I believe it's a low-level corona discharge that can happen with HV power lines, but I'm not sure. It's usually worse when the air is real humid, like in foggy conditions, and the conduction of the air is higher. Kind of like the noise from neon signs. It's not magnetostriction, so it seems like it would have to be associated with the corona discharge from the high voltage at 60/50Hz. Anybody else know for sure?
The hum is usually a 60-cycle hum, and is probably caused by the physical motion of the wires or metal coverings on transformers, instigated by the changing EM field, exerting a force on the iron/steel. I expect this is acts like a very inefficient speaker.
On powerlines in the air, there is probably a phase difference between powerlines widely spaced or on different circuits causing the air to act like part of a giant electrostatic speaker (in the hearing range).
The hum is usually a 60-cycle hum, and is probably caused by the physical motion of the wires or metal coverings on transformers, instigated by the changing EM field, exerting a force on the iron/steel. I expect this is acts like a very inefficient speaker.
On powerlines in the air, there is probably a phase difference between powerlines widely spaced or on different circuits causing the air to act like part of a giant electrostatic speaker (in the hearing range).
I think this would not be efficient enough to be audible. The current
in a high-voltage transmission line is low and conductor spacing is large.
Furthermore, the acoustical radiation efficiency of a wire at 60 or 120 Hz
is quite low.
If a 750KV transmission line carried 30 MW, the current would be around
40 Amps. While this kind of current would make conductors slap the walls
of a conduit, they would be seperated by maybe 30 meters in the transmission
system. Forces would be very low.
It's the hiss of corona discharge modulated at the 120 Hz line rate. It's
not 60 Hz because there are two peaks and zero crossings for each
cycle.
Edit: If you are in or near a power substation, then you are likely hearing the
magnetic cores of the transformers as they vibrate at 120 Hz. They are typically quite loud.
