Drones can pose a security risk to airports, critical infrastructure, military facilities, and large-scale events. However, they are difficult to detect when they fly beyond visual line of sight, for example, when visibility is impaired by terrain, buildings, or weather.

When an unmanned aerial vehicle approaches a sensitive area, every second counts. Security personnel need to determine as early as possible whether a drone is approaching, where it is located, and how the situation is developing. Conventional detection methods provide important information, but they are not equally reliable in every environment: lines of sight may be obstructed, and radio signals may be disrupted. Acoustic sensing offers an additional detection channel. As a complementary component used together with radar, cameras, and lidar, it can make existing security systems more robust and provide indications where optical or radio-based methods reach their limits.

Identifying drones based on their acoustic fingerprint

Researchers at the Fraunhofer IDMT's Oldenburg Branch for Hearing, Speech and Audio Technology HSA analyse the acoustic signals emitted by drones. Each drone produces a characteristic sound pattern—an acoustic fingerprint. Machine learning algorithms identify these patterns even in complex, noisy environments. Signals from multiple microphones can also be used to determine the direction from which the flying object is approaching. This allows drones not only to be detected, but also to be localised and spatially positioned.

“With our acoustic technology, we offer a cost-effective, low-maintenance complement to existing drone detection systems,” explains Christian Rollwage, group manager of Audio Signal Enhancement at Fraunhofer IDMT in Oldenburg. “Especially in built-up or forested areas, acoustics can provide decisive added value because it can detect drones even when they are not yet visible to other sensors.”

Passive and efficient

While radar and camera systems may involve high costs and require substantial computing power, acoustic sensing offers a particular advantage through its low energy demand: It passively receives sound emissions and vibrations from the environment without actively transmitting signals itself and thus remaining undetectable. The technology can be deployed across large areas and operated autonomously. It is also suitable as a “wake-up” component that activates additional sensors only when an acoustic indication of a drone is present.

In the future, this technology could also be applied to other acoustic events, such as vehicles or gunshots.The development is based on many years of research at the Oldenburg site. Since 2016, Fraunhofer IDMT has advanced the acoustic detection and localisation of drones, including through the joint projects AMBOS and ALADDIN. In further internal projects, algorithms, system components, and integration options have been continuously refined. Today, both easily integrable software components and a fully integrable system solution are available.

Drone Days is an event towards the end of August in Germany.  Designed for the drone technology community, it takes place at two unique locations—Bremen Airport (Bremenhalle) for presentations and networking, and Oldenburg-Hatten Airfield (UAS German Flight Center) for hands-on testing, discussions, and drone technologies.