Why does high-voltage electrical equipment buzz?

[Delvo]

Where does that sound come from? What's making the air near the cables or transformers or fuses or whatever it is vibrate? If you put your fingertip close enough without touching, could you feel whatever forces act on the air to make that sound?

[publius]

It's not so much high voltage, but AC hum. In transformers and motors and iron core stuff, the magnetic flux in the core is oscillating at line fequency. This produces magnetic forces which cause mechanical stress in the material -- for example, in transformers, the coils are pushing and pulling against other. As it oscillates, that stress waxes and wanes along with it, and that's what produces the hum. At 60Hz, you get two peaks of magnetic force per cycle, and thus that mechanical stress has a 120Hz mechanical frequency. What you hear are more harmonics of that 120Hz, actually, and it depends on the acoustics.

Under heavy current conditions, such as during a short/fault, the magnetic forces between the conductors can be enough to induce that hum even if there's no iron/steel around, but generally,it's the iron and steel of the enclosures and conduits that help amplify the magnetic forces involved.

Note it is usually the current, magnetic forces, that make the sound, not direct electric field forces alone. They are not strong enough.

Now, other sources of sound are associated with high voltage, and that's the hissing and popping of corona in regions where the electric field gets strong enough to cause ionization. That's a bad thing.

I've always been sort of an electrical nut, and one of my hobbies is welding, and thus I'm familiar with all the sights, sounds, and smells of electric arcs. AC current arcs make a prounced hum, and, playing around, if you carefully pull back to make as long an arc as you can without it going out, you'll get a very pronounced "Zooooootttttttt" riding on top of a "Mmmmmmmmmm" hum.

A DC arc is much more civilized and does not make that humming sound at all (unless it's from a rectified source with a lot of humping). I managed to acquire an old Westinghouse 50s era shipyard welder, which is roughly 15HP 3-phase motor turning a DC commutator generator. Huge, heavy thing, and they don't make 'em like that anymore. That thing makes some of the smoothest DC current you've ever seen, and that makes running some "fussy" stick rods very easy. And it will run 300A continuous output without even getting warm hardly. That thing was made to run 24/7 in heavy production.

With AC, the strength of the hum gave a little additional feedback on how much current you were pulling, and I remember burning a hole right through something when I first got the new Westinghouse, because the sound was so much more subdued and I turned up the current thinking it wasn't enough. :)

The dead constant DC sound is just a hiss with a little frying undertones, no hum whatsoever.

I put my scopemeter on the output of that old Westinghouse just to confirm how dead constant the DC was. And it was. However, the scopemeter's sensitivity was enough to pick up the slight little dips between commutator segments! I could see that right on the screen, and a little mental arthimetic with the frequency of those dips confirmed the RPM I knew it was supposed to be turning at.

[HenrikOlsen]

In some cold/cool&damp conditions, you can get the sizzling of corona when moisture builds up in the surface of the insulators, making their surface slightly conductive.
I've heard that rather pronounced a couple of times at the railroad station, where it's quite clear that it isn't induction hum, there's a distinct angry nastiness to the sound, makes me think of snarling insects :).

[astromark]

Light rain or fog can make a substation sing... and Jimmy Hendrix new a thing or two about this...

Electric guitar feedback is a close relative of this science.. Harmonic electric oscillation...

and NO.. do not be waving your finger at high power tension leeds... for it might find a quicker way to Earth... You.

Hummmm.... ZAP. Oops ! What's that burning smell, Oh, me. :)

For some years I have worked with electro magnetic soundings.. around the coastal regions of NZ..

and Yes there is oil barring rocks in shallow waters.. of the NZ continental plate..

and its a near relative of that Hmmmm... that we used. Its a whole subject of it's own.

[danscope]

" Throw the Third Switch !!! "

" Not...... the third switch, Master. "

"Throw it , I say !!!!!!!" eeennyannnnnnnnnnnn... ( long electric arcs rise from many Jacob's ladders ) .

[stutefish]

It's not so much high voltage, but AC hum. In transformers and motors and iron core stuff, the magnetic flux in the core is oscillating at line fequency. This produces magnetic forces which cause mechanical stress in the material -- for example, in transformers, the coils are pushing and pulling against other. As it oscillates, that stress waxes and wanes along with it, and that's what produces the hum. At 60Hz, you get two peaks of magnetic force per cycle, and thus that mechanical stress has a 120Hz mechanical frequency. What you hear are more harmonics of that 120Hz, actually, and it depends on the acoustics.

Is that the hum I hear when I stand underneath power transmission lines?

[korjik]

Real life application of the Rule of Cool ('http://tvtropes.org/pmwiki/pmwiki.php/Main/RuleOfCool'). Electromagnetism is just sooooo dang cool that it has a real life soundtrack.

:D

[publius]

Is that the hum I hear when I stand underneath power transmission lines?

Yep. What you're hearing is that 120Hz, the double "beating" per 60Hz cycle. Google "120 Hz tone" and play that and you'll recognize it as AC hum. My speakers on this machine won't even make a 60Hz tone audible, but I can hear it trying to do something just barely. The main thing you hear is 120Hz, and the balance of harmonics of that give it that distinct electrical hum.

On a piano -- let's see, I forget. I think A4 is 440Hz, so A2 should be 110 Hz and that will be pretty close, but all the overtone/harmonics will change things a bit. Anyway, play around with the low end of a piano and you'll find one that is pretty darn close to the AC hum.

60Hz is all over everything thanks to AC power. And 50Hz in Europe and Australia. I can take my scopemeter right now and touch one of the leads and get a nice 60Hz waveform to pop up. :) Take some of the RCA leads to one of your stereo inputs and touch the center pin. Turn down the volume, cause it will make pops and scratchy sounds as you make and break and contact, but once you have contact, turn up the volume and you'll hear a steady "Mmmmmmmmmmmm" and that's (harmonics) of 60Hz all over you.