AMT Tweeter
The Air Motion Transformer (AMT) is a type of electroacoustic transducer or loudspeaker, also
called Air Velocity Transformer (AVT) or JET transducer. It operates on a different transduction
principle than other loudspeaker designs, such as moving coil, planar magnetic or electrostati-
cally driven loudspeakers, and should not be confused with planar or true ribbon loudspeakers.
In contrast to a planar ribbon loudspeaker the diaphragm of the AMT is of pleated shape similar
to a bellows. The AMT moves air in an augmented, semi-perpendicular motion using a folded
sheet (made of polyethylene, polyester or polyimide), structured around a series of aluminum
struts positioned in a high-intensity magnetic field.
How it works
The diaphragm pushes back and forward from itself in a physical motion similar to that observed
when an accordion is pushed in and pulled out to pump air though the reed chambers, albeit
over an exceedingly smaller motion range.
The result is a dipole driver with an extraordinarily rapid response rate, enabled by the
extremely low mass of the polyester driver and the far smaller distance it travels on each
“swing” compared to a dynamic driver. In this technical respect, it shares characteristics with
the electrostatic driver.
The discernible motion of each diaphragm flexure is very small, but because of the folded
structure, more air is moved than would be by a conventional cone or electrostatic driver of
the same plotted surface area. As a matter of surface comparison, a standard 1-inch-wide
(25 mm) AMT strip has a functional driver area comparable to an 8-inch-diameter (200 mm)
circular dynamic cone. The folded driver design, combined with the small motion range, means
the AMT acts like a point source version of a larger driver, inherently resulting in lower sound
reproduction distortion. As a result of its motion pattern, the AMT “spits” the air out in a way
similar to the action of shooting a watermelon seed from your hand by squeezing it between
thumb and forefinger. The speed of the air as it leaves the diaphragm is approximately five times
faster than the speed of the actual driver structure, hence the name, Air Motion Transformer.
The AMT can reproduce sound down to 400 Hz and research is continuing to extend the range
towards bass frequencies. In the tweeter market, the AMT competes against electrostatic, ribbon
and electrodynamic tweeters. While apparently very good as a mid-tweeter or full tweeter, the
AMT is not commonly employed in lower frequency speakers. This is due mainly to the AMT’s
characteristics of a dipole sound radiator, which makes enclosure in traditional speaker cabinets
difficult without sacrificing sound quality or employing sound reflex baffles. Crossover points
are design decisions (depending on the model ESS was using a cut-off frequency of 12 db at
800 Hz, 1 kHz, or in the case of the smaller air-motion transformer unit, 2.4 kHz).
Signature SV Series 11 Owner’s Manual