This month, we’ll take a look at wire and conductivity, the electronic lifeblood behind our industry. In a rigorous sense, wire is metal that drawn through a sequence of ever-shrinking dies until it exists in fine, conductive form. If we relax that definition slightly to include any conducting element in electronics, like those on a circuit board or behind a touch screen, we have hit on a key piece of how virtually all audio equipment operates. Without electricity, and conductors to direct it, there would be no professional audio!
Let’s examine several aspects in conducting materials. We’ll introduce some physics principles behind the conductivity process. All of this will give a greater understanding of what is happening inside all the cabling that needs to be coiled after a gig. To kick off our journey into the field that scientists call electromagnetism, we need a launching point, and that point is the electron.
» Electrons!
The universe’s mediator for conduction is the electron. We will see that electrons contained in specific materials are the transit roadway for electrical effects. Nearly every early conclusion of what the electron is or does has been refined over time, so it makes sense to jump directly to the present understanding of the electron. Before we dive in, it is important to remember that physics defines things by a combination of inherent physical properties and their external effects. It doesn’t answer any deep questions about whether those properties make something “real” or concrete. The most physics says is that properties below are consistently observed at localized points in space, and we call that collection of properties an electron.
What is a reasonable modern understanding of the electron? It’s a fundamental element of nature, it has mass, it has charge, and it exhibits fields. Let’s pick apart each of these concepts. By saying the electron is fundamental, it means scientists have found nothing lurking inside an electron if we bang on it hard enough inside of a particle accelerator. This inability to further subdivide the electron makes it one of the universe’s core building blocks.
Next, when we say that the electron has mass, this means that when one tries to push an electron in a different direction, it instead desires to continue on the same path. Mass is the tendency to resist changes in the path of motion as an object moves through space. It also can be thought of as the attraction objects experience due to gravity. Scientists have found and measured that these two ways of describing mass as being equivalent. Einstein also physically described gravity as the bending of space and time. While it has long been suspected that there is some more subtle means behind gravity, that remains one of physics’ great unsolved problems. The mass of the electron is extremely small, almost 1,900 times smaller than the proton!
Now, on to charge. Charge is way of describing whether particles repel or attract. The property of charge sets up a “field.” Fields are the way that charges interact with each other. An electron has an electric charge, and that results in what is known as an electric field. The electric field moves away from the electron at the speed of light, and it causes a force (i.e., push) on any other charge it comes in contact with. If the second charge has the same sign (positively or negatively charged), then the force pushes away from the direction of the field. But if the charges have opposite signs, then the force of the electric field pulls towards the original electron. Two electrons, which have the same sign, therefore tend to repel each other.
Finally, there is one other field associated with electrons, the magnetic field. In the 19th century, scientists realized that a magnetic field results from any moving electric charge. Later it was realized that electrons also have a fundamental magnetic field, which result from the electron’s “spin.” Spin is the electron’s inherent angular momentum, and the name spin comes from the fanciful idea that electrons rotate like a top. It is this inherent angular momentum that creates the electron’s magnetic field. The magnetic field has a number of differences to the electric field, but one of the most important one is that north and south poles are always found together. Even the electron has both north and south poles. Similar to charge, opposite poles attract each other, while like poles repel.
In summary, the electron is a small “thing” that has mass, charge, and generates fields. Those two fields, the electric field and the magnetic field, are tied at the hip. Further, these fields emanate from the electron, so that charges can cause effects a distance away. All of the properties of electricity that we experience in audio start from these cornerstone behaviors of the electron.
» Moving Through Crystals
The whirlwind tour of physics above introduces the fundamental properties of the electron. This, however, is of limited utility since electrons are almost never out on their own. Instead, electrons are bound the nucleus of atoms, and those atoms are bonded together to make a “crystal lattice.” On an even larger scale, small regions of crystal lattice form “grains,” and a collection of grains is ultimately responsible for the materials we touch and interact with.
Thus, the nature of the crystal lattice, grain, and macroscopic collection of grains all influence the performance of feeder cable or a drive snake. Even with all the extra complications, electricity is still based on the aggregate behavior of many electrons, and the concepts of electric charge, electric field and magnetic field remain highly relevant.
A logical first question would be: “If electrons are bound to atoms, then how are they moving through conductors?” In answering this question, we get a first look at the importance of the crystal lattice. In many, but not all, substances, the atoms self-arrange into a regularly repeating structures. These arranged structure gives the crystal unique properties, and many of the electrical and mechanical properties we associate with materials result primarily from the lattice arrangement.
In the case of electrical properties, the electron configuration in the lattice arrangement has a great influence on the freedom of electrons to move around. Let’s imagine that the energies of electrons in the crystal are like keys on a piano. Low-energy electrons correspond to the bass octaves, and higher energy electrons to the octaves above middle C. The bass notes (i.e., low energy) represent tightly bound electrons that are near an atom, while the high keys represent electrons that are more free to move inside the crystal.
Some materials, like ceramics, have a crystal structure that allows a few low energy states, and some high energy states, but no middle states. This is like a piano missing all of the middle keys. You can play some bass notes, but you then have to slide all the way down the piano bench to get to the highs. It is this missing range of keys (i.e., energy levels) that makes electrical conduction in ceramics difficult. Unless the electron obtains certain high amount of energy, it is not free to move about the crystal lattice. These materials act as insulators (or semiconductors).
In contrast, the crystal structure of metals gives them the full keyboard. This means the electrons have lots of available energy levels that they can fill. As electrons would like to be as stable as possible, they fill the lower levels first, and then the middle levels (i.e., keys). The lowest levels remain tightly associated with an atom, but all of the middle levels are free to move about the crystal, creating a symphony of easy conduction paths. This is the property of the conducting metal strands in our pro audio cabling.
Conduction in metal crystals is a complicated phenomenon, but a fairly simple model accurately describes almost all of the observed behavior. This is known as the Drude model of electrical conduction, and was proposed by German physicist Paul Drude (1863-1906) in 1900. Drude treated the crystal lattice like the world’s tiniest pinball machine, with the electrons that are free to move bouncing about (and into) the large, immobile atomic nuclei. The commonly known Ohm’s law (see sidebar) falls squarely outside of Drude’s model.
Inside the metal crystal, there is movement of both the electric field and the electrons. The electric field moves through the metal lattice very rapidly, nearly at the speed of light. The electrons, too, bounce around with great enthusiasm due to their thermal energy. In the absence of an applied field, the net movement of the electrons is zero. Because they randomly bounce around, their average position remains the same.
» What About
Those Fields?
When an external electric field is applied to a conductor, a net shift in the position of the electrons occurs. This change in position is called the electron drift velocity, and it represents a shift away from randomly bouncing about a central point. The drift velocity is really slow by atomic standards, less than an inch per second. One way to think of the drift velocity is imagining a dancing audience that is slowly being drawn to the bar (i.e., an outside force). Each person is moving rapidly about while dancing, but the whole mass of people are shuffling ever so slowly towards the bartender.
We call the external field applied to the conducting cable voltage. Voltage is simply a measurement of the electric field that has been normalized for distance. Since the electric field strength depends on distance from the electron by the inverse square law (see sidebar), voltage is a much more convenient unit.
The applied voltage on the cable results in current flow, as the electrons are moved by the electric field. With the moving of charges comes a magnetic field. The magnetic field is perpendicular to the electric field, and can be calculated by something known as the “right hand rule.” The right hand rule is why speaker voice coils move at right angles to the direction of the voice coil windings. The cone moves in and out, but the windings form circles in the perpendicular plane.
» More to Come
So we’ve taken an article largely about theory and placed a small practical design principle at the end. If you want to design a speaker to move in and out, you must account for the right hand rule as you wind the voice coil.
In a future article, we will discuss how some of this physics relates to effects we commonly encounter, such as how fields give rise to inductance and capacitance. For now the goal was to give some perspective on what happens inside your cables, and remind people of all the various electrical phenomena that can happen are ultimately the result of electrons moving about a crystal lattice in a specific way that is shaped by how that lattice repeats.
In the end, electric fields induce moving charges, which induce magnetic fields, which drive moving cones, and result is smiling audience members rocking out at the show. Electricity can be both physically and emotionally powerful.
Ohm’s Law
Ohm’s law is quite possibly the most commonly used formula in pro audio, and says that Voltage = Current x Resistance. Ohm’s law preceded Drude’s work by about 70 years. There are many real-world situations where Ohm’s law does not hold, but it is very accurate for metallic conductors carrying direct current (DC).
Inverse Square Law
The inverse square law is commonly used in audio to talk about how speaker output levels change with distance. It also applies for how the strength of the electric field around an electron is reduced with distance. In both cases, the law results from the formula to calculate the area of a sphere, which is 4(π)r2. This means that if you double the radius of a sphere, its surface area goes up by a factor of four. Since the electric field is now spread over four times the surface area, its strength at any given point is only one-fourth of the original.
