AMT Tweeter Air Motion Transformer tweeter

What an AMT tweeter is

An Air Motion Transformer, or AMT, is an electroacoustic transducer invented by the German-American physicist Oskar Heil (1908 to 1994). It works on a different principle from a moving-coil dome or cone, from a planar-magnetic panel, and from an electrostatic panel. The diaphragm is a sheet of polyethylene terephthalate (PET) film folded into pleats like a bellows. A conductor path etched or embossed on the film acts as the voice coil, so there is no separate coil former or dome to move. Wikipedia and Aperion Audio both state that an AMT should not be confused with a planar or true ribbon driver, even though many brands sell AMT parts as "AMT ribbon" tweeters.

How an AMT works

The pleated film sits between four stacks of steel pole plates set at 45 degrees, in a strong opposing magnetic field. When the audio current passes through the conductor traces, adjacent pleats move toward each other, then apart. When two pleats close, air is squeezed out of the gap between them sideways; when they open, air is drawn in. The pleats alternate so that air is pumped in and out of neighboring gaps in opposite directions. Heil's US patent 3,636,278, filed on February 19, 1969 and granted on January 18, 1972, describes this geometry and states that it moves more air with less kinetic energy than conventional diaphragms.

The name comes from a velocity ratio. Air leaves the folds at roughly five times the speed of the diaphragm structure, which gives a mechanical advantage like a transformer. A standard 1 inch (25 mm) wide AMT strip has a working area comparable to an 8 inch (200 mm) round cone, but each pleat moves only a tiny distance. Because the folded film and its conductors have so little mass, the driver reacts very quickly, and the small excursion helps keep distortion down. The film is exposed on both sides at 180 degrees, so the AMT radiates as a dipole,, exciting front and rear sound waves at the same time.

Range and crossover points are set by the design. An AMT is suited to high frequencies but can reproduce sound down to 650 Hz, and crossover points of 800 Hz or 1 kHz are used in practice. Aperion Audio states that its Dual AMT super tweeter can be matched with speakers whose sensitivity is between 87 and 98 dB, so it is not limited to efficient designs.

AMT vs ribbon vs dome tweeters

The three are often lumped together. A dome tweeter pushes air with a rigid piston that is driven from a voice coil. A true ribbon uses a flat or lightly corrugated metal strip in a magnetic field as both diaphragm and conductor. The AMT replaces the flat strip with pleats so that a small panel moves much more air than its footprint suggests. In the tweeter market the AMT competes with electrostatic, ribbon and electrodynamic designs. The trade-off is the dipole radiation: it makes the driver hard to place in a normal closed cabinet without losing sound quality, so AMTs are common as a midrange-tweeter or tweeter in multi-driver speakers and uncommon as low-frequency drivers.

History and patents

Heil founded Heil Scientific Laboratories in Belmont, California, in 1963. Patent 3,636,278 was assigned to that company. ESS (ElectroStatic Sound Systems) of Sacramento first used the AMT in a product in 1972, the AMT1, which paired the transducer with a 10 inch ported woofer. Later ESS models included the AMT1 Tower, with a transmission-line woofer; versions with a 12 inch woofer and passive radiator; and the AMT3 "Rock Monitor," with one AMT, a 6.5 inch midrange and twin 10 inch woofers. A later patent, US 4,056,697, filed on September 3, 1976 and granted on November 1, 1977, covers a folded diaphragm with flexible hinged surrounds.

The folded design stayed bulky for years because the patents controlled it. Klaus Heinz, who met Heil in the San Francisco Bay Area in 1985, waited for the patent to expire and then built a compact version: the Accelerating Ribbon Technology (ART) tweeter, introduced in 1999 for ADAM Audio. HEDD Audio, which Heinz founded in 2015 with his son, builds its own AMT, and HEDD says compact AMT designs of this kind are reflected in Eve Audio, Unity Audio, ELAC, PreSonus and ESI products.

Where AMT tweeters show up

In pro monitors, ADAM Audio's ART tweeter and HEDD's MK2 monitors both use pleated AMTs; HEDD also builds the HEDDphone, a headphone in which an AMT with folds of varying width and depth, a layout HEDD calls Variable Velocity Transform, was unveiled in 2020. In home theater, Aperion Audio sells a Dual AMT super tweeter built around the same principle. Wikipedia notes that some of the variations in Heil's 1972 patent resemble the ELAC JET transducer, and HEDD lists ELAC among the brands whose designs echo the compact AMT. Check each model's spec sheet before assuming a given ELAC, Eve or PreSonus speaker uses a pleated driver, since brands mix dome and folded tweeters across lines.

Common confusions

"AMT" is not a synonym for "ribbon." A planar ribbon has a flat diaphragm, while an AMT is a pleated sheet with printed conductors. "Folded ribbon," "JET" and "ART" are brand names for related folded-diaphragm designs, and the fold geometry varies by brand. Finally, the dipole pattern means the rear wave has to go somewhere: room placement and the wall behind the speaker affect an AMT more than a sealed dome, so check measured off-axis response in an independent review before assuming a given speaker's treble will be smoother than a good dome's.