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Standard User Thinker27
(newbie) Sun 14-Feb-21 00:09:31
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Re: Mechanical Vibration from Fibre Cable


[re: Rhubarb] [link to this post]
 
I think that the fascia is a red herring. There is no need to postulate a soundboard or the need for a stiffer fixing. As the OP says, the sound could simply be conducted through the fairly rigid materials of the building. (The air in the room might resonate, in which case there might be louder and quieter positions in the room.)

The thing to focus on is the source of the sound which is the violent vibration of the cable, which must be caused by the wind. I wouldn't know the mechanism, but there is presumably some consistent effect (e.g. the formation of vortices which exert a periodic suction on the cable). One needs to seek a way to disrupt this response (since you can't stop the wind). Ways of doing this depend on the vibration mode, which as it is visible may be transverse vibrations. Changing the tension in the cable might make a difference, as might fixing a weight to the cable some distance out from the fixing. Or mounting the fixing on a spring and dashpot (like a shock absorber) to take the energy out of the vibration.

It will be interesting to hear if there is a 'proper' solution.

In case anyone hasn't seen the Tacoma Narrows film showing an effect of wind-induced vibration, here is one version https://www.youtube.com/watch?v=j-zczJXSxnw
Standard User Pheasant
(committed) Sun 14-Feb-21 01:18:43
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Re: Mechanical Vibration from Fibre Cable


[re: Rhubarb] [link to this post]
 
OK. Just tossing some ‘blue sky’ ideas out there...

Stockbridge dampers (tuned mass dampers):
https://www.pfisterer.com/fileadmin/pfisterer/downlo...

Anti galloping devices (for really long) power lines:
https://arproducts.org/wp-content/uploads/2021/01/AR...

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Standard User Zarjaz
(eat-sleep-adslguide) Sun 14-Feb-21 07:37:01
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Re: Mechanical Vibration from Fibre Cable


[re: Rhubarb] [link to this post]
 
I cannot imagine how this problem can be resolved but I'm wondering if anyone else has had the same and managed to do something? Does OR have any solutions that might help? I am wondering about some form of damper that could be added near the anchor. Any info appreciated.

The span is too taught. It needs to be loosened, ever so slightly.

Whereabouts in the country are you ?


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Standard User Pheasant
(committed) Sun 14-Feb-21 08:03:18
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Re: Mechanical Vibration from Fibre Cable


[re: Zarjaz] [link to this post]
 
Probably the cold snap hasn’t helped. It might have been tensioned on the high side and with the added drop in temps, it’s now too taught as you say. Add some wind at just the right speed and it’s humming.

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Standard User Rhubarb
(committed) Sun 14-Feb-21 08:49:42
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Re: Mechanical Vibration from Fibre Cable


[re: Zarjaz] [link to this post]
 
In reply to a post by Zarjaz:
The span is too taught. It needs to be loosened, ever so slightly.

Possibly so but it's lower than the other non fibre cables - I appreciate it'll be heavier. It passes over the outside corner of a bend in the road.so I don't know how much slacker it could be. The other cables are just waving around in the wind, my cable is fidgeting about erratically.
In reply to a post by Zarjaz:
Whereabouts in the country are you ?

NE Wales

Andy

Edited by Rhubarb (Sun 14-Feb-21 10:25:49)

Standard User Zarjaz
(eat-sleep-adslguide) Sun 14-Feb-21 09:46:24
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Re: Mechanical Vibration from Fibre Cable


[re: Rhubarb] [link to this post]
 
NE Wales
Ah, that’s an idea stuffed then.

It literally is the tension on the span, so if exactly right, the vibration caused by the wind makes it resonate like a guitar string.

I’d suggest contacting whoever you ordered via, and have them send Openreach back out to sort it.

Standard User Rhubarb
(committed) Sun 14-Feb-21 10:19:58
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Re: Mechanical Vibration from Fibre Cable


[re: Zarjaz] [link to this post]
 
In reply to a post by Zarjaz:
I’d suggest contacting whoever you ordered via, and have them send Openreach back out to sort it.

Yep. I'm going to get onto that tomorrow. The order's not completed yet (BT) so I hope I have a better chance of getting something done. Cheers - Andy

Quick edit: As an aside, can you tell me what the minimum height is for a drop cable over a road that could have 4.9 metre vehicles on it?

Edited by Rhubarb (Sun 14-Feb-21 10:25:01)

Standard User TheInstaller
(learned) Sun 14-Feb-21 10:45:07
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Re: Mechanical Vibration from Fibre Cable


[re: Rhubarb] [link to this post]
 
Minimum height for any drop wire going over a normal road would be 5.2m but 5.9m or higher is preferred.

To resolve the galloping of your drop wire, OR can add some twists to the drop wire, this is something that has to be done to the incoming fibre only drop wires. They say one twist for roughly every 10m of cable will do the job. This can be done easily from the pole end.

The reason for the vibration or galloping is the flat profile of the cable.

Hope this helps.
Standard User Rhubarb
(committed) Sun 14-Feb-21 11:13:59
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Re: Mechanical Vibration from Fibre Cable


[re: TheInstaller] [link to this post]
 
In reply to a post by TheInstaller:
Minimum height for any drop wire going over a normal road would be 5.2m but 5.9m or higher is preferred.

To resolve the galloping of your drop wire, OR can add some twists to the drop wire, this is something that has to be done to the incoming fibre only drop wires. They say one twist for roughly every 10m of cable will do the job. This can be done easily from the pole end.

The reason for the vibration or galloping is the flat profile of the cable.

Hope this helps.

Thanks for that.

Picking up on the 'galloping' comment, I can pretty much see down the cable from the window and in certain wind condition I can see waves forming down the line as well as seeing the profile change as the cable twists due to the irregular shape.

Andy

Edited by Rhubarb (Sun 14-Feb-21 11:38:09)

Standard User Pheasant
(committed) Sun 14-Feb-21 15:36:10
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Re: Mechanical Vibration from Fibre Cable


[re: Rhubarb] [link to this post]
 
Apparently there's a whole branch of physics devoted to the study of what's known as "Aeolian vibration" and is of great interest to grid / transmission line operators.

Usually demonstrated in one one of two ways apparently; "gallop" or "flutter". Gallop also known as 'conductor dancing' according to Wikipedia and is a low frequency (about 1 Hz) and high in amplitude oscillation - hence the dancing bit. As opposed to flutter, which is also resonant but of high(er) frequency (around 10 Hz) and low amplitude.

Your issue sounds, pardon the pun, more like "flutter" - which in larger structures and long transmission lines, apparently can be cured by fitting so called mass dampers (I always think of the millennium bridge in London) or Stockbridge dampers.

Sounds like in your case some easing of the tension in the catenary/drop, or maintaining the return twists as noted above will have a similar dampening effect on the wire.

Good luck.

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