Here's a long, slow video of the VLF activity walking across 40th street bridge in Pittsburgh. All the sounds are from the Beleboke.
Couple of interesting observations:
1. You can hear clearly the characteristic hum of the electric grid powering the bridge structure itself, but it is not as comprehensive and complicated as the sounds on the street. This makes sense: there are fewer cables and power sources across the river.
2. I realized that most but not all passing cars emit a unique signature which is captured as a fleeting high pitched singing gesture.
3. The 40 street bridge is decorated with plaques showing each of the state flags. perhaps one future walk I'll return and explore the resonant characteristics of the metal.
I did a spectrographic analysis of the sound: this is a visual representation of the frequency fundamental and harmonics present over time:
other pictures from the walk: