Eindhoven research into hummingbirds could make drones quieter in the future

Researchers from Eindhoven University of Technology, TU/e ​​spin-off Sorama and Stanford University have used new measuring technology to discover how hummingbirds make noise with their wings. That insight can contribute to making fans, windmills and drones quieter, the researchers say.

Hummingbirds are known for the distinctive sound their wings make. The hum is more like that of insects than the flapping of wings of other birds. Researchers from Eindhoven University of Technology, the company Sorama and Stanford University, led by professor David Lentink, observed the animals in a special measurement setup with twelve high-speed cameras, six printing plates and 2176 microphones. They found that the hummingbird’s wings do indeed make noise in the same way as insects.

It is the first time that researchers have succeeded in determining the origin of the wing sound of a flying animal, Eindhoven University of Technology writes in a news item. The researchers write that the sound of the hummingbird is caused by the pressure difference between the top and bottom of the wings, which changes as the wings flap up and down. These pressure differences allow the hummingbird to hover still in the air.

The hummingbird is the only bird that generates a strong aerodynamic force upwards during both the upward and downward wing beat. The upward lift is the primary reason for the bird’s characteristic wing sound. And that makes it more like how insects flap their wings. Other birds only generate lift on the downward wing beat.

The experimental test set-up in Lentink’s lab at Stanford University. Photo: Lentink Lab / Stanford

For the study, the researchers built a special test set-up where a hummingbird could drink sugar water from a fake flower. While drinking, the bird was monitored by an arrangement of cameras, microphones and pressure sensors, so that each wing beat could be recorded. This enabled the researchers to create a 3D sound map of the wing beats. The lift and resistance pressure of the wings were also measured using pressure plates. The researchers developed an algorithm that can interpret a 3D sound field from the measurements.

The way the researchers gained insight into the aerodynamics of a hummingbird’s wings, and subsequently the way the researchers were able to make sound visible, could also improve airplanes, drones and fans of laptops and vacuum cleaners, the researchers say. “If you know how an animal’s complex aerodynamic forces produce sound, you can use that knowledge to make flying or moving devices that generate complex forces quieter,” they write.