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How does an audio interface sending tiny pulses of electricity make a monitor cone vibrate violently enough to shake the air in your control room?
A moving-coil monitor driver is an electromechanical transducer โ a device that converts electrical audio signals into mechanical motion and sound pressure waves.
The Anatomy of a Moving-Coil Driver
The motor assembly creates an intense, concentrated magnetic field using a permanent magnet clamped between a steel backplate with a central pole piece and a circular top plate.
A tightly wound cylinder of enameled wire called the voice coil floats in the narrow annular gap (the circular slit) between the pole piece and top plate, bonded directly to the speaker cone.
๐Interactive diagram
The flexible spider and outer surround keep the coil centered in this narrow magnetic gap so it can slide forward and backward without scraping the metal walls.
What physical law actually forces this floating coil to push outward or pull inward when an audio current flows?
The Lorentz Force Motor Principle
In 1895, Dutch physicist Hendrik Lorentz demonstrated that an electrical charge moving through an external magnetic field experiences a perpendicular physical deflection.
When an amplifier sends an electric current through a wire situated inside a magnetic field, the conductor experiences a mechanical Lorentz force (F):
F=Bโ
Iโ
L
In this formula, B is the magnetic flux density in the gap in teslas (T), I is the audio current in amperes (A), and L is the total length of wire immersed in the gap in meters (m).