You’ve pondered the question before, but haven’t quite put your finger on the answer. How, exactly, do guitar pickups work? What goes on to change the string’s vibration to that beautiful sound that comes out of your amp? The simple and short answer is, through a process called ‘transduction’. Transduction is the conversion of one type of energy to another. Many audio devices use this process, and how they accomplish their task’s are very similar to each other. Such devices are speakers, headphones, microphones, and of course, guitar pickups.
A guitar pickup is a transducer that converts the mechanical energy of the guitar string into an electrical signal. In order to understand the process of transduction in a guitar pickup, you must first understand a guitar pickup’s anatomy. In its simplest form, a guitar pickup consists of a coil of wire wrapped around a magnet. The magnet surrounds the coil in a magnetic field. The field has lines of flux which exit the north side of the magnet, and return to the south side of the magnet (fig. 1).
When you introduce a ferrous material to the magnetic field, it becomes part of the magnetic circuit and redirects the natural lines of flux (fig. 2).
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An important thing to remember about magnetic circuits is that, even though these lines of flux are being redirected, they will ultimately end up back at the south pole of the magnet.
The manipulation of the magnetic lines of flux through the coil is where the magic comes into play. When a current is moved through a wire, it produces a magnetic field. Conversely, when a magnetic field is moved through a loop of wire, it produces a current. This phenomenon is called ‘induction‘. An electrical current is being induced in the coil by a moving magnetic field.
Now, I can see you scratching your head… “The magnets in a guitar pickup are stationary! How do stationary magnets ‘move’ through a coil?”. Though the magnets remain stationary, their magnetic fields are open to manipulation via the magnetic circuit, and the field can shift through the coil due to this manipulation.
The guitar string is a ferrous material, and we know that ferrous materials redirect the magnetic field’s lines of flux. When the string moves, the lines of flux want to follow the string. This causes the lines of flux to move within the coil, thus producing a current (fig.3).
Notice how the lines of flux expand and contract within the pickup coil due to the string’s movement? That is the moving magnetic field that makes guitar pickups work. This field oscillates at the same frequency as your strings vibration, producing a current in the coil that alternates at the same frequency. This current is send to your amplifier which magnifies, boosts, and transduces the electrical energy back to mechanical energy in order to move the speaker cone.
At this point, we’ve reached a crossroad. Those of you who want a simple grasp of how your guitar pickups work, but don’t care for the extremely technical stuff will most likely disembark here. For those of us who thirst for this kind of stuff, we’re just getting started!
Magnets, the life force of all electronics. They’re in your phone, in your car, and on you’re fridge. They are the reason that you can turn on a switch and have light, or navigate yourself home when you are lost in the woods. Most importantly, magnets are what make guitar pickups work!
We can’t really get into what causes a material to be magnetic without diving into some quantum mechanics, so let’s see if we can tackle the essentials without all the complex equations.
Scientists now believe that magnetic fields are generated by electrons, in that every electron produces it’s own magnetic field. Most element atoms contain paired electrons. These electron pairs share the same orbital of a nucleus, but have opposite spins upon their axis. This opposing rotation causes the electrons to produce opposing magnetic fields, thus canceling each other out. Some elements, however, contain unpaired electrons with the same rotation. Because there is nothing to cancel out their magnetic field, these electrons have an ‘oribital magnetic moment‘.
When cooling from a molten state, the atoms of ferromagnetic metals align themselves with other atoms that have a parallel orbital spin because of their magnetic moments. These atoms form a crystalline structure called a domain (fig 4).
Once formed, domains want to rest in a configuration that allows the magnetic fields of adjacent domains to cancel each other out (fig 5).
If you think of a group of domains as a magnetic circuit, they want to rest so that the field of each domain has the path of least resistance. To put it another way, the magnetic fields are shorting themselves out. In order to make a magnet permanent, the ferromagnetic material must be heated and cooled in the presence of a magnetic field. This aligns all the domains, and locks them in place once cooled.
That brings us to magnets that are observable and tangible in our physical realm. A magnet emits a force. This force can be represented as lines of flux, which flow from North to South (fig. 6).
These lines of flux cannot intersect, are continuous, and always form a closed loop. As you can see in figure 6, the lines of flux get much denser as you get closer to the magnets core. This magnetic flux density is measured in gauss, and reflects the number of flux lines through an area of one square centimeter. Some types of magnets can have a large magnetic field with a relatively low field density, and some can have a small magnetic field with a relatively high field density. (This is why different grades and material can have an effect on tone. More on that in a later article.)
When ferrous materials are introduced into a magnetic field, their domains align with the field and become polarized. The polarized ferrous material expands and manipulates the lines of flux from the source. Don’t forget, the lines of flux must form closed loops. The lines of flux are now going from the north end of the source magnet, through the ferrous material, and returning back to the south end of the source magnet. The ferrous material and magnet have now become a magnetic circuit. If the ferrous material is moved, the lines of flux will move with it, as seen back in figure 3.
The coil of a guitar pickup is made up of thousands of turns of fine wire. According to Faraday’s law of induction, when a magnetic field is moved in proximity of a conductor, an electromotive force is created within the conductor.
Knowing that the ferrous guitar string manipulates the magnets field with its vibration, we can surmise that the moving magnetic field is inducing a current in the pickup coil. The magnitude of this current is directly proportional to the rate of change of magnetic flux through the pickup coil.
So what does this mean for us guitar players? When you pluck the guitar string hard, the string is going to move at a higher velocity than if you had plucked it soft, while vibrate at the same frequency. The higher velocity will induce more current in the pickup coil than the lower velocity. This is because the higher velocity movement produces a greater change in the magnetic flux through the pickup coil.
Expanding on Faraday’s work, Heinrich Lenz discovered that an induced current produces a magnetic field around the conductor that always opposes the change of the magnetic force that produced it. This is important in how guitar pickups work because it allows us to determine the direction of current relative to the movement of the magnetic field. Look at figure 7. The arrow pointing down indicates the natural flow of the magnet’s flux lines from north to south. The arrow pointing to the left is showing the movement of the magnetic field, caused by the string moving down. The resulting current will travel into the screen, as perceived by the reader.
When the string is moving up, the magnetic field is contracting back into the coil, and the current moves in the opposite direction (fig.8).
This back and forth movement of the magnetic field produces an alternating current in the pickup coil that matches the frequency of the strings vibration.
In exploring how guitar pickups work, we have determined that the magnet, pickup coil and guitar string all work together to produce an alternating current via transduction. Knowing how guitar pickups work will allow you, as a player, to better understand how subtle changes in a pickup’s design or materials will affect the overall characteristics of your pickup’s signal. The output of these characteristics are what we ultimately perceive as tone or timber, which is what we’re all after, isn’t it?
In Part 2, we’ll take a look at the different pickup styles and how their construction affects the relationship between magnet, coil and string.