Cuban Embassy Attacks and The Microwave Auditory Effect

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)

Images from Frey’s paper

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.

Cheap, Full-Duplex Software Defined Radio With The LimeSDR

A few years ago, we saw the rise of software-defined radios with the HackRF One and the extraordinarily popular RTL-SDR USB TV tuner dongle. It’s been a few years, and technology is on a never-ending upwards crawl to smaller, cheaper, and more powerful widgets. Now, some of that innovation is making it to the world of software-defined radio. The LimeSDR Mini is out, and it’s the cheapest and most capable software defined radio yet. It’s available through a Crowd Supply campaign, with units shipping around the beginning of next year.

The specs for the LimeSDR mini are quite good, even when compared to kilobuck units from Ettus Research. The frequency range for the LimeSDR Mini is 10 MHz – 3.5 GHz, bandwidth is 30.72 MHz, with a 12-bit sample depth and 30.72 MSPS sample rate. The interface is USB 3.0 (the connector is male, and soldered to the board, but USB extension cables exist), and the LimeSDR is full duplex. That last bit is huge — the RTL-SDR can’t transmit at all, and even the HackRF is only half duplex. This enormous capability is thanks to the field programmable RF transceiver found in all of the LimeSDR boards. We first saw these a year or so ago, and now these boards are heading into the hands of hackers. Someone’s even building a femtocell out of a Lime board.

The major selling point for the LimeSDR is, of course, the price. The ‘early bird’ rewards for the Crowd Supply campaign disappeared quickly at $99, but there are still plenty available at $139. This is very inexpensive and very fun — on the Crowd Supply page, you can see a demo of a LimeSDR mini set up as an LTE base station, streaming video between two mobile phones. These are the golden days of hobbyist SDR.

An Unconference Badge That’s Never Gonna Give You Up

When your publication is about to hold a major event on your side of the world, and there will be a bring-a-hack, you abruptly realise that you have to do just that. Bring a hack. With the Hackaday London Unconference in the works this was the problem I faced, and I’d run out of time to put together an amazing PCB with beautiful artwork and software-driven functionality to amuse and delight other attendees. It was time to come up with something that would gain me a few Brownie points while remaining within the time I had at my disposal alongside my Hackaday work.

Since I am a radio enthusiast at heart, I came up with the idea of a badge that the curious would identify as an FM transmitter before tuning a portable radio to the frequency on its display and listening to what it was sending. The joke would be of course that they would end up listening to a chiptune version of [Rick Astley]’s “Never gonna give you up”, so yes, it was going to be a radio Rickroll.

The badge internals.
The badge internals.

I evaluated a few options, and ended up with a Raspberry Pi Zero as an MP3 player through its PWM lines, feeding through a simple RC low-pass filter into a commercial super-low-power FM transmitter module of the type you can legally use with an iPod or similar to listen on a car radio. To give it a little bit of individuality I gave the module an antenna, a fractal design made from a quarter wavelength of galvanised fence wire with a cut-off pin from a broken British mains plug as a terminal. The whole I enclosed in a surplus 8mm video cassette case with holes Dremmeled for cables, with the FM module using its own little cell and the Pi powered from a mobile phone booster battery clipped to its back. This probably gave me a transmitted field strength above what it should have been, but the power of those modules is so low that I am guessing the sin against the radio spectrum must have been minor.

At the event, a lot of people were intrigued by the badge, and a few of them were even Rickrolled by it. But for me the most interesting aspect lay not in the badge itself but in its components. First I looked at making a PCB with MP3 and radio chips, but decided against it when the budget edged towards £20 ($27). Then I looked at a Raspberry Pi running PiFM as an all-in-one solution with a little display HAT, but yet again ran out of budget. An MP3 module, Arduino clone, and display similarly became too expensive. Displays, surprisingly, are dear. So my cheapest option became a consumer FM module at £2.50 ($3.37) which already had an LCD display, and a little £5 ($6.74) computer running Linux that was far more powerful than the job in hand demanded. These economics would have been markedly different had I been manufacturing a million badges, but made a mockery of the notion that the simplest MCU and MP3 module would also be the cheapest.

Rickrolling never gets old, it seems, but evidently it has. Its heyday in Hackaday projects like this prank IR repeater seems to have been in 2012.

Hybrid Technique Breaks Backscatter Distance Barrier

Low cost, long range, or low power — when it comes to wireless connectivity, historically you’ve only been able to pick two. But a group at the University of Washington appears to have made a breakthrough in backscatter communications that allows reliable data transfer over 2.8 kilometers using only microwatts, and for pennies apiece.

For those unfamiliar with backscatter, it’s a very cool technology that modulates data onto RF energy incident from some local source, like an FM broadcast station or nearby WiFi router. Since the backscatter device doesn’t need to power local oscillators or other hungry components, it has negligible power requirements. Traditionally, though, that has given backscatter devices a range of a few hundred meters at most. The UW team, led by [Shyamnath Gollokota], describe a new backscatter technique (PDF link) that blows away previous records. By combining the spread-spectrum modulation of LoRa with the switched attenuation of incident RF energy that forms the basis for backscatter, the UW team was able to cover 2800 meters for under 10 microwatts. What’s more, with printable batteries or cheap button cells, the backscatter tags can be made for as little as 10 cents a piece. The possibilities for cheap agricultural sensors, ultracompact and low power wearable sensors, or even just deploy-and-forget IoT devices are endless.

We’ve covered backscatter before, both for agricultural uses and for pirate broadcasting stations. Backscatter also has also seen more cloak and dagger duty.

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A Fully Featured, Fifty Dollar QRP Radio

QRP radio operators try to get maximum range out of minimal power. This term comes from the QRP Q-code, which means “reduce power.” For years, people have built some very low-cost radios for this purpose. Perhaps the best known QRP kit is the Pixie, which can be found for less than $3 on eBay.

The QCX is a new DIY QRP radio kit from QRP Labs. Unlike the Pixie, it has a long list of features. The QCX operates on the 80, 60, 40, 30, 20, or 17 meter bands at up to 5W output power. The display provides tuning information, an S-meter, and a CW decoder. An on-board microswitch functions as a basic Morse key, and external Iambic or straight keys are also supported. An optional GPS can be used as a frequency reference.

The radio is based around the Silicon Labs Si5351A Clock Generator, a PLL chip with three clock outputs ranging from 2.5 kHz to 200 MHz. The system is controlled by an Atmel ATmega328P.

Demand for the kit has been quite high, and unfortunately you’ll have to wait for one. However, you can put down your $49 and learn Morse code while waiting for it to ship. While the project does not appear to be open source, the assembly instructions [PDF warning] provide a full schematic.

Antenna Basics by Whiteboard

Like a lot of people, [Bruce] likes radio controlled (RC) vehicles. In fact, many people get started in electronics motivated by their interest in RC. Maybe that’s why [Bruce] did a video about antenna basics where he spends a little more than a half hour discussing antennas. You can see the video below.

[Bruce] avoids any complex math and focuses more on intuition about antennas, which we like. Why does it matter that antennas are cut to a certain length? [Bruce] explains it using a swing and a grandfather clock as an analogy. Why do some antennas have gain? Why is polarization important? [Bruce] covers all of this and more. There’s even a simple experiment you can do with a meter and a magnet that he demonstrates.

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The Things Network Sets 702 km Distance Record For LoRaWAN

Many of us will have at some time over the last couple of years bought a LoRaWAN module or two to evaluate the low power freely accessible wireless networking technology. Some have produced exciting and innovative projects using them while maybe the rest of us still have them on our benches as reminders of projects half-completed.

If your LoRaWAN deployment made it on-air, you’ll be familiar with the range that can be expected. A mile or two with little omnidirectional antennas if you are lucky. A few more miles if you reach for something with a bit of directionality. Add some elevation, and range increases.

A couple of weeks ago at an alternative society festival in the Netherlands, a balloon was launched with a LoRaWAN payload on board that was later found to have made what is believed to be a new distance record for successful reception of a LoRaWAN packet. While the balloon was at an altitude of 38.772 km (about 127204.7 feet) somewhere close to the border between Germany and the Netherlands, it was spotted by a The Things Network node in Wroclaw, Poland, at a distance of 702.676km, or about 436 miles. The Things Network is an open source, community driven effort that has built a worldwide LoRaWAN network.

Of course, a free-space distance record for a balloon near the edge of space might sound very cool and all that, but it’s not going to be of much relevance when you are wrestling with the challenge of getting sensor data through suburbia. But it does provide an interesting demonstration of the capabilities of LoRaWAN over some other similar technologies, if a 25mW (14dBm) transmitter can successfully send a packet over that distance then perhaps it might be your best choice in the urban jungle.

If you’re curious about LoRaWAN, you might want to start closer to home and sniff for local activity.