Loudspeakers

Conventional loudspeaker diaphragms – whether cone, dome or planar – move back and forth along the axis that they radiate sound. The air motion transformer (AMT), patented by German engineer Dr Oskar Heil in 1973, operates quite differently. Pleated like an accordion, it displaces air by squeezing it out from its crevasses. Thin aluminium strips bonded to the diaphragm carry the signal current and generate the squeezing motion in much the same way as a conventional voice coil, by interacting with a static magnetic field. Oscar Heil conceived AMTs that worked at bass frequencies, and his ideas were exploited in Tymphany's compact LAT subwoofer, now discontinued.

But most AMT drivers today are tweeters, where a resurgence of interest in the technology is evident in ELAC's JET tweeter and the extensive range of AMT tweeters – including dipole models – now sold by Mundorf. Claimed advantages are a total radiating area many times greater than an equivalently sized dome tweeter, which benefits power handling and distortion performance, and a resistive impedance which simplifies crossover design. KH