Loudspeakers

It's not so long ago that Oskar Heil's Air Motion Transformer (AMT) was regarded as a historical curiosity. But in one of audio's surprising comebacks – prompted not least from its espousal by Mundorf – the technology is now increasingly used in tweeters in preference to dome, isodynamic or ribbon alternatives. What makes the AMT unique is that its principal motion is not in the direction in which it radiates sound. Instead its pleated diaphragm operates like a set of bellows, squeezing air out of its folds. In Monitor Audio's Micro Pleated Diaphragm (MPD) tweeter, the total surface area is said to be six times that of a 25mm dome tweeter, with consequent benefits to power handling and distortion performance. Plus its resistive impedance facilitates simpler crossover design.

In the Gold 100's bass-mid driver, Monitor Audio employs another modern take on an old idea: sandwich cone construction, pioneered 55 years ago by Leak. But MA's Rigid Diaphragm Technology, here in improved RDT II form, uses a sophistication of construction and materials unthinkable in the 1960s. The dished outer skin is of MA's familiar Ceramic-Coated Aluminium Magnesium (C-CAM), behind which are a core of Nomex honeycomb and an inner skin of woven carbon fibre composite. The result is a stiff but light and ostensibly well-damped diaphragm, whose breakup performance is enhanced by the coupling ring used to link it to the voice coil. KH