Although the electrodynamic drive topology incorporating two axially (not concentrically) placed coils is not a new idea and dates back to the 1950s, recent interest in the design has started a proliferation of the design conception professional and consumer markets. The fundamental design involves two coils that are opposite in phase and reside in oppositely polarized magnetic gaps, thus providing a Lorentz force in the same axial direction. The two gaps are formed from the same magnetic circuit, and both coils are wound on the same form, separated by some distance, connected or wound out of phase, and attached to a single diaphragm. Different design options with magnetic materials, magnetic circuit geometries, and voice-coil topologies are described. The focus is on performance tradeoffs and advantages in weight, power handling, power compression, and distortion relative to a single-gap design.
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