The first stop in my “mid career break” was Salt Lake City, Utah. I found out on the first afternoon that the snow was deep anywhere past about 500ft above the city, which meant I was on the hunt for flat runs in the lower elevations.
One of these was the Westline Powerline Trail. There were a few flat sections with strings of powerline structures, and I noticed that if the wind was just right, they’d sing a single pure note as I ran by. There’s a lot of wind noise, but you can hear an example in the video below at around 00:26 and again at 00:41 (not the periodic metalic squeak, or the rustling jacket!).
This is awesome! It’s an example of aerodynamic flutter, and there’s a ton of fun analysis that can be done around it. The first is just to figure out what frequency the structure sings at. I wrote a script that loads the video, pulls out the audio, does a fourier transform on a number of segments, and then plots the frequency spectrum of each segment.1 If you do this with a singing segment, you get a clear peak at 758-760 Hz (call it 759 Hz).
You can play this frequency using an online tool, or just by using python to create a tone for you:1
Kool! Moving beyond this first step, something more would be to use a sense of scale, like me standing next to the base, and some assumptions about materials and joints, to figure out which part is resonating at the singing frequency.
That’s up next.