In November, the Comprehensive Test Ban Treaty Organization (CTBTO), is holding a workshop for scientists on using ‘infrasound’. The idea is “to create an international forum for presenting and discussing recent advancements in infrasound research and operational capabilities of global and regional networks.”
Though the CTBTO’s primary mandate is to get more countries to sign the treaty — which India has not signed — it also shares the technologies it develops for monitoring nuclear tests with the industry. One such technology is ‘infrasound’, which refers to sound waves with very, very low frequencies, in contrast to the more ubiquitous ultrasound, which are sound waves of very high frequencies.
The invisible sound
Infrasound can be produced by, well, anything — a passing meteor, a storm, an aurora up north, volcanoes, earthquakes or even nuclear explosions.
The CTBTO’s International Monitoring System (IMS) uses a range of technologies to detect nuclear explosions. Its Infrasound Network (that is being built) is the only global monitoring network of its kind, with plans to build a network of 60 array stations in 35 countries. (The CTBTO is telling India, “Even if you don’t want to sign the treaty, at least allow us to set up an IMS on your soil”, but that is a different matter.) Each array contains four or more elements arranged in different geometric patterns, a meteorological station, a central processing facility and a communication system for the transmission of data. These stations are being built far from natural sources of noise, such as airports or windy coasts, with dense forests being ideal locations.
Infrasonic waves can cause minute changes in the atmospheric pressure, which can be measured by microbarometers. These noiseless sounds can travel very long distances without losing steam — a property that CTBTO finds useful for detecting distant nuclear explosions. The CTBTO website notes that the first observation of naturally occurring infrasound recorded with instruments was after the 1883 eruption of the Krakatoa volcano in Indonesia. In its aftermath, the infrasonic waves “circled the globe at least seven times, shattering windows hundreds of miles away and were recorded worldwide.”
Industrial applications
Now, it is important to note that infrasound has many industrial applications. For example, it can be used to check the structural health buildings, dams or bridges — because infrasonic waves can pass through dense materials and reveal internal stress, cracks or other defects. In the field of aerospace, low-frequency sounds generated during a rocket’s lift-off can cue the stress and behaviour of a rocket, or detect aerodynamic instabilities of an aircraft. In mining, infrasound can help check the integrity of mine shafts or determine whether a dynamite blast was successful. Infrasound has also been used in wildlife tracking, such as monitoring the movement of whales.
On the flipside, there are concerns about the so-far unknown harmful effects of infrasound on human health, a subject wide that remains open to debate.
Thus, there is a lot to learn about infrasound, so that it becomes as commonplace as ultrasound. The CTBTO workshop is an effort in that direction.