The first examples of loudspeakers using oppositely-wound dual voice coils positioned along the same axis date from the 1950's. Recent interest in the design has begun practical implementation and proliferation of this concept. 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. This paper describes different design options in magnetic materials, magnetic circuit geometries, and voice coil topologies. The focus is on performance trade-offs and advantages in weight, power handling, power compression and distortion, relative to a single gap design. System applications in the areas of high-level sound reinforcement and studio monitoring will be discussed.
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