used, but projecting it over a considerable distance and/or through walls or closed windows is difficult," Altmann said.
Sound that hurts
There are many myths about the effect that sound has in the human body. One of them is the "brown note," which is the idea that a sound frequency can make people lose control of their bowels.
While the harm caused by most acoustic weapons is minimal, some experts say it's unclear how much of the harm is temporary pain, and how much can contribute to potential long-term hearing loss.
The US employees affected in Cuba were not at the same place at the same time, but they suffered a variety of physical symptoms since late 2016 which resembled concussions, an official said.
But Altmann said he's not aware of what kind of sound may have caused those symptoms.
"I know of no acoustic effect that would produce concussion-like symptoms; according to my research strong effects on humans require loudness levels that would be perceived as very loud noise while exposed," he said.
In a 2016 report
, the International Network of Civil Liberties Organization and the nonprofit Physicians for Human Rights said there's still "little medical literature on the effects of these weapons."
But not all sound is equal.
"It (sound) doesn't really affect the body unless it's either very loud or you're exposed to it for a long time," Parker said, "though how loud and how long exactly depends on the exact frequency."