As noise-makers, volcanoes and jet engines are found similar

The similarity between volcanic sounds too low for people to hear and the audible rumble made by turbulent air in a jet's propulsion system suggests there are deeper characteristics in common between the two thunderous systems, the scientists say.

The study's findings may make an existing sound-based probe of the inner workings of volcanic eruptions more useful than it currently is, the researchers add. The new insights might also help improve volcano hazard mitigation by forecasting which eruptions are loaded with ash.

To investigate the volcanic sounds, the researchers speeded up infrasonic sound — a sound that is lower in frequency than 20 cycles per second, below the limit of human hearing — recorded from Mount St. Helens in Washington State and Tungurahua volcano in Ecuador, both of which are highly active volcanoes close to large population centers.

“We hypothesized that these very large natural volcanic jets were making very low frequency jet noise,” says Robin Matoza, a graduate student at Scripps Institution of Oceanography in La Jolla, Calif. and leader of the study.

Matoza and his international team recorded these very large-amplitude infrasonic signals during the times when ash-laden gas was being ejected from the volcano. Using 100- meter aperture arrays of microbarometers — similar to weather barometers but sensitive to smaller changes in atmospheric pressure — and low-frequency infrasonic microphones, the scientists revealed the physics of how the large-amplitude signals from eruptions are produced. They concluded that these large-scale volcanic jets are producing sound in a similar way to the turbulent processes found in smaller-scale man-made jets.

The team's results will be published on 18 April in the journal Geophysical Research Letters, a publication of the American Geophysical Union (AGU).

“We can draw on this area of research … to determine which eruptions are likely ash-free, and therefore less of a threat, and which are loaded with ash,” says Michael Hedlin, director of Scripps' Atmospheric Acoustics Lab and a co-author on the paper. Volcanoes that shoot ash clouds into the atmosphere pose a serious danger to the engines of the planes flying above.

Large-amplitude infrasonic signals from volcanic eruptions are currently used in a prototype real- time warning system that informs the Volcanic Ash Advisory Center (VAAC) when large infrasonic signals have come from erupting volcanoes. Researchers hope the new findings can improve hazard mitigation and inform pilots and the aviation industry.

“Eventually it could be possible to provide [to VAAC] detailed information, such as the size or flow rate of the volcanic jet, to put into ash-dispersal forecasting models,” Matoza says. The study was funded by a National Science Foundation grant.