Spectrogram of a police siren
Key45, via Wikimedia Commons CC SA 1.0

This week, South Korea shouted a new entry onto the list of drone countermeasures. A research project by the Korea Advanced Institute of Science and Technology (KAIST), set to be presented in DC next week, uses sound to disable the internal gyroscopes that balance drones.

The paper, entitled “Rocking Drones with Intentional Sound Noise on Gyroscopic Sensors” targets Micro-Electro-Mechanical Systems (MEMS) gyroscopes, the tiny sensors used by drones to stay level in flight. In normal function, these gyroscopes stabilize the drone against imbalances from wind or other movement. The researchers from KAIST wanted to see if enough deliberate, hostile external vibration–in this case provided by sound–could disturb the smooth flight of the drone. The answer was a resounding “yes.”

It turns out that the resonant frequencies, or sounds that vibrate at the same frequency as the targeted object, of many of the gyroscopes used in small drones are within audible range. To find that out, the researchers attached a wireless speaker four inches above the gyroscope in a target drone, and then turned it on while the drone was in stable flight. One of the targets was largely unaffected and stayed airborne, but another became unstable on all its axes and fell down to the ground.

Attaching wireless speakers to a target drone isn’t exactly a great countermeasure, so the researchers attempted other sonic attacks on drones. In a simulation they found that, if loud enough, directional speakers could disable drones from a distance of up to 120 feet, and increasing the decibels of the sonic weapon extends its range, to a point. But without the ability to track the enemy drone, it can just steer itself clear of the attack and continue to fly normally. (The kind of equipment needed to track incoming drones is more in the realm of a defense contractor than a research institute.)

Unless costs go down or power of the countermeasure goes up, we’re unlikely to see sonic weapons protecting skyscrapers from hostile drones. Fortunately, Korea has other defenses on hand. This past spring, they showed off an anti-drone system that used other small drones to attack and disable hostile quadcopters on the ground.