" " Medical and scientific researchers have found increasing evidence that sound — including frequencies that cannot be heard by humans — is linked to a variety of health problems, including hypertension, heart disease and strokes. Keith Hamshere/Getty Images

On an evening in November 2016, a U.S. diplomat stationed in Havana, Cuba, opened the doors from his living room into his garden, and noticed a loud, high-pitched sound. As he later explained in an interview with journalism organization ProPublica, the noise was so intense that the only way to block it out was to close all the doors and windows and to turn on a television. Other diplomatic personnel began to notice the sound as well. One thought it was made by cicadas, but another described it as sounding too mechanical to be an insect.

But suspicions soon arose that the sound was more than just a peculiar annoyance. In the months that followed, dozens of U.S. diplomatic staffers — as well as some Canadian personnel — began to suffer from a series of health problems, ranging from headaches, sleep disturbances and memory problems, to hearing loss and difficulties with balance. Eighty staffers and family members returned to the U.S. for medical examinations, and 21 of them were evaluated further by the University of Pennsylvania's Center for Brain Injury and Repair.

As State Department official Todd J. Brown testified in a Jan. 9, 2018, Senate hearing, U.S. officials in Havana and Washington suspected that the diplomatic staffers' injuries had been caused by a deliberate attack, intended to harass them, and the FBI eventually was called in to investigate. Accusations arose in the news media that some sort of mysterious sonic weapon had been used, and in October 2017, the U.S. ordered 15 Cuban diplomats to leave the country for what it said was Cuba's failure to protect the Americans from whatever had caused their injuries. The Cuban government denied responsibility, and the country's own experts hypothesized that stress over U.S.-Cuba relations might have triggered the diplomats' health problems.

This recording of the noise later was obtained by the Associated Press:

What happened in Havana, unfortunately, remains a mystery. In an article published Feb. 15, 2018, in the Journal of the American Medical Association (JAMA), the Penn doctors concluded that the 21 patients they sampled "appeared to have sustained injury to widespread brain networks, without an associated history of head trauma." Doctors considered explanations such as a virus or exposure to toxic chemicals but couldn't find any evidence of either one. The only apparent thing the victims had in common was that nearly all of them had heard the strange sound. Nevertheless, the Penn doctors noted that "it is currently unclear if or how the noise is related to the reported symptoms."



Sonic Damage

But whether or not the diplomatic workers were subjected to a sonic assault, medical and scientific researchers have found increasing evidence that sound — including frequencies that cannot be heard by humans — is linked to a variety of health problems. Exposure to loud sounds can damage cells, membranes and nerves in the inner ear, causing hearing loss. But as this article in the British medical journal Lancet details, numerous studies indicate that noise exposure also is a factor in hypertension, heart disease and strokes.

"For cardiovascular disease, the proposed mechanism is based on the general stress model, i.e., noise causes secretion of stress hormones (adrenaline, cortisol) that shift other blood parameters in a direction that are risk factors for cardiovascular disease (e.g., coagulation, cholesterol)," the Lancet article's lead author, Dr. Mathias Basner, explains in an email.

According to the Lancet article, noise also has been shown to cause sleep disturbances, and studies of children chronically exposed to traffic, train or aircraft noise have poorer scores on tests of reading ability, memory and other cognitive measures. Those deficits are "likely related to communication pauses, increased effort during communication, and miscommunication," Basner explains.

But there's also evidence that even sounds that humans can't hear can cause ill effects. A study by University of Southhampton researchers, published in Proceedings of the Royal Society A, indicates that people in railway stations, sports stadiums, schools and other public places are being increasingly exposed to airborne ultrasound — high frequency sound — being generated from sources such as loudspeakers, door sensors and public address systems. Other research links ultrasound, which can penetrate and heat living tissue, with ailments such as dizziness, balance disturbances, tinnitus — a persistent ringing in the ears — and fatigue.



Infrasound

Infrasound — inaudible low frequency sound — also can affect the human body. Jerry Punch, an audiologist and professor emeritus at Michigan State University, says in an email that infrasound "is typically created by large industrial equipment that causes restrictions in the flow of air, leading to modulation or pulsation, such as happens when industrial wind turbine blades pass the tower at infrasonic rates."

In his 2012 book "The Universal Sense, How Hearing Shapes the Mind," neuroscientist Seth S. Horowitz explains that infrasound's long wavelength makes it capable of bending around or penetrating the body and creating oscillating pressure. Horowitz described how a damaged elevator fan in a college building transmitted infrasound, so that even a two-floor ride would leave a person feeling sick to his or her stomach. If infrasound is the right frequency and sufficiently intense, according to Horowitz, it could cause a person to see colored flashes or even experience breathing difficulties.

But despite the growing body of knowledge about sound's effects on the human body, nothing seems to explain what happened to the diplomatic staffers in Havana, some of whom required such extensive rehabilitation therapy that they were unable to return to work, according to the JAMA article.