China sonic attack: how sound can be a weapon

There are two ways ultrasound can harm humans. The first is that it can heat up cells in the body, causing damage. The second is that ultrasound can cause “cavitation.” All sound waves are longitudinal – involving a cyclic pushing and pulling motion of molecules as the wave travels, called compression and rarefaction. This happens in air as well as when it travels through an object, such as the body. Cavitation is when the pressure difference between a strong push and a strong pull in a very loud sound causes bubbles to form.

The effects of ultrasound increase with amplitude (loudness), but heating is mainly a problem with contact ultrasound (when an ultrasonic emitter touches you), rather than waves transmitted through the air. Cavitation, by contrast, might occur in the fluid of the inner ear, in body tissue or cells. It can be transitory (the bubble forms and disappears with each frequency cycle) or sustained. In either case, it is not considered a good thing for bubbles to form in body tissue (just ask scuba divers).

The extent of these biological effects depend on how the ultrasound reaches the person being “attacked”. Any sound gets less powerful the further you are from a loudspeaker, but ultrasound loses power far more quickly with distance than audible sounds do. A single ultrasonic emitter (loudspeaker) would struggle to generate enough power to affect someone halfway across a typical room.

Ultrasound is also highly directional. Precise alignment in millimeters would be needed to steer an ultrasonic “beam” to hit someone from across a room. Every time they move, each emitter would have to carefully steer its beams accordingly.

Given that it is hard for powerful ultrasound to reach us – and that most of it then bounces right off our skin – it seems to be a strange choice of weapon.

Not a weapon but a side effect
Despite some disadvantages, ultrasound is used in various tools including motion sensors. It has also been used to detect people’s mouth movements in noisy locations, or where subjects are whispering (or miming speech). Both are useful in active surveillance, particularly when subjects are trying to avoid being overheard.

Although not deliberate, this could lead to cavitation damage. An ultrasonic loudspeaker designed to operate on a subject who is two meters away would be thousands of times more powerful at two centimeters. Just walking past, or sitting near, the active emitter for a short time could cause damage.

Multiple ultrasound emitters used for surveillance would be worse. If a subject moved their head into just the right location, waves from different emitters could combine at the eardrum, causing much higher energies. Sitting in the wrong position for too long could then cause hearing damage without the subjects noticing.

We may never know for certain what is the cause of these incidents. Given the reported symptoms, an audio related cause is likely – and if so, it is probably ultrasonic. But the nature of ultrasound suggests that these cases are probably the result of surveillance rather than a deliberate “sonic attack.”

Ian McLoughlin is Professor of Computing, Head of School (Medway), University of Kent. This article is published courtesy of The Conversation.