More than three dozen American diplomats and their families in Cuba and China have been affected by ailments that reportedly began after they heard mysterious sounds and experienced strange aural sensations. A group of Canadian diplomats has also been affected.

Most recently, nine Americans who worked with the US consulate in the southern city of Guangzhou were medically evacuated from China in May and June.

Their experience was consistent with that of the first in this string of incidents, at the American embassy in Havana, Cuba, in 2016, according to the US embassy and consulates in China.

An investigation into the cause of their ailments has yielded no results.

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Medical researchers and engineers have proposed various explanations, ranging from radio communication by-products to sonic weapons, but none have been confirmed, as detailed information on the matter remains confidential.

After comparing the reported experience of the affected diplomats with past studies on pulsed radio frequency and microwave radiation, Golomb is confident that all symptoms fit with her hypothesis.

“The sounds they reported hearing during the apparent inciting episodes, such as chirping, ringing and buzzing, are exactly what has been reported as part of the microwave auditory effect,” she said.

This aural effect, also known as “microwave hearing” or the “Frey effect”, refers to the sounds perceived by a small but significant proportion of human beings from microwave radiation, a form of electromagnetic radiation that falls within the frequency range of 300 to 300,000 megahertz.

Most people can only hear sound frequencies at a much lower range, from around 20 to 20,000 hertz.

Golomb is not the first to identify microwave radiation as the source of the reported high-pitched chirping. In January, James Lin, an electrical engineering expert at the University of Illinois at Chicago, published a study in IEEE Microwave Magazine that drew a similar conclusion, although he did not speculate on what devices may have emitted the microwave pulses.

Computer scientists Xu Wenyuan, from Zhejiang University in Hangzhou, China, and Kevin Fu, at the University of Michigan in Ann Arbor, believe the strange sensations might be an undesirable by-product of radio communication.

Their experiment showed how two signals of ultrasound – with frequencies higher than the human ear can detect – can occasionally produce a lower, audible tone.

“Ultrasonic emitters can sometimes produce audible by-products that could have unintentionally harmed the diplomats,” Xu and Fu said in their study. “That is, bad engineering may be a more likely culprit than a sonic weapon.”

These conclusions are in contrast to the view of Douglas Smith, director of the Centre for Brain Injury and Repair at the University of Pennsylvania. He recently told The Times in Britain that microwave weapons were now considered a main suspect and his team was increasingly sure the diplomats had suffered brain injury.

His research examined 21 of the affected diplomats in Cuba and was published in Journal of the American Medical Association in March, but did not mention microwave weapons as a potential cause.

“Everybody was relatively sceptical at first,” Smith was quoted as saying, “and everyone now agrees there’s something there.”

Microwave weapon suspected of mystery attacks on US diplomats in China and Cuba

Wong Hang, an electrical engineering professor and expert in microwave theory at City University of Hong Kong, said microwave radiation was a “more suitable” conclusion than the audible by-products of radio communication referred to in the Xu and Fu report. He was not involved in studies on the matter.

Microwaves are ubiquitous in modern life. They exist naturally as part of radiant energy given off by the sun, as well as machines such as microwave ovens.

However, about 1 per cent of the population can suffer side effects if exposed to a certain frequency range of microwave radiation, adding up to a considerably large group of people vulnerable to the illness.

Golomb compared the situation to people with peanut allergies. “Most people do not experience any adverse effects from peanut exposure,” she said. “But for a vulnerable sub-group, exposure can produce negative, even life-threatening, consequences.”

“It has to be pulsed radiation to create this effect, specifically within the range of 3,000 to 10,000 megahertz, as opposed to continuous radiation,” she said.

Golomb started looking into the health problems reported by the diplomats after meeting with patients who came to her laboratory with similar experiences.

She compared the diplomats’ symptoms with those reported from people affected by electromagnetic radiation in Japan, which were published in a study in 2012, and found strikingly similar patterns.

The health consequences of microwave radiation exposure have been a matter of debate since the short radio waves became a feature of modern life with the invention of cellphones, X-ray devices and microwave ovens.

More independent research should be done to address the issue, Golomb said.