Sue Pertwee-Tyr
Accuphase all the way down
On another thread, a comment was made about cable directionality.
That thread was already long enough and this particular thread drift was so far off-course that I thought it better to start a new one. The gist was that as the audio signal is AC, so the electrons don’t travel but merely wiggle back and forth, claims about cable directionality were nonsensical.
This is fair enough, at first glance, but claims about cable directionality persist so I picked the comment up to see where it took us. My query was that if the electrons don’t propagate through the conductor, how is the energy transferred? After a brief and jocular diversion into the participants’ knowledge of physics, a helpful reply was given.
I thought it might merit further discussion, hence the new thread.
The response went like this:
So, essentially, the electric current can still do work (ie heat the wire, or move the drive unit) despite the electrons merely oscillating back and forth and not actually propagating anywhere. Unlike in the commonly-accepted the water analogy, where the water molecules would physically impact on whatever was at the end of the pipe and do the required work, it’s not moving electrons doing the shoving of the voice coil or whatever, but the electrical field which is generated by the acceleration of the electrons which creates the ability to do the work. The movement of the electrical field is independent of the movement of the electrons, indeed the speed of individual electrons is slow, a few millimetres per second, whereas the electric field which propagates as current, moves at a significant fraction of the speed of light.
(Where I get hazy is in considering how this electrical field is generated. Conventional theory says that the acceleration of a charged particle generates an electrical field, though it doesn’t usually say why. So the oscillation of an individual electron in an AC circuit means it is always in a state of acceleration, hence an electric field is generated and so-on. Quite how and why it propagates along the circuit, and in what direction, is less clear. My recollection of what my physics textbooks had to say about DC circuits is that this creation of the electrical field is rather glossed-over; in a DC circuit, the electrons do drift in the general direction of the current, and the number of electrons passing a point, per second, defines the current under, IIRC, Coulomb’s Law. But if the electrons are drifting, slowly, it isn’t entirely clear how the current propagates so quickly, nor yet how the field is generated as there appears to be no net acceleration of electrons).
That’s a digression and irrelevant for our purposes, however.
What is relevant is that in the AC signal, the individual electrons are busily shuttling back and forth within the conductor. The speed at which they do this is quite low, millimetres per second, and the frequency of the directional reversals is quite high in the case of an audio signal, so they clearly don’t go very far.
Conduction and resistance in a metal is determined by various factors, including cross-sectional area, temperature and the number of available electrons free to migrate, which varies with the material. Copper and silver are good, steel and aluminium are less good. It is also, however, determined by the microstructure within the material: grain boundaries, impurities and so-forth. This is all accepted physics. More grain boundaries or impurities leads to an increase in resistance (or reduction in conductance if you prefer). These are, AIUI, literal physical obstructions to the movement of the electrons.
A cable is made by drawing, or forcing, a billet of the metal through a small hole, so the cable emerges from the hole like toothpaste from a tube. If you’ve ever played with a Play Doh Fun Factory and extruded bits of smelly gunk through shaped formers, you’ll recognise the granular irregularities such a process creates. This granularity, being a physical consequence of the drawing process, reflects the direction in which the cable is drawn so, at a microscopic level, the granularity can be seen to indicate the direction in which the cable was drawn. This might therefore correlate to a difference in conductivity depending on whether the electrons are flowing along with the grain, or against the grain. In other words, if the electrons find it easier to cross a grain boundary in one half of their oscillation, but slightly harder to jump back across the grain boundary in the other direction, in the other half of their oscillation, this would lead to an asymmetry within the alternating signal.
To my mind, this gives rise to a possible explanation for why cables might be directional. It also explains why cryogenic treatment of cables, designed to reduce the number of grain boundaries, might be beneficial.
Any takers?
That thread was already long enough and this particular thread drift was so far off-course that I thought it better to start a new one. The gist was that as the audio signal is AC, so the electrons don’t travel but merely wiggle back and forth, claims about cable directionality were nonsensical.
This is fair enough, at first glance, but claims about cable directionality persist so I picked the comment up to see where it took us. My query was that if the electrons don’t propagate through the conductor, how is the energy transferred? After a brief and jocular diversion into the participants’ knowledge of physics, a helpful reply was given.
I thought it might merit further discussion, hence the new thread.
The response went like this:
Well, start by asking yourself why an alternating current should be incapable of transferring energy. The alternating current and fluctuating voltage has a certain amount of electrical energy, and the amount of energy per unit time is power. In accordance with the 1st law of thermodynamics, this energy must go somewhere. If there was no loudspeaker in the circuit, and you effectively short circuited your power amp, this energy would be dissipated as heat and would blow up the amp. If you connect a loudspeaker, however, the drive unit will move in and out in a direction determined by the alternation of the current. Here, instead of dissipating the energy as heat, it is converted to kinetic energy in the voice coil and cone (plus a certain amount of heat); this is then transferred to air molecules... and so on.
Now, if the current did not alternate, and you passed DC through a speaker, it would not move in and out, could not convert the electrical energy to mechanical energy, would get hot and eventually die. Usually because the varnish on the voice coils melts.
So, essentially, the electric current can still do work (ie heat the wire, or move the drive unit) despite the electrons merely oscillating back and forth and not actually propagating anywhere. Unlike in the commonly-accepted the water analogy, where the water molecules would physically impact on whatever was at the end of the pipe and do the required work, it’s not moving electrons doing the shoving of the voice coil or whatever, but the electrical field which is generated by the acceleration of the electrons which creates the ability to do the work. The movement of the electrical field is independent of the movement of the electrons, indeed the speed of individual electrons is slow, a few millimetres per second, whereas the electric field which propagates as current, moves at a significant fraction of the speed of light.
(Where I get hazy is in considering how this electrical field is generated. Conventional theory says that the acceleration of a charged particle generates an electrical field, though it doesn’t usually say why. So the oscillation of an individual electron in an AC circuit means it is always in a state of acceleration, hence an electric field is generated and so-on. Quite how and why it propagates along the circuit, and in what direction, is less clear. My recollection of what my physics textbooks had to say about DC circuits is that this creation of the electrical field is rather glossed-over; in a DC circuit, the electrons do drift in the general direction of the current, and the number of electrons passing a point, per second, defines the current under, IIRC, Coulomb’s Law. But if the electrons are drifting, slowly, it isn’t entirely clear how the current propagates so quickly, nor yet how the field is generated as there appears to be no net acceleration of electrons).
That’s a digression and irrelevant for our purposes, however.
What is relevant is that in the AC signal, the individual electrons are busily shuttling back and forth within the conductor. The speed at which they do this is quite low, millimetres per second, and the frequency of the directional reversals is quite high in the case of an audio signal, so they clearly don’t go very far.
Conduction and resistance in a metal is determined by various factors, including cross-sectional area, temperature and the number of available electrons free to migrate, which varies with the material. Copper and silver are good, steel and aluminium are less good. It is also, however, determined by the microstructure within the material: grain boundaries, impurities and so-forth. This is all accepted physics. More grain boundaries or impurities leads to an increase in resistance (or reduction in conductance if you prefer). These are, AIUI, literal physical obstructions to the movement of the electrons.
A cable is made by drawing, or forcing, a billet of the metal through a small hole, so the cable emerges from the hole like toothpaste from a tube. If you’ve ever played with a Play Doh Fun Factory and extruded bits of smelly gunk through shaped formers, you’ll recognise the granular irregularities such a process creates. This granularity, being a physical consequence of the drawing process, reflects the direction in which the cable is drawn so, at a microscopic level, the granularity can be seen to indicate the direction in which the cable was drawn. This might therefore correlate to a difference in conductivity depending on whether the electrons are flowing along with the grain, or against the grain. In other words, if the electrons find it easier to cross a grain boundary in one half of their oscillation, but slightly harder to jump back across the grain boundary in the other direction, in the other half of their oscillation, this would lead to an asymmetry within the alternating signal.
To my mind, this gives rise to a possible explanation for why cables might be directional. It also explains why cryogenic treatment of cables, designed to reduce the number of grain boundaries, might be beneficial.
Any takers?