When studying engineering in university, the first few terms learned are similar for everyone — regardless of their eventual specialization into a different type of engineering. In the same way, there are certain topics that are elemental building blocks everyone in the audio and event production industry should be familiar with. At present we’ll look at conductivity, the electronic lifeblood behind our industry. Without electricity, and conductors to direct it, there would be no professional audio!
Below, we’ll examine several aspects of conductive materials. All of this will provide a greater understanding of what is happening inside all the cabling that needs to be (correctly) coiled after a gig. Behind conductivity is what scientists call electromagnetism, and the launching off point for that is the electron.
Electrons — Literally Fundamental
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. Before we dive in, it is important to remember that physics defines things via 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 listed below are consistently observed at specific locations, and we call that collection of properties an electron.
Over time, there have been many conclusions in the world of science over what the electron is or what it does. Here we will consider the present understanding of the electron. The electron is a fundamental particle, in the sense that no matter hard we try to split it apart, no other particles are found inside. It should be mentioned that, with something as tiny as an electron, the notion of a particle is hazy, even if using the word “particle” is convenient in our discussions.
An electron has mass, it has charge, and it exhibits fields. 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 its original 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 that these two ways of describing mass are 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 that enables gravity, what that remains one of the great unsolved problems.
Charge is a 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 that pushes on any other particle that also exhibits charge. Mathematically, scientists represent the attractive or repelling nature of the charge with positive and negative signs. If a second particle with charge has the same sign, then the applied push (i.e., force) is 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 electron is rotating 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 particle 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 — even when located some distance away. All the properties of electricity that we experience in audio stem from the cornerstone behaviors of the electron. Because the fields are so closely linked, this branch of physics is called electromagnetism.
Electron Paths in Crystals
The whirlwind tour of physics above introduces the fundamental electron properties. This is of limited utility, since electrons are almost never out on their own in our world. Instead, electrons are bound near the nucleus of atoms, and those atoms are further linked 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, individual grain, and macroscopic collection of grains all influence the performance of AC 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 the magnetic field remain relevant when considering lots of electrons inside of a crystal lattice.
A logical question would be: “If electrons are bound to atoms, then how are they moving through speaker wire or microphone cable?” 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 structures give the crystal unique properties, and many of the electrical and mechanical properties we associate with materials result primarily from the lattice arrangement of the component atoms.
The Piano Analogy
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). This lack of middle conduction states is a direct result of the nature of the atoms that make up the ceramic combined with the location of each atom in 3-D space.
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 pro audio cabling. The extra available middle levels for easy conduction can be calculated from quantum physics if the geometry of the crystal cell is known.
A Simpler Model for Metal Conductors
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 Paul Drude in 1900. Drude treated the metal 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” falls mathematically out of Drude’s model of electron conduction.
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 enthusiasm due their thermal energy. In the absence of an applied field, the net movement of the electrons is zero. As they randomly bounce around, their average position also 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 location. 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. As the electric field strength depends on distance from the electron, voltage is a much more convenient unit than electric field.
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.
The Conclusion, But Not The End
After all that theory, what have we learned? Electrons can move inside of materials that have certain crystal structure. Certain structures, like those of most metals, allow electrons to move without much resistance. This allows current (i.e., moving electrons) — and the result of current is a magnetic field. The magnetic field can be repelled or attracted by another magnet, and this effect can be used to generate movement in such as a speaker cone.
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. Like many other things in physics, the right hand rule exists and is consistently observed, but neither of those facts provides fundamental insight into they “whys” of the universe. If the universe instead had a “left hand rule,” then we would have to reverse the winding direction of our voice coil, or the polarity of the amplifier. In the end, the theory enables us to move speaker cones and trigger smiling audience members having a great audio experience. Electricity can be both physically and emotionally powerful.
Paul Drude
German physicist Paul Karl Ludwig Drude (1863 – 1906) wrote Lehrbuch der Optik, a landmark textbook that integrated optics with James Clerk Maxwell’s theories of electromagnetism and developed a powerful model that linked the thermal, electrical and optical properties of matter. The familiar “Ohm’s Law” falls mathematically out of Drude’s model of electron conduction. As an interesting footnote, in 1994 Drude introduced the now-standard symbol “c” for the speed of light in a perfect vacuum in 1994.
Phil Graham is FOH’s regular technical contributor and resident scientist. Email him at: pgraham@fohonline.com.
