SLOP2186 · Week 3
From Vibration to Voltage#
How a magnetic pickup turns string motion into an electrical signal.
Slop University · SLOP2186
The question
String moves.
Cable carries a voltage.
What happens in between?
The missing link
STRING → ? → VOLTAGE
Something between the string and the cable has to turn motion into an electrical signal.
Meet the pickup#
A magnetic pickup sits under the strings and does exactly one job: sense string motion and turn it into a voltage — no moving parts touch the string at all.
Photo: John Tuggle · CC BY 2.0
Pickup anatomy#
Magnet, pole piece, coil — the three physical parts
Motion changes flux#
String held still
Flux constant · no output
String moving
Flux changing · voltage induced
A still string: constant flux, no output. A moving string: changing flux, a voltage appears.
Faraday’s law#
String held still
Flux constant · no output
String moving
Flux changing · voltage induced
ε = −N dΦ/dt
The induced voltage is proportional to how fast the flux changes, not the string’s position
What’s in the voltage#
Amplitude, period, and a harmonic series — not one pure tone
Position is sampling#
A pickup only senses motion at the one point it sits at, so it samples the string’s standing-wave modes at that location.
Bridge position
Neck position
Why bridge sounds brighter#
≈ 1/5
the fundamental’s relative amplitude at a bridge-like position — weaker there, so upper harmonics dominate more of the balance
The single coil#
One magnet, one coil, several thousand turns of fine copper wire — simple, and with nothing to reject interference it picks up along the way.
Photo: William Warby · CC BY 2.0
The humbucker#
Two coils, wound and wired in opposition, sensing the same string from two adjacent points.
Photo: William Warby · CC BY 2.0
Noise cancellation#
Interference arrives alike at both coils and cancels; the string arrives opposite and adds
Recap
Single-coil vs. humbucker#
Single-coil — one coil, senses interference along with the string, brighter and more position-sensitive.
Humbucker — two opposed coils; common-mode interference cancels while the string’s signal adds. Comparatively darker, higher output, and quiet.
Listening: try it#
Open this week’s page and drag the pickup along the string in PickupLab — the sound moves with the curve, because the same expression drives both.
The full causal chain#
Next
The guitar is a circuit#
Resistance, capacitance, and the guitar’s own volume and tone controls all shape this voltage before it ever reaches a cable.