Dutch and American scientists unlock mystery of what makes the hummingbird hum

Hummingbirds beat their wings 40 times a second

Credit:  CARLOS ORTEGA/EPA-EFE/Shutterstock

Scientists have developed an acoustic model that solves one of nature’s mysteries, how the hummingbird makes its hum.

Research by Eindhoven University of Technology spin-off company Sorama and Stanford University’s professor David Lentink used 12 high-speed cameras, six pressure plates and 2,176 microphones to measure how the wings of six Anna’s hummingbirds created their soporific sound.

Previously, scientists were unsure whether it was the sound of the air through the birds’ feathers, vortices in the air flow or air pressure changes that generated the noise.

The paper, published on Tuesday in the journal eLife, explains that the hovering birds make their pleasant hum from the pressure difference between the top and underside of the wings, primarily via upward lifting pressure.

The birds beat their wings on average 40 times a second, and the team found that hummingbirds’ wings create lift both on the up and down stroke of their wings, which follows an unusual U-shape. Most birds’ wings only create lift on the down stroke.

“This is the reason why birds and insects make different sounds,” said Prof Lentink. “Mosquitoes whine, bees buzz, hummingbirds hum, and larger birds ‘whoosh’. Most birds are relatively quiet because they generate most of the lift only once during the wingbeat at the downstroke. Hummingbirds and insects are noisier because they do so twice per wingbeat.”

Using the measurements, the researchers created a 3D acoustic model of bird and insect wings to show how animals generate sound and predict the kind of volume and timbre each flap will make.  

Rick Scholte, chief executive of the acoustic consultancy Sorama, said these insights will be applied to help make household and transportation devices less noisy, and also to make their buzzing actively pleasant.

“We produced a model that explained how the sound was being produced, connecting the maths with the back-calculated 3D pressure fields to explain how the hummingbird creates its typical hum,” he said. “These insights can be literally translated to understanding how a ventilator or a drone propellor makes sound. They still need to move air, so there is no way they can make no sound.

"What we can do is really observe nature and sounds we like: the sound of the sea crashing or leaves whistling. Since most people like hummingbirds and find them soothing, we can take this model and use it to improve the sound design of products around us, like a ventilator or drone.”