The PhilaDelphia Experiment (PX) Real Projects & Mythology

Can what you don’t hear hurt you? Researchers are studying whether the largely inaudible interplay of ultrasound beams from sensors and other devices can trigger headaches and dizziness.

Those were among symptoms reported by some U.S. diplomatic personnel stationed in China and Cuba who returned home in the past year after becoming incapacitated.

Ultrasonic signals are almost everywhere but the side-effects from so many transmissions remain a mystery, several experts said. Ultrasonic sound is the workhorse of electronics, an essential part of devices that are fixtures of public spaces. They include public-address systems, smart street lights and automatic door openers. In hotels, offices and stores, air-quality sensors, motion detectors and automatic light switches often rely on ultrasonic transmitters to relay readings or trigger alarms. Typically, the signals fall outside the range of sound that all but the most sensitive listeners can hear.

Advertisers embed ultrasonic tones into commercials to track consumer behavior across smartphones, TVs, tablets and computers. Shopping-mall operators deploy airborne ultrasound to drive off loitering youngsters, whose hearing is more sensitive to high-frequency noise. At museums, ultrasonic speakers pinpoint commentary or music to a single listener in a room without disturbing anyone else. At concerts, organizers broadcast an inaudible ultrasound signal while musicians play to “water-mark” the performance.
Mixed Signals
Ultrasonic signals have many applications, including those pictured here. Researchers are studying the possible health effects of such transmission traffic.

Fire alarms

Smart street lights

Directional

speakers

TV advertising

Smartphone

apps

Self-driving cars

“We have turned very rapidly into a kind of Wild West of ultrasonic devices, vastly outstripping any kind of evidence-based guidelines for their use,” said Timothy Leighton, an authority on ultrasonic devices at the U.K.’s University of Southampton who founded a research group called Health Effects of Ultrasound in Air.

Most guidelines governing public exposure haven’t been updated since 1984—well before most of today’s devices were invented, he said. In his studies of exposure in public places, he found that ultrasonic noise occasionally caused headaches, dizziness, and nausea among unsuspecting pedestrians, but those effects were always temporary.

Scientists are hard-pressed to explain the alleged sonic assaults targeting U.S. diplomats, even as the State Department evacuated ailing personnel from Guangzhou, China, in May, and warned American citizens abroad about risks.

Two weeks ago, the State Department sent a medical team to Guangzhou to screen U.S. personnel and their families for symptoms. The State Department also created a task force to respond to the “unexplained health incidents” and last week issued a health alert for citizens traveling in China, warning of symptoms that include dizziness, headaches, tinnitus, fatigue, cognitive issues, visual problems, hearing loss and difficulty sleeping.

The State Department declined to discuss its investigation into the alleged attacks or the possibility that ultrasonic signals were a factor. “We have undertaken a government-wide effort to identify the source behind these attacks,” a State Department spokeswoman said. “The Diplomatic Security Service continues to work with the FBI and other interagency partners in this investigation.”

A medical team at the University of Pennsylvania so far has tested 24 U.S. government employees evacuated from Cuba after unexplained sound-related incidents that started in 2016. The employees “appeared to have sustained injury to widespread brain networks,” team reported, describing the ailments as a new neurological syndrome. However, neuroscientists at the University of Edinburgh and the University of Trento in Italy called evidence of the condition “almost unbelievably flimsy.”

Researchers in the U.S. and China who specialize in ultrasonic cybersecurity suggest that high-frequency noise generated by a badly engineered eavesdropping device could be at fault. In the absence of any reliable data about the incidents, their experiments raise the possibility that noise generated by the intersection of ultrasound beams is the culprit. Such beams can be transmitted by surveillance or security devices as well as normal room sensors—or by someone trying to take remote control of a smartphone.

If so, the incidents may have been entirely accidental, several experts in the physics of ultrasound said. “It could be a surveillance operation gone wrong,” said Kevin Fu, director of the Security and Privacy Research group at the University of Michigan. “There may have been no intent to harm.”


Ultrasonic signals link interactive devices. They are the electronic eyes of industrial robots and driverless cars. And they are increasingly a tool for hacking, surveillance and security counter-measures, according to experts in the cybersecurity of ultrasound.

‘Normally you wouldn’t hear ultrasonics, but if you mix the signals together—like crossing the beams in “Ghostbusters”—you get these audible sounds.’
—Kevin Fu

While most people don’t hear ultrasonic signals, Siri can.

So can Alexa, Google Now, Cortana and other speech-recognition systems, according to ultrasonic-security expert Wenyuan Xu, director of the Ubiquitous System Security Laboratory at China’s Zhejiang University. Dr. Xu and her colleagues last year demonstrated that some voice-controlled digital assistants can be hijacked using inaudible ultrasonic signals.

Targeting devices at a distance, they played music on an Amazon Echo, got Google Now to switch a phone to airplane mode, made Siri launch a FaceTime call, and even manipulated the navigation system in an Audi car, according to research presented at the Association for Computing Machinery’s annual Conference on Computer and Communications Security.

Using the same principle, Dr. Wu and Michigan’s Dr. Fu created an ultrasonic eavesdropping device that could be made to malfunction so that it produced some of the unusual noises reported by U.S. diplomatic personnel in Cuba. To design it, they analyzed sounds allegedly recorded during an incident and then made public by the Associated Press. During those incidents, people reported hearing a high-pitched whine or buffeting pressure akin to driving with the windows partly open. Others likened the sounds to chirping insects or metal dragging across a floor.

Under ordinary circumstances, almost no-one hears ultrasonic signals. But when two inaudible ultrasonic beams intersect, they can generate audible tones, at frequencies that could cause annoyance and pain, the scientists said.

“Normally you wouldn’t hear ultrasonics, but if you mix the signals together—like crossing the beams in ‘Ghostbusters’—you get these audible sounds,” Dr. Fu said. “It is so easy to do. It’s child’s play.”

Doctors at the University of Pennsylvania’s Center for Brain Injury and Repair who studied the ailing U.S. personnel from Cuba now are examining the individuals evacuated from China. The medical team declined to comment for this article.

“We are continuing to work with the Department of State to evaluate and treat personnel who have reported audible phenomena experiences,” said Holly Aue, corporate director of communications for Penn Medicine. “We are not able to provide specifics about different patient groups at this time.”