"I'd say it's fairly implausible," said Jurgen Altmann, a physicist at the Technische Universitat Dortmund in Germany and an expert on acoustics.

For decades, military researchers tried to transform sound into a non-lethal weapon that could stop enemy soldiers in their tracks.

The Pentagon funded development of loudspeakers to deliver long-range blasts of sound. The Navy uses them to ward off pirates, while the Army deploys them at checkpoints. In recent years, police have used so-called long range acoustic devices to break up crowds like those at the protests in Ferguson, Missouri.

But these weapons work because they are insufferably loud, and if one were used against diplomats in Cuba, there would be no mystery about it.

One possibility is infrasound low-frequency sound that cannot be heard by humans. A report by the National Institute of Environmental Health Sciences in 2002 noted that the military had tried to weaponise infrasound but had not succeeded because it was hard to focus the wavelengths.

The primary effect of infrasound on humans "appears to be annoyance," the report concluded.

Ultrasound is the more likely possibility. At frequencies higher than 20,000 hertz, beyond human hearing, ultrasound can damage tissue if produced with enough power.

Doctors use focused blasts of ultrasound to smash kidney stones. Decades ago, researchers created intensely powerful ultrasound beams in laboratories that can kill a mouse at close range.


Less powerful ultrasound beams do not cause injuries and have a variety of medical uses, including commonplace medical scans. But there is anecdotal evidence at certain intensities, they can make people very uncomfortable.

Researchers said, the laws of physics make it unlikely that the device could harm diplomats from afar.

"Ultrasound cannot travel a long distance," said Jun Qin, an acoustic engineer at Southern Illinois University. The further the sound goes, the weaker it gets. And, noted Garrett, humidity in a place like Havana would weaken it still more.

Moreover, a beam of ultrasound will mostly bounce off the exterior of a building. What little sound got through would be of a lower, less harmful frequency.

One way to overcome these hurdles would be to use a bigger weapon. But a massive vehicle topped with a giant sound cannon in front of diplomatic houses would probably not go unnoticed.

"If you're talking about a ray-gun rifle knocking out someone with ultrasound they can't hear at a hundred metres? That's not going to happen," Leighton said.

An ultrasound-emitting device planted inside a building, on the other hand, might be close and powerful enough to cause harm to occupants. But even an interior wall would block its waves.

A smaller emitter placed even more closely, perhaps in someone's pillow, might do the trick, Qin said. But it's hard to believe such a device could escape attention. In theory, a building could be packed with small emitters; however, experts called it unlikely.


And while ultrasound can cause many of the symptoms reported by the diplomats, there is no evidence that it can cause mild brain injury.

"I know of no acoustic effect that can cause concussion symptoms," Altmann said. "Sound going through the air cannot shake your head."

Leighton said that contagious anxiety or another psychogenic contributor couldn't be ruled out. "If you make people anxious that they're under attack from an ultrasonic weapon, those are the symptoms you'll get," he said.

The New York Times