How do hummingbirds hum?
This is what engineers at the Eindhoven University of Technology (TU/e) and others have investigated. How do these birds, with their fast wingbeat, make their characteristic low-humming sound? “The team succeeded in measuring the precise origin of the sound generated by the flapping wings of a flying animal for the first time,” reads a news article on the TU/e website.
It’s well known that hummingbirds beat their wings very quickly – up to 40 times per second. Less is known about how the sound that wingbeat creates occurs. To figure this out, the TU/e, Sorama, and Stanford University joined forces.
The researchers examined the birds in detail. They not only used 12 high-speed cameras and six pressure plates. They also used no fewer than 2,176 microphones.
Same as insects
The study shows that hummingbirds produce sound in the same way as insects. Both use their complex wings for this. Most birds stay in the air by generating a downward force. They do so by flapping their wings.
Hummingbirds, like insects, also generate flight force when they flap their wings up. “This is the reason why birds and insects make different sounds,” explains Professor David Lentink of Stanford University in the TU/e article. “Most birds are relatively quiet.”
“That’s because they generate most of the lift only once during the wingbeat. That’s at the downstroke. Hummingbirds and insects are noisier because they do so twice per wingbeat.”
An essential part of the research was an ‘acoustic camera’. Soroma developed this. This is a spin-off company resulting from TU/e research.
The TU/e article explains that to arrive at their model, the scientists examined six Anna’s hummingbirds. These are the most common species around Stanford. “They had each of the birds drink sugar water,” the article states.
“That was from a fake flower in a special flight chamber. Cameras, microphones and pressure sensors were set around the chamber. These weren’t visible to the birds.” These devices precisely recorded each bird’s wingbeat.
“We used these cameras to make the sound visible. We could then examine it in detail,” Sorama researcher and CEO, Rick Scholte says. It takes quite some doing to use a camera like that.
Sound you can see
“For this experiment, we connected an optical camera to a network of 2,176 microphones. Together, they work a bit like a thermal-imaging camera. We make the sound visible in a kind of heat map.”
“You can see the 3D sound fields in detail,” Rick says. They can use the knowledge gained from these hummingbirds to make other devices quieter. Think of planes and drones, but also laptop fans and vacuum cleaners.
“Noise pollution is becoming increasingly problematic. A decibel metre isn’t going to solve that. You need to know the noise’s location and how it’s produced. Then you can get rid of it.”
“That’s where our sound cameras come in. This hummingbird wing research provides a completely new, very accurate model form where to start. Now, we can do our job even better,” Rick concludes.
Editor: Melinda Walraven