Why do power lines buzz?

August 23, 2005 5:15 AM

ScienceFilter: Why do high-voltage power lines buzz.

Specifically, I'm looking for an explaination of how electrons passing through wire produces the compression waves that hit my ears. Bonus points if you can tell me why a buzz and not a whine or other sound.

posted by kc0dxh to science & nature (11 comments total)

iirc, it's a buzz or humm because the frequency of ac power lines is 120hz. In fact, you can check this out for yourself by using Audacity. Use Generate -> Tone and put in 120 for the frequency. Pretty much the same noise.
posted by boo_radley at 5:29 AM on August 23, 2005

It is my understanding that the physics surrounding this phenomenon get complex really quick, but I can give you a "hand waving" explanation - basically you're hearing corona discharge. The strong electric field at the surface of a high voltage power line conductor ionizes the nearby air and causes a partial breakdown of its dielectric strength. The audible noise you hear is caused by the local sound-pressure level changes due to the individual corona discharges that in turn occur as part of electron avalances. During wet weather the noise level is much higher due to the presence of drops of water on the underside of the conductors. These weather drops break up and the departing portions cause discharges in the water-to-water gap.
posted by RichardP at 5:55 AM on August 23, 2005

I just checked, I should have known that Wikipedia would have a nice article on corona discharge.
posted by RichardP at 6:05 AM on August 23, 2005

The wires physically move, as far as I know, like large strings on an instrument. They move at the frequency of the electrical current. I have no idea why, but I distinctly remember learning this from a link I found on metafilter.

Something about the air rushing over them... but... yeah... welcome to the end of my knowledge.
posted by odinsdream at 7:14 AM on August 23, 2005

But do we get credits?
posted by weapons-grade pandemonium at 7:33 AM on August 23, 2005

odinsdream:
Wires carrying power create a magnetic field, and thus will move (at the frequency of the voltage) when they are in a magnet field, and the magnetic field generated by a nearby wire can be sufficient to that task, so you get movement in both wires (I've seen a nice demonstration of this), however, I have reservations as to whether this is actually the cause of the sound - getting those cables to audibly vibrate at 60hz would (I would think) require a terrific amount of energy, meaning high power loss on the lines, meaning powerlines are almost certainly designed to avoid the effect, and it doesn't explain why the buzzing is louder in the wet.
posted by -harlequin- at 9:11 AM on August 23, 2005

Also, the sound you hear is far from a 60hz sinewave, which would be a very low hum near the end of what you could hear.
posted by delmoi at 10:40 AM on August 23, 2005

This physics bulletin board has a thread covering some of the weather issues. New Scientist talks about the hum itself, though mostly about transformers.

I personally have seen regular 120 VAC lines move when a short circuit occurs and I've heard them slap the inside of conduit. That I think is magnetic effects, however.
posted by tommasz at 10:59 AM on August 23, 2005

I stand corrected. Thanks.
posted by odinsdream at 12:01 PM on August 23, 2005

I did a google search and found the following in a brief paper about noise from power lines in Japan:

Corona noise occurs under rainfall and snowfall and comprises two sound components: one is irregular (random noise) sound, and the other is the pure sound (corona hum noise) of 'buzzing'.

The random sound has a wide frequency band because the impulsive sounds caused by corona discharge overlap randomly. The corona hum noise results from the excitation of ion groups, which was generated from corona discharge, caused by the electric field surrounding the conductors. The predominant frequency of the corona hum noise is double (100 Hz or 120 Hz) the commercial frequency.

The conductor-surface potential gradient in Japan is relatively low because of the regulation on radio noise. Accordingly, the percentage of corona hum noise in total corona noise is high, and the corona hum noise is a pure sound and the frequency of corona hum noise is low. Therefore, the corona hum noise is easily identified and is therefore likely to become a target of complaints from the public.

posted by RichardP at 12:31 PM on August 23, 2005

Also, the sound you hear is far from a 60hz sinewave, which would be a very low hum near the end of what you could hear.

Nope, 60 Hz is plenty audible. Anyone who noodles with an electric guitar or a turntable can tell you exactly what 60 cycle hum sounds like ... hmmmmmmmm ...

I loooove AskMefi, it eases the pain, but really, this thread's got a pretty high ratio of completely clueless posts. 120 Hz --who knew! Good thing RichardP stepped in early and did us up right.
posted by intermod at 8:26 PM on August 23, 2005