Why humming sound occur in extra high tension transmission lines?

The noise is better described as 'buzzing', rather than 'humming', and it is caused by the breakdown of the air's insulating properties, due to stress caused by electric fields set up in the vicinity of where the conductors are supported from insulators. The noise is accompanied by a pale blue discharge which becomes visible after dark.
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Why do power companies use high-voltage transmission lines?

High voltage lines are used for power transmission because the high voltage means that less current flows for a given amount of energy transmitted. The lower current flow mean (MORE)
In Physics

How does tension occur?

Tension is a force acting on a body and it occurs when the body is in a state of work.The direction of force acts on the opposite direction of work done.
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Why hissing sound in high voltage over head line?

EHV and VHV substations tend to have a signature hum; this is due to small electrical arcs. This is normal. Most of the time this will not be audible on VHV and EHV power line (MORE)

What is the reason for humming sound in transformer?

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Why the transmission line are at high voltage?

To reduce the power losses or conduction losses in transmission lines. - Yes its right. I will give a slightly improved answer. The size of coper conductor used for transmis (MORE)
In Physics

Does high pitch sound travel in a straight line?

Not neccessarily. Any radiation from a point source is omnidirectional by definition - it spreads in all directions. That said, it's easier to focus/concentrate higher frequ (MORE)