Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Re: Some interesting replies.

Posted by Presto (A) on April 18, 2007 at 11:02:05

In Reply to: Some interesting replies. posted by May Belt on April 17, 2007 at 02:50:57:

I don't prefer the "shock wave" analogy. Current flow is when electrons migrate. When the current is direct (DC) electrons do "flow", they migrate in one direction only. But in an alternating current with NO DC OFFSET, there is no net resultant migration - electrons simple migrate back and forth "about" where they were sitting before the voltage was applied. Although there are very short propagation delays in really long cable runs (transmission line theory), in wire runs as short as we find in home environments, the migration of electrons at the source end of a cable results in migration at the other end almost instantaneously.

So, the better way to "think" about electron migrating in an alternating current is to picture electrons moving back and forth, in unison. Their relative displacement is proportional to the magnitude of current that is flowing, which is in turn proportional to the magnitude of the applied voltage, since impedance is constant (for a given frequency). In a 120V 60Hz branch circuit seeing 1A of current, electrons (according to my calculations) move back and forth about 0.4 X 10^(-6) or 0.4 micro-meters. 4.0 micro-meters at 10 Amps. So not very far in other words. The electrons basically just "sit and wiggle".

This is why I have trouble with the directionality theory, which in my mind is non-science based on the fact that some cables rely upon telescoping shielding techniques. Instead of labelling the cable "shield terminated this end" or "source end" and "load end", they put convenient little ARROWS on cables, making it intuitive to the layperson that the arrows should "point" AWAY from the source as 'everybody knows' that "music flows from source to load". The existence of these arrows make people think that if you try and "make music flow against the arrows" you're going to get bad sound. But music does NOT "flow from source to load". The only thing that could possibly affect the sound is changing the location of the termination point of the telescopic shield from source to load. But this has everything to do with shielding techniques and nothing to do with "directionality of conductors or dielectrics".

We just said earlier that electrons do not experience a net resultant migration (save for a very very small DC component) with audio signals - they vibrate back and forth. This is how interconnects and cables work: the "wiggle" at one end is the same "wiggle" at the other. Is the wiggle at the far end smaller due to voltage drop? No. The impedance is spread out evenly throughout the cable, so all of the "wiggles" are affected equally.

So, in any case, the fact someone painted arrows on a cable makes them no more directional than a frisbee that someone painted an arrrow on. If that were true, left handed people could not play frisbee.

I think the classic "Newtons Cradle" is a much better example of the "shock wave" model of energy propagation than an electric cable. Due to the short relative length of home audio interconnects and cables, they model much more like nodes than transmission lines.

Cheers,
Presto

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