High Resolution Sound Cameras Reveal Why Hummingbirds Hum

Hummingbirds hum. It's right there in the name. And it seems fairly straightforward to say that the reason they hum is because they've got a pair of tiny wings that are whipping back and forth forty times every second. A more accurate thing to say, however, is that those lil' wings are generating sound, and that presumably they're doing so in a similar way to the wings of other birds, insects, bats, and even winged robots. But all of these sounds are very different; and why hummingbirds actually hum in that distinctive way when other flying things don't is not obvious at all.

Researchers from Eindhoven University of Technology, Stanford University, and an acoustic camera company called Sorama teamed up to study half a dozen hummingbirds using 2176 microphones. They then spent three years analyzing the data to determine the source of, as one researcher put it, a psychoacoustic auditory experience that many people find pleasing." In doing so, they've managed to come up with a model of how flapping wings make the sounds that they do, potentially leading to (among other things) quieter, more efficient drones.

Video: Lentink Lab

The motivation for this research, as far as I can make out, is pure basic scientific curiosity: Why the heck do hummingbirds sound the way that they do? And in particular, what causes the hummingbird's hum to be pleasant, and what could that level of understanding tell us about other things with flapping wings, some of which (like flies and mosquitoes) are much less pleasant to listen to.

Noise often has a spectrum that humans find annoying- this is subjective-and [it's] called psychoacoustics," said Rick Scholte, one of the authors of a paper published today in the journal eLife, when we asked him to describe the noise that a hummingbird makes. In contrast, the hummingbird's hum has a specific spectrum [comprising] many harmonic tones, frequency peaks, that are integer multiples of the first harmonic, and together result in a psychoacoustic auditory experience that many people find pleasing."

The harmonics in the hummingbird's hum are primarily caused by the pressure waves radiated from changing lift and drag forces as the wings flap back and forth. These harmonics are unique among birds, because a hummingbird generates lift with both the upstroke and the downstroke of its wings. While other birds generate a base wingbeat frequency that tops out at about 10 hertz (which we can't hear), a hummingbird's 40 Hz base wingbeat frequency is within the lower range of human hearing, giving a nice low foundation that the harmonics can build on. The real trick, though, is figuring out exactly what those harmonics are, and exactly where they came from. And for that, you need some hummingbirds, some high speed cameras, and some microphones.

Lots of microphones.

Photo: Sorama A high resolution microphone arrays, high speed cameras, and pressure sensors capture hummingbird flight from all directions.

This is an array of 12 high-speed cameras, 6 pressure plates, and 2176 microphones, which were provided by a company called Sorama that specializes in very-high-resolution acoustic imaging. At the center of it all there is, occasionally, an Anna's hummingbird. The hummingbird, which has been birdnapped for a few hours, may fly up and sip nectar from a plastic flower whenever it likes, and as it does so, a truly astonishing amount of extraordinarily high resolution data are collected about the hum that it makes. The hummingbird is then returned to the wild, and the researchers get to work synchronizing the camera, microphone, and pressure data to pinpoint exactly what part of the hummingbird's wing is generating what sounds and when. For the first time," Sorama CEO Rick Scholte says, we temporally and spatially resolved the aerodynamic source of the hummingbird's hum in exquisite detail."

Understanding those aerodynamics, embodied in the 3D oscillating lift and drag forces generated by the hummingbird's wings, allowed the researchers to develop from first principles a generalized 3D acoustics model for flapping wings. The model accurately predicts the acoustic spectrum of a wing based on wing size, wingbeat frequency, stroke amplitude and weight support profile. The model isn't hummingbird specific, and in fact, it's not even bird specific; the researchers discovered that their model can provide valuable insight into the wing sounds of insects, bats, and even flying robots that use flapping wings. This research gives the insight to make robotic hummingbirds sound more like and perform more like the real thing," Scholte says. For the robot to generate the same lift and drag force we measured during the wingbeat of a real hummingbird, the wing kinematics need to be the same. And since flight performance directly depends on the lift and drag the wing generates, basically, what you hear is what you get in terms of the aerodynamic performance of flapping wings."

Having a general mathematical model of flapping wing acoustics is obviously super important, but personally, I keep coming back to what inspired this research in the first place- that hummingbirds are uniquely pleasant to listen to. Why humans like the humming sound is not part of this research paper," Scholte told me, but since we had to listen to the humming of hummingbirds for many days and study it in detail over years, we can confirm it remains pleasing to listen to over and over again." Based on all that research, Scholte broke it down into three components:

• The first hum frequency tone, at the wingbeat frequency of 40 Hz, isn't too low- as in many larger birds-to be heard by humans. And at the same time, it is not too high, as is the case with flies and other small insects that humans find annoying.
• The volume is audible but modest for the hummingbird's size, so not overly loud. This is because the bird has large wings that carry its weight, so the wings do not need to beat too fast to generate the aerodynamic forces required to lift its weight in hover.
• The spectrum has many higher harmonics, and the first and second are of similar amplitude acoustically speaking, after which the amplitude goes down for higher harmonics, which we measured clearly up to 400 Hz (the tenth harmonic of the wingbeat frequency).

Beyond these facts about a hummingbird's hum, we start to get into the more subjective area of psychoacoustics, but that's interesting as well, if a bit more speculative, as Scholte suggests. A fair hypothesis explaining the difference in human wing sound perception is that, whereas interactions with birds do not pose an immediate threat for human health, interactions with mosquitos and some other smaller insect species do represent a threat. So, over the hundreds of millions of years during which our ancestors evolved, we humans developed a keen awareness for buzzing and whining sound of insects that may be detrimental to our health or inflict pain." This, Scholte hypothesizes, could potentially have created an evolutionary selection pressure towards humans who react negatively to insect noise.

Photo: Evan Ackerman/IEEE Spectrum

Of course, it's more than just the sound that makes hummingbirds so appealing. They're also beautiful, fantastically talented fliers that are a joy to watch. It's awesome that these tiny birds have been able to make such a huge contribution to the science of sound and of flight. And the research doesn't stop here, says Scholte. There are always more questions, but new acoustic arrays need to be developed with even finer spatial resolution. Going deeper requires going beyond the state-of-the-art, which is an ongoing technology push at Sorama. Everyone is aware of the power of a single microphone for understanding sound, the wide number of societal and engineering applications for combining thousands of microphones is particularly enticing. Because as our study shows, it can help resolve complex questions, even ones based on curiosity, like how do hummingbirds hum."

How Oscillating Aerodynamic Forces Explain the Timbre of the Hummingbird's Hum and Other Animals in Flapping Flight, by Patrick Wijnings, Sander Stuijk, and Henk Corporaal of TU Eindhoven, Rick Scholte of Sorama, and Ben Hightower, Rivers Ingersoll, Diana Chin, Jade Nguyen Daniel Shorr, and David Lentink of Stanford University, appears on 16 March in the journal eLife.