The Associated Press has released the mysterious sound that some American diplomats heard while in Cuba before coming down with horrible health problems.

The sound is high pitched, almost like nails on a chalkboard. Experts told the AP that the noise cannot harm you if you listen to it on your computer “for short durations at normal levels.” Those affected were hit by the sound “at extreme volumes.”

Experts still do not know what kind of mechanism produced this sound or who developed it. The Navy has received the sound for analysis, but did not respond for a comment.

Not everyone harmed in Cuba heard a sound, but those who did all heard the same thing. The AP continued:

Yet the AP has reviewed several recordings from Havana taken under different circumstances, and all have variations of the same high-pitched sound. Individuals who have heard the noise in Havana confirm the recordings are generally consistent with what they heard.

“That’s the sound,” one of them said.

The recording being released by the AP has been digitally enhanced to increase volume and reduce background noise, but has not been otherwise altered.

The sound seemed to manifest in pulses of varying lengths — seven seconds, 12 seconds, two seconds — with some sustained periods of several minutes or more. Then there would be silence for a second, or 13 seconds, or four seconds, before the sound abruptly started again.

The AP used a spectrum analyzer, which showed reporters that there are “20 or more different frequencies, or pitches” within that one sound:

To the ear, the multiple frequencies can sound a bit like dissonant keys on a piano being struck all at once. Plotted on a graph, the Havana sound forms a series of “peaks” that jump up from a baseline, like spikes or fingers on a hand.

“There are about 20 peaks, and they seem to be equally spaced. All these peaks correspond to a different frequency,” said Kausik Sarkar, an acoustics expert and engineering professor at The George Washington University who reviewed the recording with the AP.

Those frequencies might be only part of the picture. Conventional recording devices and tools to measure sound may not pick up very high or low frequencies, such as those above or below what the human ear can hear. Investigators have explored whether infrasound or ultrasound might be at play in the Havana attacks.

Something has affected 22 people in Cuba since late 2016. From The New York Times:

Of the people who have become ill, 17 were government employees and four were spouses. Three of the spouses worked at the embassy. For some, the injuries appear permanent, with symptoms including hearing loss, dizziness, tinnitus, balance and visual problems, headache, fatigue, cognitive issues and difficulty sleeping.

Some of those affected reported hearing odd sounds in particular rooms of their homes, leading some experts to speculate that some kind of sonic weapon or faulty surveillance device may have been at fault.

The AP has spoken to those affected and they provided details about the attacks:

The blaring, grinding noise jolted the American diplomat from his bed in a Havana hotel. He moved just a few feet, and there was silence. He climbed back into bed. Inexplicably, the agonizing sound hit him again. It was as if he’d walked through some invisible wall cutting straight through his room.

Soon came the hearing loss, and the speech problems, symptoms both similar and altogether different from others among at least 21 US victims in an astonishing international mystery still unfolding in Cuba. The top US diplomat has called them “health attacks”.

The AP said other symptoms have been “brain swelling, dizziness, nausea, severe headaches, balance problems and tinnitus, or prolonged ringing in the ears.” Almost all diplomats have felt better once they returned to the states.

These attacks have caused more strains between Cuba and the U.S., who have tried to normalize relations since 2015. Cuban officials insist the government has not attacked the diplomats and are trying to help out. They have allowed in our FBI and other intelligence services to investigate.

As the number of affected have risen, the State Department has no choice but to take action to protect our people. Secretary of State Rex Tillerson decided in late September to bring home all non-essential personnel from the U.S. Embassy in Havana, thus cutting the staff by 60% and halt citizen travel to the island.

A few days later, Tillerson decided to expel 15 diplomats from the Cuban Embassy in Washington, D.C. He stated:

On October 3, the Department of State informed the Government of Cuba that it was ordering the departure of 15 of its officials from its embassy in Washington, D.C. The decision was made due to Cuba’s failure to take appropriate steps to protect our diplomats in accordance with its obligations under the Vienna Convention. This order will ensure equity in our respective diplomatic operations.