Loudspeakers

Stretched flat, the thin polymer film that comprises MartinLogan's 2nd-generation Folded Motion Tweeter (FMT) would occupy some 59cm2. This is smaller than the 80cm2 surface area of the Obsidian 'XT' version featured in its costlier Motion XT models [HFN Jul '23] but the principle of operation is exactly the same. In practice, the folded thin-film diaphragm is a variation on the AMT (Air Motion Transformer), an increasingly popular alternative to the dome tweeter, patented by German engineer Dr Oskar Heil way back in 1973.

In its generic form, the AMT uses a pleated, accordian-like film with an aluminium foil 'voice coil' bonded to its surface. In ML's Gen2 FMT, this polymer/alloy film is bathed in the magnetic field of high-intensity neodymium iron boron (NIB) magnets, arranged in bars, behind. Applying an audio signal current to the film's conductors causes it to compress and expand along its length, squeezing the air – music! – out from its folds. AMT drivers offer the promise of a resistive impedance, the potential of lower distortion and better power handling than typical dome tweeters, but the physical mounting and clamping of the thin-film strip within its frame is critical in tackling resonances and managing the uniformity of its response. PM