How do hummingbirds hum?

Hummingbirds are so named owing to the sound they create as they hover by beating their wings extremely rapidly, up to 80 times a second. Now, with the help of high speed cameras, extremely sensitive force detectors, and a vast array of microphones, the physical mechanism that causes a hummingbird to hum has been unpicked. And Eva Higginbotham heard how from David Lentink...

David - We were curious about how hummingbirds hum. So whenever you see hummingbirds fly in front of a flower, you hear this characteristic humming sound, and we were curious what was causing it. And so what we did is we developed a special measurement set up with more than 2000 microphones so that we could record the acoustic field in 3D and really figure out where it is coming from, what the origin is.

Eva - How did you get a hold of 2000 microphones? Was this like a 3d stadium that you put a hummingbird in to try and make it make its noise?

David - I love that comparison. It is a little bit like a stadium. We developed a special flight chamber and we collaborated with a company called Sorama in the Netherlands, who develops these microphones. And so invited the CEO to come to California where I worked at the time where we did this research at Stanford University, for some coffee and fun experiments where we would just record the humming sound with all these microphones. And that's really where it took off. And then the key thing is that we didn't stop with just making those recordings, but we also ended up recording the forces that the hummingbirds generate while they're hovering. And by forces I mean the aerodynamic forces that enable them to stay aloft, so the lift force, but also to drag force that they have to overcome to generate this lift force that lifts their body weight up in the air, enables them to hover perfectly still in front of the flower. And that was also a first where we measured these forces. And then we were able to show that the fluctuations of these forces as the wings beat back and forth, that those really nicely predict the acoustic field, the humming sound that we hear.

Eva - Wow. So you found that you could, by measuring the forces, you could predict what kind of sound you were going to get out the other end?

David - Yes, and that's the key thing. And to predict this we actually used a mathematical model. Now the wonderful thing about acoustics is that it's basically just governed by Newton's law of motion, but then for fluids, right? So for air. And of course it's not super simple, but it is manageable. So we had this mathematical equation that predicts how the forces would generate sounds based on first principles and what that acoustic field would look like. And then we compared that with our recordings and they matched up.

Eva - One thing I'm wondering is how did you manage to get the hummingbird to stay in your experimental setup? You can't tell it to hum, I assume, it's going to do what it wants!

David - Yes. We prefer the animals to do what they want! But the way this works is you provide a twig on which they can perch and you provide an artificial flower with unlimited sugar water that they know how to find, and then they'll just fly back and forth every 10 minutes. And as they fly over, we could already, through the acoustic microphones, see where it was flying. Because if you have about 2000 microphones, you basically have an acoustic camera. You can see where the sound is coming from. So we would see on the computer screen how the hummingbird was flying because of how its sound was moving in space. And then basically it hovers in front of the flower, and then we made our recordings. We have these over 2000 microphones, but also multiple high-speed cameras, tracking cameras. So we had a really good idea what the hummingbird was doing.

Eva - How do you go about measuring the force created by a hummingbird's wing? Because presumably it's not a very high level of force.

David - Yes, that's a wonderful question. What we did is we had a floor, but also ceiling and all sidewalls were instrumented with a very large panel made out of carbon fibre. And that was connected to four sensors that were recording the forces very fast. And by combining the pressure force exerted by the hummingbird below the hummingbird, above the hummingbird, on the sides and front and aft, by combining all of that, we have the net force it is generating within a wing beat, and wing beat resolved. And this was the first time we had a set up that could measure these forces in 3D. And the reason why we were really confident it worked is because we were able to point to the forces as being the source, and those are really easy to understand. So it wasn't the feathers rubbing that was the most important, it wasn't the turbulence generated that was the most important. It wasn't the feathers whistling or something. What we found is it's really just the wing moving back and forth and reorienting the aerodynamic force in space that generates acoustics waves, so pressure waves, and that is generating this humming sound that we hear. And so what you can do is, if you know the forces that the animal generates in flight, you can predict the sounds it's generating. And that's really the exciting part that we got out, we now have this really elegant model that can explain where wing sounds are coming from in flying animals.

Eva - What sorts of applications do you imagine this being used for?

David - What's so wonderful about this mathematical model, it's like we really developed it for a very complex wing. Bird wings are exquisite compared to, for example, the wings of aircraft or drones, but all of these wings generate noise. And what's really cool about the model is that it predicts the noise for a hummingbird wing that moves very complex, generates really complex forces, really well. So now we have a model that can be applied much more generally. If we want to make drones more quiet or fans more quiet, or actually anything that just rotates and generates forces, you can also think about things like wind turbines. So what you can now imagine is that we can design wings to have the right fluctuations in forces so that their sound, or humming sound, is more pleasing. And this is really, I think, the future for making our environment, our own environment, a little bit more pleasant. Where we design the sounds of the products and systems that we use instead of just accepting the sounds that they make so that we can enjoy more of the pleasing sounds like the hummingbird hum.