Why Do Power Lines Buzz?

Power lines produce an audible sound or buzz because they are producing something called a corona discharge that is interacting with the surrounding air. The corona discharge is a side-effect of the electric field the power line generates by carrying electricity. The discharge can be greater, and the buzzing louder if there is increased moisture or pollutants in the air.

Electric Fields
Any medium that conducts an electric current will produce an electric field. This is true of electronic devices, power cords, and power lines. An electric field is similar to the more familiar magnetic field. In fact the two types of fields are deeply related because they are both generated by the electromagnetic force.

The movement of electrons through the power line is called current, the electrons themselves carry an electromagnetic charge. An electric field radiates perpendicular from the direction of the current. A power line is a very long cylinder so the field radiates in a cylindrical shape. The strength of the field drops off in inverse proportion to the radius. In other words, the electric field is strongest nearer the line and drops off dramatically farther away.

Interaction with the Air
The electric field contains enough electricity to ionize the air. Specifically, it will ionize oxygen and nitrogen in the air. This can produce a low energy plasma, the corona discharge. A plasma is a fourth state of matter (after solid, liquid, and gas). It is like a gas or liquid, but molecules are separated into atoms, and the outer electrons are stripped off and are freed into the plasma. In comparison, the sun is essentially a big ball of very hot plasma.

The current carried in the power line alternates direction, usually 60 times a second (60Hz). This propagates to the electrical field, affecting the plasma, and producing the audible vibration of air.

Safety
The buzzing of power lines or the electric field is generally not a safety problem. You won't get zapped just by being beneath a power line. However, there is ongoing debate about the effects of long term exposure. There has been widespread paranoia about power lines causing leukemia in children, but decades of research has proven the connection inconclusive. Corona discharge is actually safe enough that it is used in home ozone generators that clean room air of contaminants, as well as air conditioners and photocopiers.

Corona discharge is undesirable though difficult to remove entirely from power lines. The sound produced requires energy, reducing the amount of energy delivered across the lines. This is part of line loss, electricity lost to the environment. Direct current has more line loss than alternating current, which is one of the main reasons alternating current is employed by electrical utilities and direct current is confined to small consumer electronics. Thicker, less weather-worn insulation helps but will not fully prevent line loss