Moving-coil loudspeaker

The moving-coil loudspeaker crystallized when four technological streams converged. Ernst Siemens described the moving-coil transducer principle in 1874, recognizing that an electrical coil in a magnetic field could produce mechanical movement. Oliver Lodge refined the concept in 1898 with his "bellowing telephone." Peter Jensen and Edwin Pridham manufactured the first practical moving-coil speakers in 1915. But these early attempts relied on resonance to achieve volume—sacrificing sound quality for loudness.

Three conditions materialized in the early 1920s. Radio broadcasting exploded—KDKA Pittsburgh became the first commercial station in 1920, and by 1922 there were 1,000,000 sets in use. Vacuum tube amplifiers matured, providing sufficient power to drive speakers without resonance. Manufacturing techniques for permanent magnets improved.

Chester Rice and Edward Kellogg of General Electric articulated the breakthrough in their September 1925 paper "Notes on the Development of a New Type of Hornless Loudspeaker." They built laboratory amplifiers capable of real power, eliminating the resonance trap entirely. Their design featured a lightweight paper cone attached to a wire coil in a strong magnetic field, with a butyl rubber surround—details still recognizable in modern speakers. The patent was filed April 20, 1925. RCA commercialized it as the Radiola Model 104 in 1926.

The moving-coil defeated competitors through economics and physics. Horn speakers were efficient but enormous. Balanced armature drivers offered precision but limited frequency range. Electromagnetic field coils worked but consumed constant power. The moving-coil's advantages compounded: wide frequency response in one driver, scalable from pocket radios to concert halls, simple manufacturing, compatible with new vacuum tube amplifiers.

Nearly a century later, Rice-Kellogg's design remains the industry standard. Neodymium magnets replaced electromagnets, digital signal processing corrects nonlinearities, but the fundamental mechanism—coil, magnet, cone—persists unchanged. Every smartphone, car, television, and concert venue relies on descendants of the 1925 patent. The invention constructed a niche that subsequent inventions must inhabit.