Folded motion tweeters are a variation of speakers that is used to produce high frequency sounds. They differ quite a bit in design and function from a standard dome tweeter. They are constructed of 4 main parts, the front plate, diaphragm, magnet and back plate. Though they sound simple, constructing them is much more complicated. They are a some what delicate part and take careful planning to sound their best.
The parts and their purpose
Lets take an in depth look at those parts. The plates on the front and back hold the assembly together like a sandwich. Between those, there is a diaphragm made typically of polyethylene, it has a pair of electrical circuit traces along it, one positive and one negative. The change in charge along them causes the folds of the tweeter diaphragm to move like an accordion. Each fold, as it squeezes together forces air to move outward from the fold. This is why the generic term for the motion tweeter is Air Motion Transformer. Directly behind the diaphragm of the tweeter are magnets. They type, lay out, and number of them depends on the manufacturer. Martin Logan uses multiple neodymium iron boron magnet strips. The strength of the magnet will effect how much force is exerted upon the air in the folds.
Folded Motion Tweeter Video
What about bass
Folded motion tweeters do not produce bass well at all. In almost all applications you will have a hybrid speaker, meaning there are also standard drivers installed in the cabinet for either mid bass or low bass, sometimes both. They produce incredible neutral highs due to the effective surface area of the driver. A 1 inch wide strip has about the same area as an 8 inch circular shaped driver. It is able to move much faster though which allows it to produce such detailed sounds.
Are they a ribbon tweeter?
Some people may confuse the Folded Motion Tweeter with a ribbon tweeter, though similar, they are not the same. A ribbon tweeter uses an aluminum ribbon that is suspended in the electromagnetic field. The positive and negative current that is passed across the aluminum diaphragm cause it to move in an out. Some ribbon tweeters are flat, some pleated and some have the same shape as an Air Motion Transformer. The design and function is very similar so confusing them is easy. Some people seem to use the term Ribbon tweeter as a blanket for all motion designs as well, sort of like Wise Grips for any locking pliers.
Folded Motion Tweeter Summary
The air motion transform, or Folded Motion Tweeter as called by Martin Logan is a great sounding component for almost any listening environment. It produces detailed and directional sound that creates a beautiful stage. They are also fast, and neutral, two great qualities of any speaker. When combined with a typical mid bass driver the whole package covers a wide range of frequencies. The only down side is that in smaller speaker set ups such as the LX 16 that I own, you may want to add a subwoofer to fill in the lower frequencies. This is much less an issue with something like the Motion 60 that has larger bass drivers. One concern many listeners do have is the brightness. They can be very bright and fatiguing at times. If you prefer a warmer tone you may want to avoid anything with a Folded Motion tweeter. I, myself actually prefer brighter speakers, so they work perfectly in my room.
If you would like to see more you can check out Martin Logan’s Page explaining their design.