For a year or so, U.S. staff had been reporting a variety of debilitating symptoms. These ranged from nausea and dizziness to deafness. Nearly two dozen U.S. staff and several Canadians were affected. U.S. officials claimed to not know the means, the methods, or how the attacks were being carried out. Cuba denied knowledge of any attack, but in retaliation, the United States expelled the Cuban diplomats and withdrew two-thirds of its own staff from the embassy in Havana. Here is a short clip of the sound embassy workers reported hearing, as well as a description of the symptoms they experienced: The sounds varied, including high-pitched chirps and low-pitched hums. One person might be woken by the sound, while the person next to them heard nothing. Sometimes the sound could be heard only in one part of the room and disappeared when you walked away from it. There was also a feeling of vibration, like the rapid flutter of air when windows of a car are partially rolled down. The Cubans suggested it was the sound of crickets. Others concluded it was a deliberate attack using some kind of sonic weapon. But a third explanation, put forward by Kevin Fu, an associate professor of electrical engineering and computer science at the University of Michigan, suggested an alternative scenario: bad engineering. Fu’s theory is that the sound heard is the interference generated by two devices, each producing an inaudible sound, and that one of them might be a covert surveillance unit. This interference, called intermodulation distortion, produces a sound that is audible and potentially harmful. While there is no evidence that this mechanism could produce the physiological effects described in the reports, there is an interesting component of the theory, and that is the use of ultrasound. Ultrasound describes the qualities of noises broadcast at frequencies too high for humans to hear. And as Fu and his colleagues point out in their paper, it has some interesting uses—for example, hacking electronic devices. The DolphinAttack is one such hack, described in research carried out by a team at Zheijiang University in China. In this research, the team demonstrated that it was possible to broadcast inaudible commands that could activate devices and applications, like Siri or Google Now, that listen for audio commands. Our always-on listening devices can lead to unexpected consequences. There was the Facebook message that went viral asking everyone to say, “Hey Siri, 108.” Do that and your phone will immediately start dialing emergency services. And there was the San Diego news station retelling the story about a child who said, “Alexa, order me a dollhouse,” unwittingly broadcasting the same command to Alexa owners everywhere. Better switch off Alexa before watching this video clip of the story. According to Amazon, “purchasing is enabled by default”: The ramifications of using hidden commands to activate technology are immense. It has already been explored by the marketing industry, where the idea of ultrasonic beacons latching onto passing mobile phones seems very attractive. In an interview with TechCrunch in 2014, a company called SilverPush said that it had already developed technology that used inaudible sounds, via websites or television, to retrieve data from the phones of its app users. What seemed like a good idea to the marketing industry was a disturbing thought to virtually everyone else. In 2016, the Federal Trade Commission issued a warning to app developers on the Google Store who were employing the “audio beacon” technology to monitor users. SilverPush subsequently said it was no longer using its audio beacon software. Despite this, research carried out in 2017 by the Technische Universitat Braunschweig in Brunswick, Germany, revealed that several stores had installed audio beacons, and more than 200 Android apps were set up ready to receive hidden messages. Maybe the marketing industry is waiting for Facebook’s privacy issues to blow over. It’s easy to imagine, in a Black Mirror world, how secret signals could be used to switch off all the smartphone cameras during a protest. The first secret audio signal I became aware of as a magician was a remarkable demonstration of “telepathy” that took place in Wales during the 1950s. It was conducted by S.G. Soal, a noted figure in parapsychology. Thinking that young rural minds made better subjects than city-dwelling adults, Soal sought to find some evidence of psychic ability in children. He found it in the case of Glyn and Ieuan Jones, two 13-year-old cousins from Snowdonia in Wales. Mentalist Frederick Marion with psychical researchers Samuel Soal (middle) and Harry Price (right). Photo: Josef Kraus, public domain, via Wikimedia Commons Soal carried out a series of card-guessing tests with the cousins. One boy would be shown a picture card, and the second boy would try to guess what it was. At first, children being children, they tried all sorts of wheezes to convey the information, using coughing and sniffing as a crude attempt at a secret code. Undeterred, Soal continued the tests, offering them money for successful guesses. The financial motivation seemed to spur them on, and soon the cousins managed to communicate the identity of cards without kicking each other under the table. Conditions for the tests were made stricter to prohibit secret communication, but even when separated by a screen and placed 100 feet apart, the boys could still transmit the identity of the card from one to the other. Over the course of several years, they earned themselves about £200, a considerable sum in those times. Soal told the story in his 1959 book, The Mind Readers. It created quite a stir and was greeted with favorable reviews, including one in the Times. Not everyone shared those views. C.E.M. Hansel, a psychologist, was skeptical of the Jones boys’ results. He had found a way that the two boys could communicate—a way that would baffle everyone present. Hansel tried it using two young girls as his volunteers, and they successfully managed to duplicate the Jones boys’ success. The secret was simple and one that Hansel thought two boys from the sheep-filled hills of Wales would be familiar with. It was a dog whistle.