“Acoustic rainbows” sound psychedelic. But the idea is no acid trip: By dispersing sound waves, noisy objects can actually be made silent.
That’s why U.S. Special Operations Command is seeking acoustic rainbow emitters, or ARE, for its drones, according to a SOCOM Small Business Innovation Research solicitation.
“The ARE would provide a means to redirect the acoustic signature of a UAS and change the frequencies of the acoustic signature,” notes the solicitation, which closes March 25.
Though tactical drones can be small and hard to spot, the noise of their propellers can warn potential targets and put the drone itself at risk to hostile fire. “Acoustic sensors are becoming more prolific on the battlefield,” SOCOM noted.
The project aims to attach acoustic rainbow emitters to Army Special Operations Forces tactical drones. The emitters “should redirect the acoustic signature of a UAS towards the atmosphere and away from the ground,” the solicitation said. In addition, they “should filter the frequency of the sound emitted towards the ground to levels inaudible to the human ear.”
The goal is to reduce a drone’s acoustic signature by at least 50%. The device should weigh no more than 1 kilogram for a Group 1 UAS (under 20 pounds) and 3 kilograms for a Group 2 drone (under 55 pounds). The ARE must also “be colored to maintain a low visual signature.”
SOCOM seems particularly interested in silent attack drones. The solicitation mentions “a low-cost sensor employed from a UAS during terminal attack or when directed from the operator.”
Phase I of the SBIR is a Technology Readiness Level 3 feasibility study to see whether the concept is viable. Phase II calls for a prototype.
Acoustic rainbows have generated much buzz in recent years. The concept is similar to rainbows in the sky, when sunlight passing through water droplets becomes separated into different wavelengths and thus colors.
“Although acoustic rainbows are less well-known, they follow the same principle, being the spatial decomposition of sound in free space where waves oscillating at different frequencies propagate in different directions,” explained a Physicsworld article.
Some animals, such as bats and even humans, have the ability to do this naturally, which helps them to localize sounds. But doing this artificially across a wide range of frequencies has been a problem.
However, Danish and Spanish researchers unveiled last year a 3D-printed acoustic rainbow emitter that splits broadband sound — which would normally radiate in all directions — into separate frequencies that can be individually redirected. The device works on the principle of “passive scattering, where the acoustic system is purely driven by interactions between the hard plastic surface and sound waves, requiring no electricity,” noted Phys.org.
Michael Peck is a correspondent for Defense News and a columnist for the Center for European Policy Analysis. He holds an M.A. in political science from Rutgers University. Find him on X at @Mipeck1. His email is [email protected].
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