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Monday, March 05, 2018

Ultrasound listening devices in Cuba? Unlikely

A discussion of the University of Michigan's new theory about the Cuban embassy sonic attacks

K evin Fu of the University of Michigan has hit on the same idea many people discussed a few months ago: two ultrasonic beams separated by 7kHz would create a focusable sonic weapon. If they passed through a nonlinear medium, you'd get audible sound at the frequency by which the beams differ. It's called a virtual end-fire array, and I discussed the pros and cons of the theory here.

Ultrasonic transducer
Ultrasonic transducer sold commercially for 159 € + VAT tax and shipping. The sales blurb says: “The mic+600/D/TC M30 cylindrical ultrasonic sensor from Microsonic with a transducer frequency of 80kHz and a PNP switching output that is programmable as NCC or NOC has an operating range of 600 to 6000mm.” Source

They're typically used as security alarms and (when submerged) to measure water levels. To use it to bug an embassy, you'd need some way to form a collimated beam. That can be done, crudely, with a horn, a refractive lens or a parabolic reflector, but most likely it would be an array like this.

In their article, they suggest a new twist: the Cubans are amplitude-modulating an ultrasonic beam, using it as an old-fashioned AM radio bugging device, only with sound instead of radio. In their report On Cuba, Diplomats, Ultrasound, and Intermodulation Distortion. Tech. Report CSE-TR-001-18 (available here) they point out, quite correctly, that if there were two of these transmitters operating 7 kHz apart, they could interfere with each other. This could easily create an audible noise.

When they tried to demodulate the existing audio recording, all they got was a low frequency 180 Hz noise, which the theory doesn't explain. As they point out in their article, there are many sources of ultrasound, including sonic burglar alarms and cleaners. DSLR cameras use ultrasound to keep dust off the sensor by vibrating it. But the theory that ultrasound would work as a bugging device doesn't really hold up.

Put yourself in the place of the Cubans. You want to know what the Yanquis are saying. Is ultrasound a good way? It could be done, but there are formidable challenges.

  1. Ultrasound is rapidly attenuated in air. Using ultrasonic sound to relay information out of a solid building would face a number of technological obstacles. First of all, it would require enormous amounts of energy. Ultrasound doesn't easily pass through objects like solid walls and commercial windows; most of the energy reflects off and is scattered. And it doesn't go very far in air. That's why those of us who search for bat calls usually do it outside.

    An ultrasound transmitter would have to be exposed to the outside air (or better yet, under water) in order to have a chance of being received. If the sound waves are bouncing around inside a room, it is seriously malfunctioning.

  2. Radio is much cheaper. By contrast, a radio frequency transmitter would be many thousands of times more efficient. Modern FHSS (frequency hopping spread spectrum) transmitters can be fiendishly difficult to detect, especially in bugging devices that are only activated for short periods of time, or digital ones that store up their data and burst it at, say, three in the morning.

  3. Interferometry. Suppose, as an alternative, you bounced a sound beam off a window, created some sort of standing wave, then collected the reflected energy. Some of the sound might scatter into the room, creating the squeaky sound we heard on the recording. But the amount of information you'd get back would be infinitesimal. It reminds me of a scheme someone emailed me about EM fields being used as a way of remotely reading people's brain waves. It just will not work. EM waves don't interact with each other in free space; they just add to each other. Likewise, a faint low-frequency sound wave, like a voice, won't modulate a high-frequency one in the open air.

    You might imagine using the reflected beam as a way of isolating the signal. Many electronic devices use modulation in this way. It makes it easier to filter out DC and low frequency noise. But as a listening device, it would be like blasting somebody with a car horn and expecting to pick up reflections of somebody whispering. You'd be better off just eliminating the car horn.

    Here again, infrared lasers are the method of choice. It's a well known technique: slap a retroreflector on the window (nowadays you can buy retroreflecting paint), then reflect an invisible beam of light off it. With a lot of tweaking, you can use it to bug the room. Maybe a million times more efficient.

  4. Feedback. If a bugging device creates sound, the sounds would be picked up by the microphone and create feedback. It wouldn't necessarily sound like the feedback squeals we're all familiar with, but it should be there.

To me the scheme sounds almost like an attempt to exculpate the Cubans, which might explain why the news media like the theory so much.

The US government surely has ways of detecting ultrasound. Whether it's an ultrasonic burglar alarm gone rogue, or Cubans who are so interested in listening to our ambassadors bloviating that they try to beam information out of the embassy in the most inefficient way imaginable—through a sound beam that would have to be so intense as to cause thermal brain injury in order to be detectable across the street—even the FBI could have detected it.

The available evidence suggests that it's a fake mystery. Somebody knows the source and is embarrassed to tell us. Maybe somebody's ultrasonic cleaner is acting up. Or maybe they installed ultrasonic window cleaners to save money. Or maybe they blanket the room with ultrasound to jam any microphones. Or maybe it's not even sound at all.

If the sonic transmitter theory is true, it should be easy to prove it: find the device. If it existed, the US government surely would have found it by now.

mar 05 2018, 5:23 am; figure added mar 08 2018. last edited mar 09 2018, 5:12 am.

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