![]() ![]() The Motion 40i is a tall, narrow “tower” speaker of the type so popular these days. For this review, we are interested in the next to the largest floor stander, the Motion 40i. MartinLogan makes an entire line of loudspeakers employing the AMT tweeter, and they range in price and size from the Motion 2i, a small bookshelf model starting at $US200 each all the way up to the large floor standing “flagship” 60XTi at US$1750 each. Judging by the number of speaker companies both in the USA and in Europe who utilize AMT tweeters, I’d say that the technology has become quite popular of late. Searching on the Parts Express web-site, I see that they carry a number of AMT tweeters ranging in price from about US$20 each to US$120 each. While ESS still makes speakers employing this technology (down to around 800 Hz), the patents have expired and many companies now build variations on this theme. Heil invented this concept in the early 1970’s and applied it a line of speakers built by the firm of ESS in South El Monte, California. When they do that at an audio rate, sound is produced. When the pleats are squeezed together, on one half of their cycle, they compress the air between each fold, displacing it and when the pleats expand again, on the other half of the cycle, they rarefy the air as it rushes in to refill the voids between each fold. The electromagnetic “motor” is arranged so that instead of moving a disc-shaped diaphragm in and out like a normal speaker driver, this one squeezes the pleats in a manner that is perpendicular (rather than parallel to) the desired air displacement. The diaphragm in the tweeter consists of a pleated Polyamide sheet arranged so that the pleated area is facing the listener. Of course, the same thing is occurring inside the bellows as well and that out-rush of air is what powers the instrument and vibrates the various reeds to play the notes. The compressing pleats push air out from between the pleats as that space is eliminated by the compression. If you place your hand close to the accordion’s bellows while someone is playing it, you will feel a rush of air as the accordion is squeezed together. When one pulls the bellows back apart, those spaces re-appear. In an accordion, when one squeezes the bellows together the space between the pleats goes away. The AMT differs from conventional tweeters in that instead of using a piston-like diaphragm the way most magnetic tweeters operate, this technology works similarly to the way an accordion functions. Instead, the Motion series is a cone-based speaker system that employs Oskar Heil’s “Air Motion Transformer” (AMT) technology for it’s high-frequency driver unit. The Motion series is a departure from MartinLogan’s traditional offerings in that this line of speakers does not utilize electrostatic drivers. Readers will remember that several months ago this reviewer wrote about the company’s bookshelf Motion™ 4i speakers. MartinLogan is an American speaker manufacturer (although its main factory is in Canada) long known for its hybrid electrostatic/cone loudspeakers.
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