The hummingbird is named after its pleasant humming sound when it hovers in front of flowers to feed. But only now has it become apparent how the wing creates the hummingbird’s namesake sound when it is beating rapidly at 40 beats per second.
Researchers in Eindhoven University of Technology, Stanford University, and Sorama meticulously observed hummingbirds using 12 high-speed cameras, six pressure plates, and 2,176 microphones. The group of engineers succeeded in measuring the precise origin of the sound generated by the flapping wings of a flying animal for the first time.
They found that the soft and complex feathered wings of hummingbirds create sound in a fashion similar to how the easier wings of insect perform. The new insights could help make devices like fans and drones quieter.
The hummingbird’s hum originates from the pressure difference between the topside and bottom of the wings, which affects both in magnitude and orientation as the wings flap back and forth. These pressure differences across the wing are essential, since they furnish the net aerodynamic force that permits the hummingbird bird to liftoff and hover.
Unlike other species of birds, a hummingbird wing generates a strong upward aerodynamic force during both the downward and upward wing stroke, so twice per wingbeat. Whereas both pressure differences due to the lift and drag force acting on the wing contribute, it turns out that the upward lifting pressure difference is the principal source of the hum.
The difference between whining, humming, and wooshing
Professor David Lentink of Stanford University said, “This is why birds and insects make various sounds. Mosquitoes complain, bees buzz, hummingbirds hum, and larger birds ‘woosh’. Most birds are rather quiet because they generate most of the elevator only once during the wingbeat at the downstroke. Hummingbirds and insects are noisier because they do so twice per wingbeat. ”
To arrive at their version, the scientists analyzed six Anna’s hummingbirds, the most frequent species around Stanford.
One by one, they had the birds drink sugar water from a fake flower in a special flight chamber. Around the chamber, not visible to the bird, radios, cameras, and pressure sensors were set up to exactly record each wingbeat while hovering in front of the flower.
During a follow-up experiment, six highly sensitive pressure plates eventually was able to capture the lift and drag forces generated by the wings as they moved up and down, a first.
The researchers eventually was able to condense all their various benefits in a simple 3D acoustic version, borrowed from the world of planes and mathematically adapted to flapping wings. It predicts the noise that flapping wings radiate, not only the hum of the hummingbird, but also the woosh of different birds and birds, the buzzing and whining of insects, as well as the noise that robots with flapping wings create.
Making drones quieter?
Even though it wasn’t the focus of the study—published in March in the journal eLife—the knowledge gained may also help improve aircraft and drone rotors as well as notebook and vacuum cleaner fans. The new insights and tools can help make engineered devices that generate complex forces like animals do quieter.
This is just what Sorama aims to do: “We create sound visible in order to produce appliances quieter. Noise pollution is becoming an ever-greater problem. And a decibel meter alone is not going to address that. You need to know where the noise comes from and how it’s produced, in order to have the ability to eliminate it. This ’s what our sound cameras are for. This hummingbird wing research gives us a completely new and very accurate model as a starting point, so we can do our work much better, concludes CEO and researcher Rick Scholte of Sorama, a spin-off of Eindhoven University of Technology.
If the motions of hummingbirds can give us quieter technology from the future? Well, we’re absolutely here for it.
(WATCH the investigators ’ video all about how hummingbirds hum)
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