Hummingbirds are compact, energetic flying machines belonging to the family Trochilidae. They typically measure 7.5–13 cm (3–5 in) in length, boast colorful, almost jewel-like plumage, and feed on flower nectar. They are also known for agility, flexibility, and speed, which is how they came to inspire the QPS logo. The birds eat eight times their body weight each day by flying to and from 1,000 flowers and drinking the nectar they need. They are also uniquely adaptable flyers, as the only birds that can move backward, forward, in place, sideways, and upside down.
This flying style involves wingbeats between 40 and 200 beats per second and gives off a soft, characteristic humming sound — quite different than the sound of other birds or insects, and often much softer and more pleasant than propellers on drones, airplanes, and even fan blades.
How are these unique sounds produced? Scientists from Sorama, a spinoff of Eindhoven University of Technology, and Stanford University discovered the secrets behind the hummingbird’s hum by making detailed observations with 12 high-speed video cameras and 2,176 microphones. Their results showed how the unique structure of the hummingbird’s wings, coupled with pressure differences between the top and bottom of wings during flight, produces its humming. The study, which appeared in the March 16 issue of eLife, could help engineers develop quieter propeller-driven machines, such as drones or fans.
The scientists looked at (and listened to) six Anna’s hummingbirds, which are commonly found on Stanford’s California campus. While the birds drank sugar water in a flight chamber, the microphones and pressure sensors took a precise record of every beat of the bird’s wings. The high-speed cameras, meanwhile, could catch an image of exact wing movement, one photographic frame at a time. The researchers also designed pressure plates that could measure lift and drag, forces generated by the wings as they moved.
After collecting sound, pressure, and video data, the researchers had to precisely align each location, sound, and pressure change with each video frame, so they could determine which wing position resulted in which sound, and how pressure differences during flight produced the sounds.
Manipulating the data (which amounted to terabytes) required artificial intelligence. The Eindhoven researchers developed an algorithm that interpreted the data inputs and created an acoustic model.
The researchers discovered that hummingbirds have a flying style like no other bird. When a hummingbird beats its wings, it generates an upward force during both its downward and upward wing strokes. Other birds only generate pressure differences during the downward stroke. The upward lifting pressure produces the hum, known as a harmonic. Harmonics are a series of sound frequencies that blend together to produce the familiar sounds of a musical instrument.
Hummingbird wings and insect wings behave in similar ways. Mosquitoes, flies, and moths also produce characteristic sounds associated with higher harmonics. “Most birds are relatively quiet because they generate most of the lift only once during the wingbeat at the downstroke,” said David Lentink, assistant professor of mechanical engineering at Stanford and one of the paper’s authors. “Hummingbirds and insects are noisier because they do so twice per wingbeat.”
The research also showed the complex aerodynamic pressures and movements that all add up to the hummingbird’s unmistakable sound during flight. “The distinctive sound of the hummingbird is perceived as pleasant because of the many overtones created by the varying aerodynamic forces on the wing,” Lentink said. “A hummingbird wing is similar to a beautifully tuned instrument.”
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