Loudspeakers

Before there were permanent magnets, the first loudspeakers utilised electromagnets. A large 'field coil', energised by hundreds of volts of DC through a second pair of connections to the driver, created a temporary magnetic field. It wasn't until after World War II that advances in permanent magnet materials, and their subsequent affordability, saw the decline of field coil drivers for consumer audio applications – although recent years have seen a resurgence of interest among audiophile hobbyists, with various driver models available for DIY speaker builders from the likes of Feastrex, Supravox, Voxativ and Lowther America.

Varying the voltage to Focal's 400mm electromagnetic low-bass driver alters both its sensitivity and electromagnetic damping, allowing its output to be tuned to suit a range of different room conditions.