There can be many attributes that affect the sonic result of a guitar signal chain. One of the most important, yet typically overlooked, parts of the sound are the electromagnetic devices that convert the motion of metal strings into electrical signals: the pickups.
They serve a uniquely significant purpose because they define the character of the instrument, impacting the subjective ‘feel’, aural subtleties and overall physical and electronic response of the instrument. It is the only portion of the guitar that has a direct, causal connection with the physical action of playing the guitar. Despite this, very little appears to be understood in the way of that the engineering considerations involved in pickup design affect their properties.
All pickups work essentially the same way, a magnet generates a magnetic field which is directed and shaped by the pole pieces, and when the metal strings of the guitar is plucked, their movement disturbs the magnetic field and these disturbances induce a current in the coil. This current constitutes the raw signal. The coil is typically wrapped around a plastic bobbin that holds it in place, and the bobbin is attached to the base plate (which can be made from a variety of materials).
However, in modern times, pickups are typically in a humbucker configuration, meaning that they have two coils of wire inside them, wound opposite directions so that there is a destructive interference that eliminates noise created by the interaction of the pickup and the magnetic environment in which the pickup is used. In single coil guitars, the bridge and neck pickup are wound one direction, while the middle pickup is wound the other so that similar humbucking properties emerge when the neck-middle, middle-bridge combinations are used. The wiring of a humbucking pickup is illustrated below, courtesy of 1728.com.
Unfortunately, the field of pickup engineering is filled with design elements whose functional value can be quite hard to determine. While there are elements of the pickup that have a direct impact on the function of the pickup, such as the size, strength and/or shape of the magnet in the pickup, that have generally accepted functional meanings, the relationship between these meanings and the physical behavior of these elements is little understood.
However, noise elimination is one property of the pickup that the device is designed to maximize. One common practice to reduce noise involves a process called ‘potting’, where the coil is dipped in molten wax so that the movement of the wires in the coils are minimized. This, reduces noise caused by subtle vibrations in the coil due to playing the instrument. Beyond that, little can be done in the construction of the pickup (as far as contemporary pickup design is concerned) to reduce noise, although gate and filter pedals can be used to reduce noise further down the chain.
One of the primary components of the pickup, across all designs, is the magnet. It can be made of various materials, but the most common are alnico (aluminum, nickel and cobalt) and ceramic (althought the exact composition of these magnets isn’t much understood), and in some niche cases neodymium magnets are used. The various compositions of alnico magnets, each differing in elemental composition, have different magnetic properties, making some produce a more ‘vintage’ sound (low output) and other produce a more ‘modern’ sound (higher output). The physical shape of the magnet also affects the sound of the pickup, as its topology has a direct impact on the magnetic field through which the strings must pass.
In order to better understand the magnetic properties of the pickup, I would like to draw on Dr. A Scott Lawing’s brilliant series of articles on his blog (link: https://lawingmusicalproducts.com/dr-lawings-blog) as well as the work featured on Liam’s Research (link: https://sites.google.com/site/liamsresearch/). Both of these sources provide immense physical insight into the behavior of pickups.