Geophysicist Phil Blom

If a tree falls and no one hears it, does it make a sound?  That’s the question Phil Blom of Los Alamos National Laboratory's seismoacoustics team is researching…but with a national security twist. Blom wants to know if a missile launches or a nuclear device explodes or a supersonic aircraft flies by, and no one hears, do those things make sound? 

The answer, of course, is yes (anything that moves air creates sound, which travels in waves), and Blom researches the details using atmospheric acoustics—the study of how sound waves propagate in the atmosphere—and infrasonics—the study of sound waves with frequencies too low for humans to hear. What type of missile launched? How big was the nuclear explosion? Where did that supersonic aircraft go?

“Infrasonic waves travel large distances,” Blom says. “But they’re detectable 24/7 by microbarometer sensors on the earth’s surface that inexpensively and precisely measure fluctuations in air pressure.” A large area can be monitored without a dense network of sensors, and sensors don’t have to be near the source (of a missile launch, for example) to acquire data.

Groups of microbarometers can detect the direction sound moves from its source. But doing that for a fast-moving source can be tricky, so Blom turns to atmospheric acoustic modeling—predicting how acoustic energy spreads through the atmosphere. “We use modeling to better understand how infrasonic waves propagate from the source to the sensor; then we can learn where the infrasonic signal came from and what produced it,” Blom says. “This potentially enables us to discern the type of missile, how it was launched, and its trajectory.”

Because of infrasound’s relatively slow propagation speed, Blom’s research will likely never be a warning system for incoming missiles. Instead, it can be used to retroactively detect, track, and characterize missiles that have already launched. “We can help characterize missile performance,” he says, “what occurred during launch, flight, and reentry.” Infrasound also has increasing battlefield applications, such as tracking aircraft, localizing the source of gunfire, and detecting tunnels.

Combined with seismic, electromagnetic, or other data types, “infrasonic signatures contribute to a more complete picture and improve our confidence in characterizing foreign weapons systems,” Blom explains. Blom hopes to use supercomputers to improve modeling and characterization. “The future will bring even finer characterizations of missiles and supersonic aircraft, as well as explosions and other phenomena,” he says. “Infrasound is proving very useful for national security and nuclear nonproliferation.” ★