Infrasound is any acoustic oscillation that occurs below 20Hz, the threshold of human hearing.  Infrasonic sound can be manifested by avalanches, earthquakes, volcanoes, meteors, alligators, whales, hippos, giraffes, wind turbines, explosions, and some very specially designed loud speakers.  Because of their long wavelengths, infrasonic waves can propagate hundreds of miles from their source.

The Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) has constructed a global infrasound monitoring network consisting of 60 infrasonic monitoring stations in 35 countries.  While the intent of this network is to detect unauthorized nuclear weapons testing, the CTBTO  provides data from their International Monitoring System (IMS) to scientists around the world.  

When a volcano erupts, the pressure wave erupting from the crater creates infrasound.  The infrasonic waves are detected by monitoring stations, recorded as electric signals, and then sent via radio waves to satellites linked to the CTBTO.  The infrasonic signature of a volcano is very different from that of a nuclear explosion, so the data is then sent to any number of scientists who have requested access to IMS data in support of their vulcanological research.

One of those scientists is Dr. Milton Garces, the Founder and Director of the Infrasound Laboratory at the University of Hawai’i at Mānoa.    Dr. Graces has studied the infrasound produced by volcanoes since 1995 and was one of the designers of the IMS.  A facet of his research involves processing the electrical signals received by the CTBTO infrasound monitoring stations at the time of volcanic eruption into sounds that can be heard by human ears by “speeding up” the recordings by a factor of 100x to 400x.   This edition of Special Collections draws upon 15 years of Dr. Graces’ research to present a sonic portrait of volcanic eruption.