Sound waves with a sufficiently high frequency which are inaudible to humans are called ultrasound. It has been used in medicine for therapeutic purposes due to its non-invasive nature. Early uses include the treatment of tendinitis or bursitis and breaking down kidney stones. It is also useful in diagnoses such as through imaging, and in communication. The possibilities for haptic feedback and levitation of small objects have also been demonstrated in recent years. Thus sound is no longer a privileged domain of our ears.
As well as its benefits, safety issues arise. Despite being inaudible, ultrasound is still capable of causing hearing loss at high volumes. It can also produce heating and cavitation effects. Ethical concerns also become apparent in recent experimental uses including its application on the brain. The technique known as transcranial pulsed ultrasound may help treat conditions ranging from depression to Parkinson’s disease. However, its more sinister potential is also apparent with its basis in stimulating high neuron activity to manipulate brain waves.
Along with the general trend in the development in audio technology such as the use of sonic booms in the military and sound canons for crowd control by law enforcement to “mosquito” devices to repel teenage gangs, the potential use of sound as a weapon and its political context require serious consideration. In these examples, sound’s relationship to space becomes a form of asserting power and putting public space under the domain of authority and control.
In Localising Borders, a set of parametric speakers and a laser are placed in two rooms, with the sound and the light pointing in the same direction. In the first room, its position is fixed; in the second, they are suspended from the ceiling and left to rotate. The parametric speakers emit modulated ultrasound which is highly directional, creating a “beam” of sound more similar to light than conventional audio. The sound remains inaudible until it is reflected off an object such as a wall or a person at which point it is demodulated and sound appears to be emitted from the reflecting object. The point at which the sound is reflected (and is illuminated by the laser) marks out the interiors of the space in a process known as echolocation and used in sonar. The sound played are recordings of ultrasonic bat calls used for such purposes which have been transposed to the audible range of human hearing.