New measurement technique unravels what gives hummingbird wings their characteristic sound
Eindhoven University of Technology
Researchers from Eindhoven University of Technology, Sorama, a TU/e spin-off company, and Stanford University meticulously observed hummingbirds using 12 high-speed cameras, 6 pressure plates and 2176 microphones.
They discovered that the soft and complex feathered wings of hummingbirds generate sound in a fashion similar to how the simpler wings of insect do. The new insights could help make devices like fans and drones quieter.
The team of engineers succeeded in
measuring the precise origin of the sound generated by the flapping wings of a
flying animal for the first time. The hummingbird's hum originates from the
pressure difference between the topside and underside of the wings, which
changes both in magnitude and orientation as the wings flap back and forth.
These pressure differences over the wing are essential, because they furnish
the net aerodynamic force that enables 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 primary source of the
hum.
The difference between whining, humming, buzzing and wooshing
Professor David Lentink of Stanford
University: "This is the reason why birds and insects make different sounds.
Mosquitoes whine, bees buzz, hummingbirds hum, and larger birds 'woosh'. 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."
The researchers combined all measurements in a 3D acoustic model of bird and insect wings. The model not only provides biological insight into how animals generate sound with their flapping wings, it also predicts how the aerodynamic performance of a flapping wing gives the wing sound its volume and timbre.
"The distinctive sound of
the hummingbird is perceived as pleasant because of the many 'overtones'
created by the varying aerodynamic forces on the wing. A hummingbird wing is similar
to a beautifully tuned instrument," Lentink explains with a smile.
High-tech sound camera
To arrive at their model, the
scientists examined six Anna's hummingbirds, the most common 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,
cameras, microphones and pressure sensors were set up to precisely record each
wingbeat while hovering in front of the flower.
You can't just go out and buy the
equipment needed for this from an electronics store. CEO and researcher Rick
Scholte of Sorama, a spin-off of TU Eindhoven: "To make the sound visible
and be able to examine it in detail, we used sophisticated sound cameras
developed by my company. The optical cameras are connected to a network of 2176
microphones for this purpose. Together they work a bit like a thermal camera
that allows you to show a thermal image. We make the sound visible in a 'heat
map', which enables us to see the 3D sound field in detail."
New aerodynamic force sensors
To interpret the 3D sound images, it
is essential to know what motion the bird's wing is making at each sound
measurement point. For that, Stanford's twelve high-speed cameras came into
play, capturing the exact wing movement frame-by-frame.
Lentink: "But that's not end of
story. We also needed to measure the aerodynamic forces the hummingbird's wings
generates in flight. We had to develop a new instrument for that." During
a follow-up experiment six highly sensitive pressure plates finally managed to
record the lift and drag forces generated by the wings as they moved up and
down, a first.
The terabytes of data then had to be
synchronized. The researchers wanted to know exactly which wing position
produced which sound and how this related to the pressure differences. Scholte:
"Because light travels so much faster than sound, we had to calibrate each
frame separately for both the cameras and the microphones, so that the sound
recordings and the images would always correspond exactly." Because the
cameras, microphones and sensors were all in different locations in the room,
the researchers also had to correct for that.
Algorithm as a composite artist
Once the wing location, the
corresponding sound and the pressure differences are precisely aligned for each
video frame, the researchers were confronted with the complexity of
interpretating high volume data. The researchers tackled this challenge
harnessing artificial intelligence, the research of TU/e PhD student, and
co-first author, Patrick Wijnings.
Wijnings: "We developed an
algorithm for this that can interpret a 3D acoustic field from the
measurements, and this enabled us to determine the most probable sound field of
the hummingbird. The solution to this so-called inverse problem resembles what
a police facial composite artist does: using a few clues to make the most
reliable drawing of the suspect. In this way, you avoid the possibility that a
small distortion in the measurements changes the outcome."
The researchers finally managed to
condense all these results in a simple 3D acoustic model, borrowed from the world
of airplanes and mathematically adapted to flapping wings. It predicts the
sound that flapping wings radiate, not only the hum of the hummingbird, but
also the woosh of other birds and bats, the buzzing and whining of insects and
even the noise that robots with flapping wings generate.
Making drones quieter
Although it was not the focus of
this study, the knowledge gained may also help improve aircraft and drone
rotors as well as laptop and vacuum cleaner fans. The new insights and tools
can help make engineered devices that generate complex forces like animals do
quieter.
This is exactly what Sorama aims to
do: "We make sound visible in order to make appliances quieter. Noise
pollution is becoming an ever-greater problem. And a decibel meter alone is not
going to solve that. You need to know where the sound comes from and how it is
produced, in order to be able to eliminate it. That'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 even better,"
concludes Scholte.
This research appears on March 16 in
the journal eLife, under the title "How Oscillating
Aerodynamic Forces Explain the Timbre of the Hummingbird's Hum and Other
Animals in Flapping Flight."