Low sounds detect meteor blast

One of a planned network of 60 infrasound listening posts

By BBC News Online science editor Dr David Whitehouse

One of the first stations of what will be a global "infrasound" listening network, has detected a meteor that exploded over the Pacific Ocean with the force of a small nuclear blast.

There is a lot going on in the atmosphere that we need to know more about

Dr Michael Hedlin, Scripps Institute

"Infrasound" refers to sound waves that fall below the 20 hertz lower level of human hearing. The new detectors record signals that are too faint, and vary too slowly, to be detected by humans.

The global network is designed to monitor clandestine nuclear tests but scientists say it will have many scientific uses as well.

It will be able to detect previously unsuspected meteor entries into the atmosphere, volcanic eruptions, and the formation of hurricanes.

Nuclear blast

One of the first significant signals received by the infrasound array built by the Scripps Institute of Oceanography at the University of California, San Diego, was of a meteor that came crashing into the Earth's atmosphere on 23 April.

The first data from the array comes through

Estimated at between 2-3 metres (8 - 10 feet) across, it exploded with a yield of a few thousand tonnes of TNT, about a quarter of the force of the atomic weapon that was dropped on Hiroshima.

"If this rock had come into the atmosphere at a slightly different time, it might have exploded not over the Pacific, but over a large metropolitan area," said Dr Michael Hedlin of the Scripps Institute.

"With this global listening network we can develop much better statistics on large meteors and get a better idea of how often these massive objects enter the atmosphere."

Large explosions send part of their acoustic energy into the audible range, but those signals dissipate rapidly. But they also emit large amounts of energy into the infrasonic range in signals that decay slowly across vast distances.

The 23 April explosion occurred 1,800 km (1,118 miles) away from the Scripps detector. It was also detected by an infrasound array in Germany, 11,000 km (6,835 miles) away.

'Unprecedented opportunity'

As well as meteors, infrasonic sound is generated by supersonic aircraft, tornadoes, earthquakes and volcanoes.

The infrasound detector up close

According to Hedlin, scientists have already discovered that volcanic eruptions produce strong infrasonic signals, "seismic and infrasound data taken together give a much fuller account of activity inside the volcano that might be indicative of an impending, significant eruption."

Scientists are also planning to build a new infrasonic array at Cape Verde in western Africa, near to a region where hurricanes develop and emit infrasonic signals.

"There is a lot going on in the atmosphere that we need to know more about. The infrasound network will offer us an unprecedented opportunity to better understand these phenomena on a global scale.

"We anticipate that this global network of listening posts that monitors Earth's fluid exterior shell where we live will someday become as indispensable as the global seismic network that monitors the Earth's solid interior for seismic activity."