Pickups are essentially the "microphone" for an electric guitar. They capture the sound of the vibrating strings and send it to the amplifier where it's converted into a sound. There are many different kinds, but they all function in largely the same way. Understanding the physics of how a guitar string vibrates can help you get the most from your pickups.
Guitar pickups are essentially electromagnets. Just like the experiment from school science class where a length of wire wrapped around a nail became a magnet when connected to a battery, pickups are lengths of wire, coiled around a magnetic core, that generate an electromagnetic field when they're connected to a power source, in this case, an amplifier. The movement of the steel strings disrupt the field, creating variations in the current. This is detected by the amplifier and turned into sound.
There are two main types of electric guitar pickups: single coils and double coils, often referred to as "Humbuckers." Single coil pickups, as the name suggests, have one coil of wire around a single magnet. They're twangy and punchy, often seen in country and blues music. Double-coils feature two parallel magnets, with two separate coils wound in opposite directions. The opposing magnetic fields help to filter out much of the magnetic interference from nearby electrical devices. This allows a much higher output and a fatter sound. They're commonly used in heavier rock music.
Most electric guitars have two or more pickups, often identical in design, spread between the neck and bridge of the guitar. The different vibration characteristics of the string at different points along its length create different tonal properties. Positioning multiple pickups, even identical ones, can radically change the tone of the pickup's output. Placing it near the neck emphasizes bass frequencies, giving a round and warm tone. Positioning it closer to the bridge, where the vibrations are much smaller, emphasizes the high frequency overtones. This creates a sharper, more piercing sound.
Other Pickup Types
In addition to the traditional electromagnetic pickups, there are also piezo microphones. They contain quartz crystals sensitive to contact vibration. When a current is passed through them, they react like the diaphragm of a microphone. A piezo pickup can be as thin as paper and installed very discreetly. On acoustic guitars, they are placed under the bridge saddles. Roland uses a piezo pickup for its guitar-synthesizer, which is glued to the body behind the scratch-plate. The vibrations are translated into MIDI data, which is used as an input for synthesizer module.
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