Saturday, March 03, 2018

The Affair of the Sonic Weapons Attack: A Sonic Cocktail with a Kick--(Kevin Fu, Wenyuan Xu, and Chen Yan, “On Cuba, Diplomats, Ultrasound, and Intermodulation Distortion,” IEEE Spectrum (March 2018)

The Affair of the Sonic Weapons Attack continues to percolate away in the back burner while the chattering classes' attention is focused on momentarily more fashionable events.  That distraction provides a space within which scientists in the U.S. can continue to develop ideas about what might have caused injury.

A recent higher profile report by two scientists,  Kevin Fu, a computer scientist at the University of Michigan, his frequent collaborator, Wenyuan Xu, a professor at Zhejiang University, in Hangzhou, China, and her Ph.D. student Chen Yanand, suggest yet another explanation for the sonic attacks--not as attacks but intermodulation distortion, as an unfortunate consequence of a cocktail of sonic devices with cross purposes  (Kevin Fu, Wenyuan Xu, and Chen Yan, “On Cuba, Diplomats, Ultrasound, and Intermodulation Distortion,” IEEE Spectrum (March 2018) available on the Security and Privacy Group’s website [PDF]).

The theory has yet to be discussed by the scientific community (at least in public).  Some questions remain worth thinking about. First, while U.S. (and it appears Chinese) scientists continue to work on the issue, one hears nothing from Cuba and its scientists. Second, intermodulation distortion is traditionally understood as an unfortunate and unwanted effect of unwanted additional signals when two or more signals interact.  That is true enough and well known.  Yet one can only wonder whether it is possible to weaponize such a phenomenon.  If someone understands what happens when certaion signals are mixed, can one deliberately create certain intermodulation distortions deliberately in a targeted way?

Thus, while it is possible to suggest that "bad engineering" caused the injury, it might also be possible to suggest that a good knowledge of the effects produced by certain cocktails could be quite "good engineering" if instead the object was to cause injury. The science can provide the answer to "how" but it cannot help us understand either intention or objective. For that a different forensics will be in order.  All of this remains to be determined, of course.  But it does appear that the Affair continues to present interesting problems for resolution.

Portions of recent reporting follows.

(Pix Credit: Intermodulation Distortion (

"Finally, a Likely Explanation for the “Sonic Weapon” Used at the U.S. Embassy in Cuba": Researchers say bad engineering, not a deliberate attack, may be to blame
By Jean Kumagai

Last August, reports emerged that U.S. and Canadian diplomats in Cuba had suffered a host of mysterious ailments. Speculation soon arose that a high-frequency sonic weapon was to blame. Acoustics experts, however, were quick to point out the unlikeliness of such an attack. Among other things, ultrasonic frequencies—from 20 to 200 kilohertz—don’t propagate well in air and don’t cause the ear pain, headache, dizziness, and other symptoms reported in Cuba. Also, some victims recalled hearing high-pitched sounds, whereas ultrasound is inaudible to humans.

The mystery deepened in October, when the Associated Press (AP) released a 6-second audio clip, reportedly a recording of what U.S. embassy staff heard. The chirping tones, centered around 7 kHz, were indeed audible, but they didn’t suggest any kind of weapon.

Looking at a spectral plot of the clip on YouTube, Kevin Fu, a computer scientist at the University of Michigan, noted some unusual ripples. He thought he might know what they meant.

* * *

He discussed the AP clip with his frequent collaborator, Wenyuan Xu, a professor at Zhejiang University, in Hangzhou, China, and her Ph.D. student Chen Yan. “We saw it as an interesting puzzle,” says Xu, whose lab works on embedded security, including the use of ultrasound and radio waves to fool voice-recognition systems and self-driving cars. “It was a lot of fun to try to solve it.”

* * *

Yan and Xu started with a fast Fourier transform of the AP audio, which revealed the signal’s exact frequencies and amplitudes. Then, through a series of simulations, Yan showed that an effect known as intermodulation distortion could have produced the AP sound. Intermodulation distortion occurs when two signals having different frequencies combine to produce synthetic signals at the difference, sum, or multiples of the original frequencies.

When signal processing equipment behaves in a nonlinear way, it can cause this type of distortion. For example, Fu says, microphone circuitry can exhibit nonlinear behavior, and waves propagating through air can also behave in a nonlinear fashion. “As acoustic waves containing multiple frequencies travel through a nonlinear system, you can get these bizarre ripples in the spectrum of the signal,” he explains. “At the same time, intermodulation distortion can produce lower-frequency signals than the original signals. In other words, inaudible ultrasonic waves going through air can produce audible by-⁠products.”

Yan followed up the simulations with lab experiments, in which he used two ultrasonic speakers, one emitting a signal at 25 kHz and the other at 32 kHz. When he crossed the two signals, it produced the telltale high-pitched sound at 7 kHz, which was equal to the difference between the two speakers’ frequencies—and the same frequency as in the AP audio. In a nod to the Internet meme “rickrolling,” Yan was even able to embed an ultrasonic version of the Rick Astley song “Never Gonna Give You Up,” which became audible at the point where the two signals crossed.

Having reverse engineered the AP audio, Fu, Xu, and Yan then considered what combination of things might have caused the sound at the U.S. embassy in Cuba. “If ultrasound is to blame, then a likely cause was two ultrasonic signals that accidentally interfered with each other, creating an audible side effect,” Fu says. There are existing sources of ultrasound in office environments, such as room-occupancy sensors [see, for example, “How an Ultrasonic Sensor Nearly Derailed a Ph.D. Thesis”]. “Maybe there was also an ultrasonic jammer in the room and an ultrasonic transmitter,” he suggests. “Each device might have been placed there by a different party, completely unaware of the other.”

One thing the investigation didn’t explore was whether the AP audio could have produced the wide range of symptoms, including brain damage, that afflicted embassy workers. “We know that audible signals can cause pain, but we didn’t look at the physiological effects beyond that,” Fu says. At press time, the FBI had yet to announce the results of its investigation. A panel of Cuban scientists and medical doctors, meanwhile, concluded that a “collective psychogenic disorder” brought on by stress may have been at work.

Fadel Adib, a professor at MIT who specializes in wireless technology for sensing and communications, calls the study by Fu and his colleagues “a creative take on what might have happened.” Adib, who wasn’t involved in the research but reviewed the results, adds that wireless signals can and do interact with one another. “And if that happens, you’ll hear signals you wouldn’t expect to hear,” he says. “Given all the possible explanations, this definitely seems the most plausible and the most technically feasible.”

Fu is careful to offer a caveat: “Of course, we don’t know for certain this was the cause. But bad engineering just seems much more likely than a sonic weapon.”

This article appears in the March 2018 print issue as “Reverse Engineering the ‘Sonic Weapon.’ ”

Editor’s note: An article by Kevin Fu, Wenyuan Xu, and Chen Yan about their research will be published by IEEE Spectrum in March. Their technical report, “On Cuba, Diplomats, Ultrasound, and Intermodulation Distortion,” is available on the Security and Privacy Group’s website [PDF].


Can sound be used as a weapon? 4 questions answered 
March 1, 2018 6.37am EST
Editor’s note: Government and academic investigators continue to probe reports from Cuba that, starting in 2016 and continuing through 2017, U.S. and Canadian diplomats and tourists may have been subjected to a “sonic weapon,” damaging their hearing, causing nausea, speech problems and potentially even mild brain injuries.

Electrical engineering and computer science professors Wenyuan Xu from Zhejiang University and Kevin Fu from the University of Michigan explain their research, which suggests a more likely scenario of sloppy engineering, and what ultrasound frequencies (which can be used to transmit information gathered by listening devices) traveling through the air can – and can’t – do.

1. What is ultrasound useful for?

The most commonly known use for ultrasound – high-frequency sound waves human ears can’t hear – is a medical device used for examining a fetus during pregnancy. But there are plenty of other uses.
Many offices have occupancy sensors that use ultrasound to detect movement and keep the lights on when someone is in a space, and off when nobody is around. These sensors operate at frequencies such as 32 kilohertz, far above what the human ear can hear – which is a range from 20 hertz to 20 kilohertz.

Other products use ultrasound to deliver targeted sound, for instance allowing a museum to play a recording for visitors in one area of an exhibit without disturbing others nearby. Electronic pest repellents use ultrasound to keep rodents or insects at bay.

A similar product can even be used to disperse teenagers; aging tends to reduce people’s ability to hear higher frequency sounds, so a noisemaker can annoy young people without adults even noticing. (This has also let teens create smartphone ringtones their elders can’t hear.)

2. What can go wrong with ultrasound?

Airborne ultrasound is not inherently bad. But things can go wrong. A former colleague of Kevin’s used to hear strange sounds from his hearing aid when in rooms with occupancy sensors, likely because the hearing aid’s electronics improperly converted the ultrasound into audible noises. These noises were annoying, but not harmful. A similar problem tainted one of our students’ research, conducted in a room that, unbeknownst to him, had an ultrasonic room occupancy sensor in the ceiling.

Both ultrasound and human-audible sound can also affect electronics. For instance, one of us has conducted research in which carefully crafted ultrasonic signals secretly activate voice-control systems, even unlocking an iPhone with a silent “Hey Siri” command, and telling it to make a FaceTime call.

Sound can also affect the physical world, as when a singer shatters a wine glass. Microelectrical mechanical sensing chips – such as accelerometers used in car airbag systems and smartphones, and gyroscopes in drones – are susceptible to the same interference. Those systems can be attacked with sound, crashing a drone mid-flight, or fooling a smartphone about whether it’s moving.

3. Should people worry about ultrasound causing bodily harm?

It’s well-known that sounds that are too loud can damage people’s ears and hearing. However, there’s little evidence of ultrasound causing bodily harm without prolonged, direct physical contact at high intensity. If you are accidentally subjected to extremely intense ultrasound (such as when holding an ultrasonic arc welder), you could experience an annoyance like a headache or temporary loss of balance.

Academics disagree about safe levels of airborne ultrasound. The U.S. Occupational Safety and Health Administration warns of potential health risks from audible subharmonic byproducts of ultrasound, more so than the ultrasound itself.

Many animals can hear higher frequencies than humans. Dogs can hear higher-pitched whistles, for instance. One of our students noticed that his pet turtles would begin to dance rhythmically when he performed ultrasound experiments!

4. What might have happened in Cuba?

In early 2017, U.S. diplomats in Cuba reported hearing strange metallic sounds, and suffering hearing loss and other neurological harm. Later reports of similar effects came from Canadian diplomats and tourists from both Canada and the U.S. Possible explanations have varied: Some have alleged Cuba used an unknown sonic weapon, while others have blamed “mass hysteria.”

Our research offers a new explanation not previously considered by others: The true cause could have been equipment trying to listen in on the diplomats’ and visitors’ conversations.

We were able to use ultrasonic tones to create sounds like those that were described and recorded in Cuba. No single ultrasonic tone would do this, but as with musical combination tones, combining more than one can create audible byproduct sounds, including by accident.

Further, we created a proof-of-concept eavesdropping device that would record audible conversations and transmit the recordings to a nearby surveillance team over an inaudible ultrasonic link. When we placed a second inaudible ultrasonic device in the area, we were able to create interference – technically called “intermodulation distortion” – between the two signals that made similar sounds to those recorded in Cuba. We were even able to control the volume of the audible sounds by varying the strength of the ultrasonic signals.

Without additional evidence, our research does not identify what actually happened in Cuba, but it provides a plausible explanation for what might have happened, even if the eavesdroppers were not trying to harm people.

Disclosure statement

Kevin Fu receives funding from the National Science Foundation CNS-1330142. He is affiliated with the Computing Research Association’s Computing Community Consortium (CCC). The views and conclusions contained in this paper are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of NSF.

Wenyuan Xu does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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