The global proliferation of drones has spurred a parallel race in counter-unmanned aircraft systems (C-UAS). While radar, electronic warfare, and kinetic interceptors dominate headlines, one of the most promising–yet often understated–developments has been the rise of acoustic sensors. These passive listening devices, once confined to research projects and niche applications, are now being deployed at scale on battlefields, tested by Western militaries, and integrated into multi-sensor defense systems. They offer a low-cost, scalable, and resilient way to detect threats that are increasingly cheap, numerous, and elusive.

Acoustic sensors operate by capturing and interpreting sound waves generated by aerial objects such as the hum of propellers, the whine of engines, or the distinct resonance of missiles in flight. Using arrays of microphones, they measure differences in the arrival time or amplitude of sounds, then process those signals to estimate the direction of the source. With multiple nodes or advanced algorithms, rough localization becomes possible. Their greatest advantage lies in their passivity: unlike radars, they emit no signals, making them harder to detect or jam. They are also inexpensive compared to high-end radars or interceptors, enabling widespread deployment across broad geographic areas.

This affordability and scalability come with limitations. Sound attenuates quickly in the atmosphere, which means acoustic sensors cannot match the range of radar. They are highly sensitive to environmental conditions, with wind, terrain, humidity, and urban noise all complicating detection. Distinguishing a small drone from birds, cars, or industrial machinery requires sophisticated signal processing and machine learning. Precise three-dimensional tracking, particularly altitude, remains elusive. For these reasons, acoustic sensors are best viewed not as a replacement for radar or electro-optical systems but rather as a complementary layer in a multi-modal network, cueing other sensors and providing redundancy when high-end systems are degraded or overwhelmed.

Ukraine has provided the most vivid real-world demonstration of acoustic detection at scale. Following the destruction of much of its radar infrastructure in the early phases of the Russian invasion, Kyiv turned to acoustic networks as a low-cost and resilient way to detect incoming drones and missiles. Systems such as the so-called Sky Fortress or Zvook networks now comprise thousands of microphones deployed across Ukrainian territory. Reports suggest that between 6,000 and 10,000 units are active, each costing only a few hundred dollars to manufacture. They are cheap enough to be deployed in large numbers, creating a mesh of “listening posts” that cover wide swaths of the country.

The network has reportedly proven effective against Iranian-made Shahed loitering munitions, cruise missiles, and other low-flying threats. Acoustic detections are integrated with radar and visual observations, then pushed to mobile anti-air teams via apps and tablets, guiding them toward interceptions. In several mass-drone attacks, the system has been credited with enabling rapid responses that neutralized the majority of incoming threats. Ukraine has also begun testing acoustic payloads mounted on reconnaissance drones themselves, such as the lightweight Weles Acoustic Sensor Payload, which can detect small arms fire at a few kilometers and artillery out to 15 kilometers. These experiments illustrate that acoustic detection is not confined to static ground nodes but can also be extended into mobile and airborne platforms, further enhancing coverage and flexibility.

The Ukrainian model has attracted close attention from NATO allies. U.S. commanders have publicly praised the effectiveness of the system and expressed interest in replicating aspects of it. American forces in Europe are already studying Ukrainian acoustic deployments as a low-cost option to augment existing air-defense networks. The appeal is clear: while interceptors and radars are expensive and often scarce, acoustic nodes can be produced and deployed at scale, filling in the detection gaps left by high-end systems.

Europe, too, has taken notice. The European Union is discussing the creation of a “drone wall” along its eastern flank to counter Russian incursions. EU defense officials have explicitly cited Ukraine’s acoustic network as a model, noting that its low cost and scalability make it a logical foundation for a continent-wide early-warning layer. The envisioned wall would integrate radars, cameras, electronic warfare, kinetic interceptors, and passive acoustic sensors, with the latter providing continuous listening coverage in low-altitude corridors where radar struggles.

Industry has also begun to integrate acoustic sensing into commercial counter-drone systems. Robin Radar, a Dutch company originally known for bird-detection radars, now incorporates acoustic nodes into its ELVIRA system, pairing radar, microphones, and optical sensors for comprehensive drone detection and classification. By combining modalities, Robin seeks to overcome the limitations of any single sensor: radar provides range and altitude, acoustic nodes confirm and classify, while cameras allow visual identification. The company has supplied radar systems to Ukraine and continues to develop hybrid solutions that fuse radar, acoustic, and AI-driven analytics. This approach reflects a broader industry trend, in which acoustic sensors are embedded in multi-sensor architectures rather than deployed as standalone detectors.

The promise of acoustic detection lies in its ability to serve as the foundation of layered defenses. Its limitations in range and precision are offset by its ubiquity, affordability, and resilience. A dense network of microphones may not tell defenders everything, but it can provide enough information to trigger other sensors, alert troops, and enable faster reactions. In a battlespace where drones cost only a few thousand dollars and arrive in swarms, traditional air-defense systems alone are insufficient. The future will likely belong to layered networks in which cheap acoustic nodes act as the first listening line, feeding data into radars, electro-optical systems, and interceptors.

The war in Ukraine has transformed acoustic detection from a research curiosity into an operational reality. Western militaries and defense industries are now grappling with how to adapt those lessons to their own contexts. The EU’s “drone wall” discussions and U.S. experiments with Ukrainian acoustic concepts suggest that listening networks may soon become a fixture in global defense. Quiet though they are, acoustic sensors may prove to be one of the most important innovations in the era of drone proliferation.