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After several months of investigation, the US has concluded that its diplomats in Cuba have been “exposed to an advanced device that operated outside the range of audible sound and had been deployed either inside or outside their residences,” reports the . The effect was so severe that some of the diplomats had to return home from Cuba early due to hearing loss.

Last week, Rex Tillerson, the US secretary of state, elaborated that the diplomats had been the victims of “health attacks”. At least one Canadian diplomat in Cuba has also been treated for hearing loss.

The Cuban government has said it wasn’t responsible for any possible attack, but that it has launched an “exhaustive, high-priority, urgent investigation”.

Part of the difficulty in unravelling what may have occurred is that the history of the development of acoustic weapons is shrouded in rumour and conspiracy, says Steve Goodman, the author of .

There is some research showing that frequencies just outside the human hearing range can have effects including hearing loss, concussion and nausea.

Beyond detection

“There are basically two ways that hearing loss could be effected without people knowing,” says at Manchester Metropolitan University in the UK.

The first is through sound waves at a frequency below the range of human hearing. But at these frequencies, generating sounds loud enough to do damage would require a large array of subwoofers, which wouldn’t be very covert.

The other option is to use ultrasound, creating sound waves at a frequency above the range of human hearing. For this application, there are already a number of speakers But while these can be directed into an ear quite precisely, “one needs a clear path to the target”, says Heys.

Exactly who might be behind the possible attack and whether it was directly aimed at the diplomats is far from clear.

Until more details are uncovered, it’s hard to shed much light on the events. “Overall, I would be pretty circumspect about the claims to be honest – it is all very Philip K. Dick territory,” says Heys. “That said, we are living in a fairly surreal world right now.”