How Does a Pickup Really Work?
/In the first couple of installments, I outlined my early efforts at pickup development, up to the point at which the Zexcoil® platform as it currently exists was more or less solidified. Before we continue with the Zexcoil story, let’s set the stage. We’ll back up a bit and re-consider for a second what an electric guitar pickup is and how it really works.
An electric guitar pickup is an inductive sensor that consists, in its simplest form, of a coil wrapped around a permanently magnetic pole piece or pole pieces. This is the architecture of some of the most important and popular pickup designs, including the conventional Stratocaster® pickup (Figure 1). This inductive sensor sits below a ferromagnetic string. When the ferromagnetic string vibrates, a signal is generated in the coil. It is this signal that gets amplified to create the sound of an electric guitar.
But how, exactly, is this signal generated? The most common way you’ll see the string-pickup interaction represented is from a perspective that is centered on the magnetic field of the pickup (Figure 2). In this interpretation, the magnetic field of the pickup gets “disturbed” by the vibrations of the string. The ferromagnetic string, because of its magnetic permeability (which we will talk about in more detail in a later post) kind of pulls the pickup’s field around as it vibrates, causing the coil to generate a signal in response to this changing magnetic field. This is the perspective on pickup function that you’ll generally see in books, websites and forum posts written by pickup winders, guitar players and gear heads. Go ahead and search, “how does an electric guitar pickup work?”, and this is almost all of what you’ll get, in one form or another. It’s easy to see why, given the prevalence of this perspective, that the role of the magnetic field geometry of the pickup itself has been given so much emphasis in the discussion of pickup design, tonality and performance.
But there’s another perspective, one that is more prevalent in books about pickups written by physicists and engineers (Figure 3). It’s also the one put forth by the National High Magnetic Field Laboratory (but, hey what do they know, they’re just Scientists). This interpretation is centered on the string itself as a magnet. The magnetic field of the pickup creates a magnetic dipole in the string in the region above the pole piece. When the string vibrates, it becomes a source of flux, essentially a magnet moving in the vicinity of the coil. In this model, the coil is simply a receiver of the magnetic flux being generated by the moving magnetized portion of the string.
These are two fundamentally different mechanisms of signal generation. In the first, the important part of the equation is the magnetic field of the pickup, and it’s the movement of the pickup’s own field lines that create the signal. The second mechanism, where the string itself is a vibrating magnet, doesn’t really require that the pickup has a magnetic field of its own at all, as long as the string becomes magnetized.
But which is correct? Let’s do an experiment to find out. We built a Zexcoil pickup, complete with everything but magnets. It still has the coils, ferromagnetic pole pieces and structural elements, just no magnets and no magnetic field of its own. We take this no magnet pickup and we suspend it above the guitar, so that it is the same distance from the strings as the magnetized pickup installed in the guitar. Then we switch between them.
What do you think happens?
If the pickup centric model is correct, then the no magnet pickup should generate a much weaker signal than the magnetized pickup installed in the guitar since it is sitting in a region of lower magnetic field strength, as illustrated in Figure 4. The strength of the field generated by the magnetized pickup decays rapidly with distance, and since the no magnet pickup has no field of its own, it would only be getting signal from the fluctuating field lines of the relatively distant magnetized pickup.
If the string centric model is correct, the signals from the two pickups would be equal in strength, as illustrated in Figure 5, since they are both equidistant from the vibrating magnetized string, the source of the magnetic flux.
Watch this video for the answer:
Surprise! Much of what you thought you knew about how pickups work is wrong, or at least incomplete. The most important aspect of electric guitar pickup function appears to be as a receiver of magnetic flux, not a generator of magnetic field. In fact both of these electromagnetic phenomena occur in real life, and may be important to varying degrees in different situations. But as the video shows, a Zexcoil pickup with no magnet at all generates a nearly identical signal level and tonal response compared to the standard Zexcoil pickup, indicating that the string-as-a-vibrating-magnet mechanism is more important in 6 string electric guitar pickups.
So we can see that the role of the pole piece as a magnet, as well as the role of the magnetic field generated by the pickup, has been greatly exaggerated. Our simple experiment shows that the most important function of the pole piece as a magnet is to magnetize the string, and the most important magnetic field to be concerned with is the field associated with the vibrating string, not the static field associated with the pickup itself. In some sense, and for the purposes of this analysis, as long as the string is magnetized we can effectively neglect that the pickup encompasses a magnet at all. The most important role of the pole piece in shaping tone then, is not as a generator of magnetic field, but as a receiver of the magnetic flux radiated from the vibrating string. In this string centric view, the importance of the pole piece as a concentrator and filter of the magnetic flux becomes much more obvious. I will get into more detail about the exact role of the pole piece in coloring the tone and the physics of how and why this works in future posts.
That’s a lot to digest, and I’m sure some of your heads are exploding based on my shattering your pickup centric view of the magnetics, so we’ll leave it there for now and pick up again next time.