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
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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 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)
For what it's worth: I counted about 85 or 86 "clicks" in 10 seconds. It's a loud click followed by a quieter click, like as if it's oscillating towards and away from you. The sound of the click itself is loudest at about 2.6 khz - whether that is simply the sound of friction, or some sort of electrical phenomenon, I don't know.
The fuzzy area at the bottom half of the spectrogram is the dull roar of distant wind. The clicks themselves show up as spikes, and the intense colors on the right are from where the voice starts speaking. The dark band above 10K is just the data lost from audio compression.
Well, I can say definitively that I know what is making that clicking sound. It's hard to see since the cable is in silhouette, but there are silver-colored spirals wound around the cable, and the sound is made by the plastic sheathing of the black cable wobbling inside of those metal spirals. The spirals are made of aluminum, I'm pretty sure. Those spirals are put there to stiffen the the hanging cable, and appear on the hanging cable between every set of poles (not just these, that are wiggling). There are two spirals mounted on each cable between the poles. I assume the spirals are mounted there to provide damping, just in case the wind does cause the cable hanging between the poles to swing too much. But, there was no wind blowing when I shot this video (Dec 5, 2023). The voice you hear is mine, just speculating on what might be causing the oscillating cable...
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.
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.
You're sure there's no squirrel hiding there taunting you and your dog?
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 :)