The mysterious “sonic attack” on US diplomats based in Cuba raises questions about what form an acoustic weapon might have taken and the prevalence of such devices in the military.
The US State Department claims that the “attacks” started in autumn 2016 and ended in April this year and had affected at least 16 individuals. Officials said that the symptoms, including hearing loss, headaches and loss of balance, appeared to be the result of sophisticated devices operating outside the range of audible sound. No device nor any perpetrator has been discovered, however.
A sonic weapon operating outside the human hearing range implies one emitting either very low (infrasound) or high (ultrasound) frequencies. In the second world war, the German military considered deploying an infrasound device, called the Wirbelwind Kanone (Whirlwind Cannon), aimed at knocking enemy bombers out of the sky using a vortex of sound. Targeted at people, infrasound can resonate with the stomach cavity, causing people to suddenly feel anxious or nauseated.
However, low frequencies are difficult to target and the symptoms in the Cuban case do not appear consistent with infrasound attacks.
Robin Cleveland, a professor of engineering science at the University of Oxford, said: “What’s probably happening in the Cuba situation, is ultrasonic – higher frequencies above above 20 kHz.”
Tim Leighton, professor of ultrasonics and underwater acoustics at University of Southampton, agreed: “If you want to produce a tight beam of energy that you can point at someone, ultrasound is the one to go for.”
There is good evidence that hearing loss can result from longterm exposure to ultrasound, based on studies of people working in factories where ultrasound is used to weld plastic parts.
Cleveland said that building an ultrasound emitter would not be hard. “You can buy transducers on the internet that emit these frequencies,” he said. “Anybody with a bit of engineering background could put one together.”
A device the size of a kitchen matchbox could emit high enough amplitudes at close range to induce feelings of anxiety or difficulty concentrating.
However, putting together something powerful enough to affect hearing would be more challenging as it would require a large amplifier, may require a focused beam, and would need to be placed in the close vicinity of the target. High frequency sound does not travel well through barriers such as walls, curtains, or even human skin.
“If you want to put a lot of power into it so you could produce a beam that could go through windows, it starts to look more like a suitcase,” said Leighton. “In order to generate hearing loss at 50 metres away, you’d be looking at a car-sized device.”
Another ethical issue, specific to ultrasound weapons, is that they are difficult to target and tend to affect women and to a far greater degree children, more severely than middle-aged men.
Source: The Guardian