If you’ve been paying attention to the news, you may have seen a series of articles coming out about US staffers in Cuba. It seems that 21 staffers have suffered a bizarre array of injuries ranging from hearing loss to dizziness to concussion-like traumatic brain injuries. Some staffers have reported hearing incapacitating sounds in the embassy and in their hotel rooms. The reports range from clicking to grinding, humming, or even blaring sounds. One staffer described being awoken to a horrifically loud sound, only to have it disappear as soon as he moved away from his bed. When he got back into bed, the mysterious sound came back.

Cuba has denied any wrongdoing. However, the US has already started to take action – expelling two Cuban diplomats from the US in May. The question though is what exactly could have caused these injuries. The press has gone wild with theories of sonic weaponry, hidden bugs, and electronic devices, poisons, you name it. Even Julian Assange has weighed in, stating “The diversity of symptoms suggests that this is a pathogen combined with paranoia in an isolated diplomatic corps.”

So what’s going on? Bizarre accidents? Cloak and dagger gone awry? Mass hysteria among the US state department, or something else entirely?

The most common theory passed around is some sort of auditory or sonic weapon. Acoustic (ultrasonic) non-lethal weapons like the Long Range Acoustic Device (LRAD) are well known due to their use by law enforcement to disperse protests, or on oceangoing ships to deter pirates and environmentalists. LRAD devices emit an extremely loud focused beam of sound. Usually, the sound is a siren, though the system can be used as a giant megaphone as well. Anyone in the beam is motivated to get out of it.

The thing about LRAD devices is they are not small or light. Even with ultrasonics, you can’t beat physics. Making a lot of noise means vibrating a lot of air. That takes a relatively big loudspeaker. The smallest portable device is roughly fifteen pounds. Since LRAD is still vibrating the air, it wouldn’t work very well through walls. LRAD style devices are also not very clandestine. They emit a beam 30 to 60 degrees wide, so definitely not a sound laser. They also have plenty of spill — operators standing behind the device always need to wear hearing protection.

Unwrap Your Tinfoil Hat

One theory I haven’t seen passed around much is the microwave auditory effect. This is a phenomenon where RF energy directed at a human head is converted to sound perceivable by the target. The first paper published about the effect was by Allan H. Frey in 1961. Frey worked at the General Electric advanced electronics center at Cornell University in NY.

I should note that microwave here refers to the wavelength of the RF signal being transmitted. Microwaves include any signal from 1-meter wavelength (300 MHz) to 3mm wavelength (100 GHz)

Frey’s article describes how test subjects were able to hear buzzing, clicking, hisses and even knocking when transmitters were pointed at their skulls. Strangely, some of the test subjects were partially deaf, and still were able to hear the microwave sounds. What’s more, subjects could feel the effects from the microwave beam. Depending on the transmitter settings, subjects felt “severe buffeting of the head”. Further transmitter changes resulted in subjects reporting “pins and needles” sensations.

The purpose of the paper was to call attention to the phenomenon. Frey didn’t have the resources to completely explore the microwave auditory effect, so he wanted others to start working on it. It’s the scientific equivalent of saying “Hey, this is neat, you should check it out!”

If you haven’t guessed yet, the power levels required to hear microwave sounds were rather high. Frey used several transmitters at different power levels. The transmitters were pulsed, like magnetrons, so while average power was low, peak power was high.

As an example – the weakest transmitter Frey used was able to output a power density of 4 w/m² at 1310 Mhz. The peak power was 2670 w/m². The US guideline for human exposure at that frequency is 6.55 w/m². A different transmitter Frey used measured 71 w/m² at 425 MHz, with peaks at 2540 w/m². Compare this to the FCC guideline of 2 w/m² at that frequency.

What exactly causes the RF energy to be converted to sound? The mechanism behind the microwave auditory effect has not been scientifically proven. The leading theory is pulsed RF energy heats the tissues of the inner ear, causing them to expand quickly. These expansions cause tiny shockwaves which are then interpreted as sounds by the brain.

Frey noted that “one can shield, with 2-inch square piece of fly screen, a portion of the [temple] and completely cut off the RF sound.” Fly screen would be the fine metal grid used in screen doors. Frey may not have known it, but he was providing all the proof the tin-foil hat crowd needed.

Of course, a technology like this can’t exist without someone trying to build a weapon out of it. In the early 2000’s, the US Navy funded research on Mob Excess Deterrent Using Silent Audio (MEDUSA). This was a “less lethal weapon” which would use the microwave auditory effect for crowd control. It utilized an electronically steered antenna which allowed it to transmit a wide or narrow RF beam. MEDUSA could even “spotlight” multiple targets simultaneously.

MEDUSA never became a fieldable weapon. The initial results of the project were promising, but there were questions about its safety. At the high power levels used, could the micro shockwaves actually damage sensitive brain tissue? What about the RF exposure to sensitive neurons? The project was eventually canceled.

Coming back to the present day, could the microwave auditory effect be at play in Cuba? It’s quite possible. The technology is definitely there – the effect has been demonstrated with 1960’s era transmitters. With sufficient power and a narrow beam antenna, the attackers wouldn’t even need to be in the same room or building as their targets. Power levels high enough to be audible or even cause pain might also cause dizziness, nausea, and even traumatic brain injury. All we can do is wait for the results of the current investigations, and keep a tin foil hat handy.