From the ESS website:
The HEIL air-motion transformer is recognized by the audio industry as the most significant loudspeaker breakthrough of the last century and is licensed EXCLUSIVELY to ESS. Once you’ve heard it, you’ll agree that the ESS HEIL air-motion transformer indeed delivers “SOUND AS CLEAR AS LIGHT”.
"The Heil Air Motion Transformer (AMT) uses a folded pleated Mylar Diaphragm of a mere 0.0005” thickness! The voice coil is a serpentine pattern of precision etched aluminum chosen for light weight and high power handling. Unlike a normal piston loudspeaker which excites the air by alternately pushing and pulling the diaphragm, the AMT displaces the air by alternately squeezing and expanding the pleats, moving the air at a very high velocity. Because the moving mass of the AMT is extremely low, and the magnetic structure very powerful, this combination results in extremely high acceleration yielding amazing transients. The AMT's low mass and high output allow for a unusually high combination of efficiency and bandwidth to be attained in a single loudspeaker.
Ess is the manufacturer of the original and still best Air Motion Transformer invented by Dr. Oskar Heil over 30 years ago. The AMT is able to efficiently reproduce frequencies over the very wide range of 500 to 20,000 Hz. The minimum recommended crossover frequency is 800 Hz at 12db/octave or more. Impedance is 4 ohms nominal, 3.5 ohms minimum. Maximum sustained RMS power is 400 watts RMS band limited pink noise, 160 watts maximum peak. Sensitivity is 92db for 1Watt @ 1 Meter."
They have met my every expectation.