For months, Russia unleashed waves of Iranian-made Shahed drones against Ukrainian cities. Small, slow and flying low, they often slip under radar. But they have one weakness: the unmistakable buzz of their gasoline engines. Ukraine has turned that sound into a weapon.

Across the country, thousands of low-cost acoustic sensors, some originally built from smartphones bolted to 6-foot poles, now form a vast listening network. Trained with artificial intelligence to recognize the Shahed’s signature hum, these sensors detect drones that radar can’t see.

The data is fed, in real-time, to mobile teams armed with heavy machine guns and anti-aircraft cannons. Instead of million-dollar missiles, defenders are using bursts of relatively inexpensive projectiles to bring the drones down.

Now, Europ

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