What is an AMT tweeter?
For DIY audio enthusiasts looking for a unique tweeter with excellent performance, the AMT tweeter is an interesting option. AMT stands for Air Motion Transformer and was invented in the 20th century by scientist and physicist Dr Oskar Heil. The AMT tweeter differs from other loudspeaker designs such as moving coil, planar-magnetic or electrostatic driven speakers in that it uses a different transduction principle. An AMT tweeter should also not be confused with a planar tweeter or true ribbon tweeters.
Unlike a planar ribbon tweeter, the AMT's diaphragm has a bellows-like pleated shape. Using a folded sheet of polyethylene terephthalate (PET) film with metal etching, the AMT moves air sideways in a perpendicular direction. An AMT tweeter can reproduce sound up to 650 Hz and is particularly suitable for high frequencies. In high-end multi-driver speakers, the AMT tweeter is sometimes used as a midrange tweeter or in combination with horns.
How does an AMT tweeter work?
To pump air through the reed chambers, the diaphragm physically moves back and forth in a motion similar to that of an accordion being pulled in and out, albeit over a much smaller range of motion. Due to the extremely low mass of the polyethylene substrate and the significantly shorter distance it travels with each "swing" compared to a dynamic driver, the result is a dipole driver with an incredibly fast response. In a technical sense, it shares characteristics with an electrostatic driver. Order high-quality AMT tweeters from well-known brands such as Dayton Audio, Monacor, Mundorf, Dynavox and SB Acoustics at SoundImports.
For the DIY'er diving deeper into the technology: each diaphragm bending moves very little air, but due to the folded structure, more air is moved than by an electrostatic or conventional cone driver with the same expanded surface area. In terms of area comparison, the functional driver area of an 8-inch (200 mm) round dynamic cone is comparable to a standard 1-inch (25 mm) wide AMT strip.
Due to its folded driver design and small motion range, the AMT behaves like a point source version of a larger driver, which naturally leads to lower sound reproduction distortion. The name "Air Motion Transformer" comes from the fact that the speed of air as it leaves the diaphragm is about five times that of the driver structure.