February 20, 2014
Measuring wind with microphones
To many who live along the road, the roar of traffic on the Diagonal Highway between Boulder and Longmont, Colo. is nothing but a nuisance. But to a small team of physicists in Boulder, the noise proved inspirational. The group, led by Oleg Godin of CIRES, used the traffic noise on “The Diagonal” to accurately measure wind speed. The scientists reported the proof-of-concept study this month in the Journal of the Acoustic Society of America Express Letters.
“We have demonstrated for the first time that we can use ambient noise to measure wind speeds,” said Godin, lead author of the new study and a senior research scientist with NOAA’s Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder. Godin works in NOAA’s Earth System Research Laboratory (ESRL). Eventually, the method could be used to cheaply measure wind speed and direction in the atmosphere, critical information for weather forecasts, he said, or even to study the rotation of Earth’s core.
The technique he and his colleagues use is called acoustic tomography, and it is similar to the use of sonar (sound navigation and ranging) to map underwater objects and measure temperatures below the ocean surface.
Traditional sonar techniques require beaming loud signals through water bodies and recording their arrival at underwater microphones. Sound travels faster or slower, depending on water temperature. A few years ago, Godin and his colleagues figured out how to use ocean noises such as breaking waves, singing whales and passing ships to measure underwater temperatures without relying on loud and potentially disruptive sounds.
Now, they’ve applied that research to the atmosphere. In the air, wind speed and direction affect the propagation of sound waves.
During a series of experiments in 2012, the team set up four to five microphones, about 50-150 feet away from one another, in the median of Diagonal Highway close to Boulder. They also set up a conventional weather station, to provide “ground truth” information on wind speed and air temperatures between the microphones.
As cars rushed by, all microphones recorded the sounds, and Godin and his colleagues captured the tiny differences in when sounds arrived at each microphone. Using sophisticated mathematical techniques, they matched up the signals and arrival times to calculate the speed of wind between the microphones. Their measurements were extremely close to those recorded by the conventional weather station, within 1-3 miles per hour.
In concept, the same kind of “noise interferometry” method could be used to better measure the mass and heat carried by the Gulf Stream and even to understand the rotation of Earth’s core, Godin said. By comparing noise recorded simultaneously by two microphones, he and his colleagues can measure tiny differences between the time it took sound to travel in opposite directions between the microphones. This travel time difference, which scientists call “acoustic non-reciprocity,” is a very sensitive and robust measure of ocean current velocity or Earth’s core rotation.
Authors of the new paper, “Passive acoustic measurements of wind velocity and sound speed in air,” are Oleg Godin (CIRES scientist working in NOAA's ESRL) and Vladimir Irisov and Mikhail Charnotskii (Zel Technologies, LLC scientists working in NOAA's ESRL).