Infrasound is atmospheric based acoustic waves with a frequency lower than humans can detect (less than 20 Hz). Infrasound is typically generated by large sources such as hurricanes, explosions, tornados, and atmosphere – ocean interactions. Because the attenuation in this frequency range is very low, these waves travel great distances, although the propagation path is complicated by the layered and dynamic structure of the atmosphere. By understanding the physical mechanisms of how the sound is generated, we can determine information about these large sources such as location and energy scales.
The infrasound group at NCPA, currently led by Dr. Roger Waxler, has been very active in investigating a number of aspects of infrasound. This group has developed a new type of infrasound sensor array with improved signal to noise capabilities and frequency range. They have also studied infrasound signals generated by hurricanes in the Atlantic and us arrays along the eastern seaboard to track hurricanes. They are currently working to understand the propagation of infrasound through the hurricane in an effort to use infrasound to gather information about the internal structure and wind speeds inside the hurricane. This group has also studied distinct acoustic signatures of tornados and are working to understand the mechanism well enough for tracking and early warning. Finally, our infrasound group works with the international community to analyze signals of weapons tests recorded by infrasound arrays located around the globe. A significant facility is an infrasound calibration chamber designed by NCPA researchers.
- FY2008-2009 saw significant changes for infrasound at NCPA. Since 1998, funded largely by Congressional initiatives, NCPA has been leading the national resurgence of the development and application of infrasound research and technology. NCPA played a decisive role in the development of the U.S. infrasound component of the United Nations Comprehensive Test Ban Treaty Organization’s International Monitoring System (IMS) and has been, and remains, the lead institution for the U.S. Infrasound Research and Development Team.
- While the IMS was intended primarily for the detection of clandestine nuclear weapons tests, for the last decade the U.S. has been focused on using the infrasound network for geophysical research. During this period, researchers at NCPA and other laboratories in the U.S. team have established themselves as the world experts in various areas of infrasound research including the radiation of infrasound by hurricanes, the propagation of infrasound through the atmosphere, the generation of infrasound by volcanoes, and the development of infrasound sensing technology.
- In 2008, the U.S. Department of Defense initiated the Nuclear Arms Control Technology (NACT) program whose mandate is to prepare the U.S. for the possibility that the Comprehensive Nuclear Test Ban Treaty (CTBT) will go into force. The University of Mississippi, and in particular NCPA, was chosen to direct the infrasound research and development component of the NACT program. In partnership with NCPA, the Geophysical Institute of the University of Alaska Fairbanks was chosen to direct the operation and maintenance of the U.S. infrasound stations in the IMS. The funding for the NACT program comes directly from the Department of Defense.
- In addition to the NACT program, NCPA is leading the Remote Infrasonic Monitoring of Natural Hazards project for the National Oceanic and Atmospheric Administration. This program is being conducted by a consortium consisting NCPA, the University of Hawaii, the University of Alaska Fairbanks, and the University of California, San Diego to develop technologies for detecting and monitoring hurricanes, volcanic eruptions, tornados, severe surf conditions, and tsunamis.