Headphones

The planar magnetic drive unit – comprising a flat 'voice coil' laid out on the surface of a thin, flat plastic diaphragm, with bar magnets arrayed close to one or both faces to create a substantially linear magnetic field around the conductors – was patented as long ago as 1961 by Israeli inventors Gamzon and Frei.

But as a practicable alternative to the moving-coil driver in headphones it was hobbled in particular by the low-strength magnetic materials of the time. In 1972 Wharfedale introduced the first commercially available planar magnetic headphone – the Isodynamic – but even with samarium cobalt magnets it had a specified sensitivity of just 95dB SPL for 30mW input, equivalent to just 89.4dB for 1V. Planar magnetic technology has come a long way in the interim. KH