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Piezo or "contact" pickups:
These use a piezo-electric element that basically produces an electrical charge when squeezed. Quartz crystals do this. Sound is compression waves travelling through the air to your ear, and travel through any material, like the body of the instrument, which you can feel as vibrations. The shockwaves compress the piezo element as they pass through it, and the piezo element converts these waves into electrical waves that are transmitted down the cable as a signal to the amplifier. The nature of the tone is controlled by what materials the vibrations pass through before they are absorbed by the piezo element, and what frequencies are filteredby those materials. I have heard of pizo elements being used to create all sorts of pickups, using diferent materials, sizes and shapes, and ways of attatching the pickup or suspending it near the instrument.
Microphonic pickups:
These use the same principle as a speaker, in reverse. And in fact a speaker can be used as a microphone. A mobile diaphram moves in respose to the sound waves in the air, a coil or a magnet on the diaphram reacts with a fixed coil or magnet, the motion between them is converted into electrical waves in the coil, which pass down the wire to the amp. Some microphones are passive, others use a power source to excite the coil and boost the sensitivity by creating an active electromagnetic field.
Coil pickups:
These are what you see on typical electric guitars. They only respond to steel strings, which contain iron, so react to and cause reactions in magnetic fields. When a fixed coil with fixed magnets is placed next to vibrating steel strings, the string's vibration induces waves in the magnetic field generated by the fixed magnets. The waves in the magnetic field then induce electric waves in the electo-magnetic field of the coil of copper wire. This is the signal that is transmitted down the cable to the amp. A coil with many turns produces a louder signal, but excentuates the higher frequencies. Guitar coils usually run 6 K (thousand) to 10K turns. As I uderstand it so far, too few turns and the signal is weak and lacks highs that give clarity and bite, too many turns and the signal gets too hot and brittle, lacking depth and tone from midrange frequencies. A pickup with a weak signal can be boosted by using a power source like a battery to excite the coil and boost the sensitivity by creating an active electromagnetic field, so the induced vibrations are riding on the already present but unmodulated (no waves) current produced by the battery.
The type of magnet and placement affect the sound of the pickup, and magnetic and metal posts are used on the inside of the coil to direct the magnetic field toward the strings, and adjustable height posts allow the response of each string to be fine tuned. The signal can also be boosted using stronger magnets or moving them closer to the strings, but at a certain point they start to actually damp the motion of the strings themselves.
An important principle to understand in coils is what changes tone. One aspect is the shape of the coil. For the same number of turns, a wider (and thus flatter) coil will sense the signal over a wider area of the string, while a narrower (and taller) coil will sense a smaller area of the string. This relates to another important principle which is placement. The difference in tone on a multi-pickup guitar is often not from different pickups, the pickups are often the same, but they are sensing the string in different places. Why this matters is harmonics. If you have ever played the harmonics on a guitar, then you know that you can get a related tones, harmonics, to the open string by plucking it while gently damping it at specific points. At the exact point you are touching it, the string isn't moving for that harmonic. A plucked string vibrates with all those harmonics as it vibrates, so a pickup placed right under one of thos harmonic points won't sense that harmonic because the string isn't moving directly above it as it sounds that harmonic. This might take some thinking to grasp, but if the pickup is placed under the position of the 3rd harmonic, it doesn't sense that harmonic, and thus, gives you the sound of the string minus the third harmonic. That is why the tone is different depending on whether the pickup is placed under the 2nd harmonic, or the first, or none at all.
"Humbuckers" and "switched coils":
A coil can also pickup signals and fields generated by other coils. This means transformers in amplifiers for example. The "hum bucker" was designed to use the principle of phase canceling so avoid induced signals. Basically, two coils are set side by side or on top of each other, then are wired to be 180 degrees out of phase, opposite, each other. When two signals are exactly the same and 180 degrees out of phase, they cancel each other out, while two different signals just add together into one signal. In a humbucker, the two coils are sensing the string in slightly different places, so have different signals in regards to the string. However, an induced hum, like a radio signal, is exactly the same in both coils so cancels out, thus the "hum bucker".
You can also get an effect by placing any coils or their signals out of phase, called phasing, of course. Which leads to the final possibility of coils, which is different ways of switching and combining the signals from the pickups. Such as using a coil that has a loop pulled out halfway through the winding process. By placing a switch there, you can switch from the coil acting as a single high wind pickup (in series), or acting as two coils each with half the windings of the entire pickup, and a different tone (in parallel and/or out of phase).
This page is part of a section dedicated to building pickups for electric hammered dulcimers, which is part of my extensive home site, where you'll find lots of info on me, my music, and the electric and acoustic hammered dulcimers I have designed and built. There are pages on my recordings, travels, ARCHIVES of tour journals, background information, recent news, and lots more.