Sound Science: Pickup the Pieces

Face it, a solid body guitar on its own doesn’t sound like much. In order to get anything really useful out of it you need pickups.  Well, you need an amp too, but that’s another article.  Some might argue that you also need talent, but… there’s plenty of evidence to the contrary. Anyway, let’s get to the good part.

An electric guitar pickup is essentially a magnetic core (or set of individual magnetic pole pieces), wrapped with wire. That may sound overly simple, but really it’s true.  There are lots of variables that contribute to the tone of the pickup, like the size, strength, and type of magnet(s), the resistance and inductance of the coil, the number of turns in the coil, and so on, but the general structure is usually the same.  Great, but how does it work?

Well, first let me introduce you to electromagnetic induction.  Electromagnetic induction is a principle linking electricity and magnetism.  Specifically, it states that a change in magnetic flux induces an electromotive force in a closed circuit.  To make this less complicated let’s skip the physics and learn about it by looking at a real world example.

Example: The induction flashlight

Sometimes called a “shake and shine” flashlight or “forever flashlight”, this device is actually powered by shaking the flashlight back and forth.  Inside the flashlight there is a magnet that slides up and down the length of the tube.  See Figure 1 below.  There is also a coil of wire that is permanently mounted inside the tube.  The center of the coil is open so that the magnet can slide through the coil.  When the flashlight is shaken, the magnet moves back and forth through the coil. This changing magnetic field causes a small current to flow in the coil.  The charge from the current is collected and stored so that it can be used to light up the LED.

 

Figure 1: Induction Flashlight

What does this have to do with pickups? It turns out that the effect taking place in the induction flashlight is similar to what takes place in a guitar pickup.

As we stated earlier a pickup is basically a magnetic core surrounded by a coil of wire.  This magnetic core creates a magnetic field which extends into the space surrounding the pickup. This space also includes the guitar strings which are normally made of steel or nickel or some alloy thereof. As shown in Figure 2, the magnet is surrounded by a dense coil of wire.  So there are basically three elements to the system: the strings, the magnet and its magnetic field, and the coil of wire. (The bobbin is the frame around which the coil is wound.


Figure 2: Single Coil Pickup Components

The strings are sitting in the magnetic field.  When a string vibrates, it disturbs the magnetic field in proportion to the vibration of the string. Since the coil is also sitting in the magnetic field, the change in the magnetic field induces a current in the coil of wire, similar to what’s happening in the flashlight example.  It’s this electrical signal that is sent out from the pickup.

Think about it this way: when the open A string is played on a guitar, that string vibrates at 110 Hz. The vibration of the string disturbs the magnetic field around the pickup at the same frequency.  This variation in the magnetic field induces an alternating current in the pickup coil at 110 Hz.  It’s this electrical signal that gets fed to the amplifier and sounds like an open A.

You may have noticed that string height has a large influence on how the guitar sounds.  This is because the height of the string determines the degree to which the magnetic field is disturbed.  The lower the string, the deeper into the field, and the more effect the string has on the output signal.  Putting it another way: a lower string means the string is closer to the magnet, so it generates a stronger current in the coil. The down side of setting the strings close to the pickup is that the magnet exerts a stronger downward pull on the string, which reduces sustain. In the words of Rush: “You can’t get something for nothing.”