Did you measure the distance between the poles? I suspect it's different from all the other spans, so this one happens to have a resonant frequency that exactly matches whatever vibration source is already there (could be the tension too). As for sources of vibration, wind is probably it, even if it's not strong. If it just happens to create the right frequency, the cable will vibrate just like a violin string.
this post was submitted on 21 Dec 2023
37 points (95.1% liked)
Mildly Interesting
17145 readers
631 users here now
This is for strictly mildly interesting material. If it's too interesting, it doesn't belong. If it's not interesting, it doesn't belong.
This is obviously an objective criteria, so the mods are always right. Or maybe mildly right? Ahh.. what do we know?
Just post some stuff and don't spam.
founded 1 year ago
MODERATORS
I immediately thought of the original Tacoma Narrows Bridge, which collapsed after wind caused a resonance to build up and literally shook itself apart: https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)
No, I've never tried to quantify the variables in that way. Just out walking the dog, and notice this strange behavior from time to time. I always assumed the poles were placed a specific distance apart, but honestly, I'm not sure. I suppose if I ever have the urge to pace it out to get a good estimate, I will...
I work with telecom cables. This happens when the cable isn't rotated when hung. If it's hung without any rotations the wind catches it and it'll dance like this
I guess my only question is, why is this happening when there is literally no wind blowing?
Found this: https://electronics.stackexchange.com/questions/253607/why-weights-on-cables-between-utility-poles
Probably some combination of electromagnetic effects in that specific section. Happens to oscillate just right. Though these weights can be used for wind issues, I'm seeing a lot of references to dampening other kinds of vibrations. Might help you find a better answer.
Hmm. It's odd that this isn't ALWAYS vibrating, but it IS only between these two poles that I've noticed the vibration. But why not between other sets of poles? And since the cable vibrating the most isn't a power cable (I believe it's fiber, but it could be copper - but it's definitely telecom, since it's not insulated on the top crossbar, as the other power cables are), it can't really be much power going through it. I'm kind of hoping someone else has seen similar behavior somewhere else. You can see the anti-wind-twist devices (don't know what else to call them) bolted to the other telecom cable (sort of diamond-shaped) and they will cause the cable to stabilize when the wind is really blowing, but you can see the wind isn't blowing at all, and the cable continues to vibrate quite noticeably. I really do hope someone else has seen something like it elsewhere...
In regards to it only being between those two poles, I'd wager it simply comes down to tension. The other cables are tensioned correctly and this section is either too loose or intentionally loose as some sort of expansion/contraction relief.
You're sure there's no squirrel hiding there taunting you and your dog?
Look up the following: “aeolian vibration“.
This is most likely due to a differing tension between that span and the spans on either side of it. It has more “sag” (even if only fractionally different) therefore it is more susceptible to what I referenced above.
I assume that line is communication, and the tension on them is enormous. Did you happen to get a closer look at the spiralling on it? I can’t tell by this video if it’s armouring or a grip.
This is indeed very interesting. I can reccomend asking the telecom company that owns the line with a quick email as to whats going on here, they usually like to answer such questions. Maybe post in a sub thats more familiar with mechanics/electronics or even general asklemmy?
Perhaps that section is exactly centered between two different sources of vibration, so the vibrations meet and amplify at that point?
i love that this is a mystery that the internet cannot easily solve, thank you for sharing :)