In an analog signal it is alternating currents that mimic the sound wave; so in my mind DC should never produce audio but is that true? In a ground loop the noise is caused by differences in ground potential of either side of the loop allowing current to flow but is it AC or DC?

Is a ground loop caused by voltage potential between the grounds only or that plus EMI combined?

To add to the question clearificarion. If I have an unbalanced output from a mixer to an input with a different ground I will get noise on the receiving input even when no signal is being sent from the mixing desk. What is making the current alternate between ground references? This was the case recently with a church that asked me to fix a buzzing in their video feeds. Fixed it with an 8 dollar transformer from amazon because they did not want to convert to balanced equipment and proper interfaces but this is beside the point of my question. I’m just trying to actually understand physically how it works rather than just knowing how to prevent it

  • a) upvote removed & downvote given for completely changing the question after three people bothered to answer it & b) my answer removed for arguing over my initial complaint. – Tetsujin Jan 5 at 17:25
  • "Is a ground loop caused by voltage potential between the grounds only or that plus EMI combined" Mark answered the first half, I answered the second. You then moved the goalposts, introducing a new theme entirely, a specific product issue not even touched upon before three of us had wasted our time. I asked you to raise that as a separate question. You argued & continue to. I shall no longer respond to you. – Tetsujin Jan 5 at 17:59

Sound is derived from vibrations which in turn are generated or represented by changing current levels. Sound can never be derived from a static voltage or current. Your statement that "DC should never produce audio" is - to all intents - true.

A ground loop is the unexpected flow of current in a circuit caused by a potential difference between two points in that circuit. Effectively what you have is two points in a circuit which you might believe to be ground, but which actually have a potential difference between them and cause unintended currents to flow in the direction of the potential difference.

A ground loop can become audible if the potential difference alternates as this then becomes part of the signal.

Ground Loops can be AC or DC - remember any instantaneous change in DC should be considered AC for that change therefore a change in DC can generate unwanted sound in a ground loop scenario, however once the DC stablises, then there is no longer an AC component present and therefore no sound will be induced.

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Short a 9V battery across a speaker's input tabs and watch what happens.

The cone will immediately move out (or in) and stay there until you remove the battery. But don't let it run for more than a couple of seconds.

If you reverse the connection, it will instead move in (or out).

(You might hear a brief click when the connection is made or broken, but that's an artefact of irregular pressure while the metal parts are brought into contact, so ignore it.)

As long as the DC current is flowing, the cone will silently remain in a constant displaced position.

If you were incredibly skilled, and could quickly reverse the connection 880 times a second, the cone would repeatedly move back and forth between the two extended positions, and you would hear a sound, something like a middle "A" for tuning an instrument.

But of course, that repeated DC reversal is exactly what AC is.

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    The problem with answers like this is that there will always be one idiot that will actually try this. Best not eh. – Mark Jan 6 at 10:14
  • @Mark, well this idiot tried it, connected for 5 seconds with no problem. Measured: 9.6V, 7.4 Ω, .7A. Interestingly the current was about half of what Ohm's Law would predict for a pure DC circuit, which I guess is due to the battery's limited power. – Ray Butterworth Jan 6 at 14:25
  • Oops… it was a powered monitor… Really, don't recommend tests that can damage equipment. – Tetsujin Jan 6 at 20:47
  • @Tetsujin, by "speaker" I mean the simple component with the magnet, coil, and cone. – Ray Butterworth Jan 7 at 0:31

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