Low Impedance Voice Coils for Improved Loudspeaker Efficiency

In modern audio systems, utilizing switch-mode amplifiers, the total efficiency is dominated by the rather poor efficiency of the loudspeaker.For decades voice coils have been designed so that nominal resistances of 4 to 8 Ω is obtained, despite modern audio amplifiers, using switch-mode technology, can be designed to much lower loads.A thorough analysis of the loudspeaker efficiency is presented and its relation to the voice coil fill factor is described.A new parameter, the drivers mass ratio, is introduced and it indicates how much a fill factor optimization will improve a driver's efficiency.Different voice coil winding layouts are described and their fill factors analysed.It is found that by lowering the nominal resistance of a voice coil, using rectangular wire, one can increase the fill factor.Three voice coils are designed for a standard 10" woofer and corresponding frequency responses are estimated.For this woofer it is shown that the sensitivity can be improved approximately 1 dB, corresponding to a 30% efficiency improvement, just by increasing the fill factor using a low impedance voice coil with rectangular wire.