HANOVER, N.H. – The U.S. Army Engineer Research and Development Center’s Cold Regions Research and Engineering Laboratory’s (CRREL) signature physics branch is obsessed with sound -- or more specifically, the way it travels through the atmosphere and interacts with terrain – and methods for extracting information from sound signals.
The way sound is transmitted can be incredibly varied depending on the environment. Deserts, forests or frozen tundra can all impact sound in unique ways. Currently, there is very little research in the way sound travels through arctic terrain, which is why CRREL researchers from the signature physics branch and several universities went to Fairbanks, Alaska, to collect data for developing new methods for detecting, localizing and identifying aerial drones.
Small, inexpensive drones are a rapidly growing threat to Department of Defense forces and facilities. It is important to develop a better understanding of what technologies are best suited to detecting and tracking drones, particularly in extreme environments such as the Arctic where little data is currently available.
“We went specifically to Fairbanks because they have interesting atmospheric conditions,” said Matt Kamrath, a CRREL research physical scientist. “They have very strong temperature inversions, and when you have interesting things happening in the atmosphere, the sound does not travel in straight directions.”
Kamrath and the CRREL team were looking at identifying and locating drones with a microphone array to help determine the direction of the sound or what is known as the direction of arrival.
“If the sound is coming from a certain direction, when it reaches the array, we infer that it's coming from that direction,” said Kamrath. “But when you have interesting things happening in the atmosphere in terms of temperature and wind, it doesn't travel in a straight line, and that can affect how accurately we can determine where something is. And so, we're trying to determine how big of an error the extreme conditions in the Arctic could have on that kind of accomplishment.”
As part of the Arctic, Alaska has some very interesting atmospheric features.
“I study how sound travels, and it's a very interesting atmosphere in Alaska,” said Rachel Romond, a CRREL research physical scientist who studies the propagation of sound in the atmosphere. “I don't think the Arctic has been studied a whole lot.”
“It's very easy to go to the desert southwest and do an acoustic propagation study,” she continued. “The atmosphere there is very well known, and others actually choose to test there because the atmosphere is benign. It's well known. It's well characterized, and the models work very well in that kind of atmosphere.”
The study of how sound travels through the atmosphere in Arctic regions is relatively unknown, Romond says. It's an area that hasn't been studied very much, and it intersects really well with CRREL’s mission, which is to study environmental effects of extreme environments.
“The differences in how sound travels also affects how far the sound is audible,” said Romond. “Any sound in a desert situation that would have gone straight up into the air, in an Arctic situation under these inversion conditions, could go up and then come back down.”
There are many variables that can impact sound in cold regions. Snow absorbs acoustic energy very well, and sound traveling near the ground can be quickly absorbed by snow.
“The ambient environment is different,” said Romond. “We've all been outside on a quiet, snowy night. It impacts the background noise levels, and therefore, the detectability and notability of any particular sound that you hear that you think might be the background but is not.”
This effort from CRREL researchers, like Romond and Kamrath, in cooperation with several universities, is groundbreaking in that they are developing better sensing systems and better prediction systems for when early detection systems are trying to detect drones.
“If we know the science of how the sound will propagate better than we know now,” said Kamrath. “It'll make those types of detection systems and possibly deterrent systems better.”