A group of American diplomats in Havana, Cuba have suffered severe and unexplained hearing loss over the past year, which U.S. officials believe was caused by a covert and advanced sonic device.

The severity of some of the diplomats’ symptoms has forced them to cancel their Cuba tours early and return to the U.S. for treatment, the Associated Press reports.

But what exactly is a sonic device, how common are they, and how much damage can they actually cause? Here’s what to know.

What weapons were used to hurt diplomats in Cuba?

State Department spokeswoman Heather Nauert said the government does not “have any definitive answers about the source” of the attack. It is continuing to investigate what may have caused the diplomats’ injuries.

However, U.S. officials who spoke anonymously to the AP said that the hearing loss was caused by a device deployed either inside or near the diplomats’ residences. The devices emitted a sound that was not audible to human ears, they added. That would indicate it was most likely an infrasonic or ultrasonic weapon.

Infrasonic weapons can cause physical pain without detection, though they usually target the entire body rather than just the eardrums.

How much damage can acoustic weapons do?

The damage caused by acoustic weapons can vary from minor irritation to death, theoretically speaking. The effect can depend on the type of weapon and the range involved. Ryan Littlefield, who published a paper with the University of Portsmouth on the topic, says the fatality of such a device is unproven.

Police have used infrasound for riot control around the world, using it trigger vertigo, imbalance and other medical disruptions, according to Littlefield. In other cases, it’s been shown to cause vomiting and defecation. In still others, the technology has been used to disrupt targets’ hearing.

How long have we been using sound as a weapon?

For decades. It’s believed that, towards the end of the Second World War, Hitler’s chief architect Albert Speer was working on a device called an “acoustic cannon.” The weapon was designed to deafen and potentially kill targets by creating a series of more than 1,000 explosions per second.

According to Motherboard, the device was designed to ignite a mixture of methane and oxygen in a resonant chamber, which would send out a deafening and focused sound that could potentially kill someone within a 300 foot radius in around half a minute. The weapon was never used, but it paved the way for the use of sonic weapons in warfare.

Various types of sonic warfare has occurred worldwide over the past century, from “audio-harassment” campaigns carried out by the U.S. in Vietnam in the 1970s to the employment of a device called “The Scream” by the Israeli army during a demonstration in 2005. “Sound cannons,” meanwhile, have been deployed in as many as 70 countries around the world.

What’s a sound cannon?

The LRAD (Long Range Acoustic Device) sound cannon was developed by the U.S. military in collaboration with the LRAD corporation following the USS Cole suicide bombing. During the October 2000 incident, an explosion ripped a hole in the ship’s hull while in a Yemen port, killing 17 U.S. sailors.

Following the attack, the LRAD corporation developed the LRAD in the hope of keeping small boats away from warships. But sonic devices also have a history of being used to disrupt protests in the U.S. They are thought to have been used for the first time publicly in 2009, when Pittsburgh officials fired a sound cannon during a protest over a two-day G20 meeting. According to The New York Times, the cannon “emitted shrill beeps, causing demonstrators to cover their ears and back up.”

“Other law enforcement agencies will be watching to see how it was used,” said Nate Harper, the Pittsburgh police bureau chief at the time. “It served its purpose well.”

How do they work?

LRAD devices produce a deafening sound that can be heard up to 5.5 miles away. According to Gizmodo, anyone within 100 meters of the device’s sound path will experience extreme pain, and anyone within 15 meters can experience permanent hearing loss. One version of the device can cause severe headaches in anyone within a 300-meter range.

LRAD systems are now used in more than 70 countries around the world. They have various uses, including maritime security, wildlife control and preservation, and border and port security. LRAD Sound Cannons are also deployed at airports to deter birds from flying in the path of aircraft.

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Write to Kate Samuelson at kate.samuelson@time.com and Justin Worland at justin.worland@time.com.