The moving-coil principle commonly used today in speakers was patented in 1925 by Edward W. Kellogg and Chester W. Rice issued as US Patent 1,707,570. Apr 2, 1929. The key difference between previous attempts and the patent by Rice and Kellogg is the adjustment of mechanical parameters so that the fundamental resonance of the moving system is below the frequency where the cone's radiation Impedance becomes uniform. About this same period, Walter H. Schottky invented the first ribbon loudspeaker together with Dr. Erwin Gerlach.