Air Motion Transformer
Encyclopedia
The Air Motion Transformer (AMT) is a loudspeaker
mechanism, or audio
transducer
, invented by Dr. Oskar Heil
. It operates on a different principle than both electrodynamic and electrostatic speaker drivers. 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 magnet
ic field.
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 inches (25.4 mm) AMT strip has a functional driver area comparable to an 8 inches (203.2 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.
Although it is possible to operate the diaphragm without the magnetic field, using piezoelectric technology, this method has been used only by TakeT for their headphone and tweeter products.
a California company based near Sacramento. The first speaker was called ESS AMT-1. Looking like a truncated, four sided pyramid, the system combined the AMT driver with a 10-inch woofer and bass port. There were several models that followed. AMT-1a. This redesign of the AMT-1 replaced the smaller woofer with a 12-inch woofer and passive radiator. This gave the speaker better bass response characteristics. The AMT 1 Tower was a speaker design with 1 AMT and a 10-inch woofer with a "transmission line labyrinth." The AMT 3, also called Rock Monitor, a design with 1 AMT, a 6.5-inch mid-range and two 10-inch woofers.
At the end of the 1970s, ESS and Dr. Heil introduced the ESS Transar using one high-frequency AMT and one low-frequency AMT.
It was possible to get ESS speakers from Sacramento until 2006.
The most common use for the AMT driver in consumer electronics today is as a midrange-tweeter
or tweeter in high-end multi-driver speakers, sometimes paired with horns, or in the case of Precide
's speaker products, with an upward-firing woofer driver. There are a couple of companies producing Heil AMTs: Precide (Switzerland) who calls their version the AVT (Air Velocity Transformer), ELAC (Germany) who calls their version of the transducer JET, ADAM
(Germany, under the name Accelerating Ribbon Technology), FAL (Japan), ETON (Germany) and MartinLogan (USA, under the name Folded Motion Tweeter). The Tetra (Canada) 606 uses the Mundorf (Germany) Air Motion Transformer. In Germany, it is possible to get ESS speakers (new or classic designs) as well as the ESS AMT drivers (e.g. Great Heil). Precide uses an AMT driver for their top of the line headphones.
http://www.google.com/patents?id=Yjw1AAAAEBAJ&printsec=abstract&zoom=4#PPP1,M1&as_drrb_ap=q&as_minm_ap=1&as_miny_ap=2006&as_maxm_ap=1&as_maxy_ap=2006&as_drrb_is=q&as_minm_is=1&as_miny_is=2006&as_maxm_is=1&as_maxy_is=2006
US#3,832,499 issued Aug. 2, 1974.
US#4,056,697 issued Nov 1, 1977.
US#4,039,044 issued Aug 2, 1977.
http://www.google.com/patents?id=IpQTAAAAEBAJ&printsec=abstract&zoom=4#PPP1,M1&as_drrb_ap=q&as_minm_ap=1&as_miny_ap=2006&as_maxm_ap=1&as_maxy_ap=2006&as_drrb_is=q&as_minm_is=1&as_miny_is=2006&as_maxm_is=1&as_maxy_is=2006
US#4,107,479 Aug 15, 1978.
http://www.google.com/patents?id=km8zAAAAEBAJ&printsec=abstract&zoom=4&dq=ininventor:oskar-heil&as_drrb_ap=q&as_minm_ap=1&as_miny_ap=2006&as_maxm_ap=1&as_maxy_ap=2006&as_drrb_is=q&as_minm_is=1&as_miny_is=2006&as_maxm_is=1&as_maxy_is=2006
US#4,160,883 Jul 10, 1979.
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
mechanism, or audio
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
, invented by Dr. Oskar Heil
Oskar Heil
Oskar Heil was a German electrical engineer and inventor. He studied physics, chemistry, mathematics, and music at the Georg-August University of Göttingen and was awarded his PhD in 1933, for his work on molecular spectroscopy.-Personal life:At the Georg-August University in Göttingen, Oskar Heil...
. It operates on a different principle than both electrodynamic and electrostatic speaker drivers. The AMT moves air in an augmented, semi-perpendicular motion using a folded sheet (made of polyethylene, polyester
PET film (biaxially oriented)
BoPET is a polyester film made from stretched polyethylene terephthalate and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties and electrical insulation.A variety of companies manufacture boPET and other...
or polyimide), structured around a series of aluminum struts positioned in a high-intensity magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
ic field.
How it works
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 inches (25.4 mm) AMT strip has a functional driver area comparable to an 8 inches (203.2 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.
Although it is possible to operate the diaphragm without the magnetic field, using piezoelectric technology, this method has been used only by TakeT for their headphone and tweeter products.
Application/use
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 as much as one may expect in lower-end 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. The AMT can reproduce sound down into the low midrange fully competently, making design decisions regarding crossover points difficult (depending on the model ESS was using a cut-off frequency of 1, 1.5, or 2.5 kHz).Consumer products
The AMT was first used in 1970 by ESS (ElectroStatic Sound)ESS speakers
ESS was a company that was based in Sacramento, California. Their original speaker designs were a hybrid of conventional woofers, passive radiators, and electrostatic tweeters in bookshelf and tower configurations. They were one of the only manufacturers of Dr...
a California company based near Sacramento. The first speaker was called ESS AMT-1. Looking like a truncated, four sided pyramid, the system combined the AMT driver with a 10-inch woofer and bass port. There were several models that followed. AMT-1a. This redesign of the AMT-1 replaced the smaller woofer with a 12-inch woofer and passive radiator. This gave the speaker better bass response characteristics. The AMT 1 Tower was a speaker design with 1 AMT and a 10-inch woofer with a "transmission line labyrinth." The AMT 3, also called Rock Monitor, a design with 1 AMT, a 6.5-inch mid-range and two 10-inch woofers.
At the end of the 1970s, ESS and Dr. Heil introduced the ESS Transar using one high-frequency AMT and one low-frequency AMT.
It was possible to get ESS speakers from Sacramento until 2006.
The most common use for the AMT driver in consumer electronics today is as a midrange-tweeter
Tweeter
A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...
or tweeter in high-end multi-driver speakers, sometimes paired with horns, or in the case of Precide
Precide
Precide is a Swiss company that makes high-end audio equipment. Precide is one of the few companies in the high end audio market who use Air Motion Transformer or AMT drivers in their products. They are one of only two companies ever to use this driver to produce a headphone and are the only...
's speaker products, with an upward-firing woofer driver. There are a couple of companies producing Heil AMTs: Precide (Switzerland) who calls their version the AVT (Air Velocity Transformer), ELAC (Germany) who calls their version of the transducer JET, ADAM
ADAM Audio
ADAM Audio is a loudspeaker manufacturing company, based in Berlin, Germany. The company offers home loudspeakers and studio monitors. The latter range from nearfield to big main monitors. Both ranges may be supplemented with ADAM subwoofers...
(Germany, under the name Accelerating Ribbon Technology), FAL (Japan), ETON (Germany) and MartinLogan (USA, under the name Folded Motion Tweeter). The Tetra (Canada) 606 uses the Mundorf (Germany) Air Motion Transformer. In Germany, it is possible to get ESS speakers (new or classic designs) as well as the ESS AMT drivers (e.g. Great Heil). Precide uses an AMT driver for their top of the line headphones.
Dr. Oskar Heil's "Air Motion Transformer" (AMT) US Patents
US#3,636,278 issued Jan 18, 1972.http://www.google.com/patents?id=Yjw1AAAAEBAJ&printsec=abstract&zoom=4#PPP1,M1&as_drrb_ap=q&as_minm_ap=1&as_miny_ap=2006&as_maxm_ap=1&as_maxy_ap=2006&as_drrb_is=q&as_minm_is=1&as_miny_is=2006&as_maxm_is=1&as_maxy_is=2006
US#3,832,499 issued Aug. 2, 1974.
US#4,056,697 issued Nov 1, 1977.
- Movable diaphragm connector method flexible hinge diaphragm surround and... http://www.google.com/patents?id=aMkwAAAAEBAJ&printsec=abstract&zoom=4#PPP1,M1M1&as_drrb_ap=q&as_minm_ap=1&as_miny_ap=2006&as_maxm_ap=1&as_maxy_ap=2006&as_drrb_is=q&as_minm_is=1&as_miny_is=2006&as_maxm_is=1&as_maxy_is=2006
US#4,039,044 issued Aug 2, 1977.
http://www.google.com/patents?id=IpQTAAAAEBAJ&printsec=abstract&zoom=4#PPP1,M1&as_drrb_ap=q&as_minm_ap=1&as_miny_ap=2006&as_maxm_ap=1&as_maxy_ap=2006&as_drrb_is=q&as_minm_is=1&as_miny_is=2006&as_maxm_is=1&as_maxy_is=2006
US#4,107,479 Aug 15, 1978.
http://www.google.com/patents?id=km8zAAAAEBAJ&printsec=abstract&zoom=4&dq=ininventor:oskar-heil&as_drrb_ap=q&as_minm_ap=1&as_miny_ap=2006&as_maxm_ap=1&as_maxy_ap=2006&as_drrb_is=q&as_minm_is=1&as_miny_is=2006&as_maxm_is=1&as_maxy_is=2006
US#4,160,883 Jul 10, 1979.
- Acoustic transducer and method of making same http://www.google.com/patents?id=35g5AAAAEBAJ&printsec=abstract&zoom=4&dq=ininventor:oskar-heil#PPP1,M1&as_drrb_ap=q&as_minm_ap=1&as_miny_ap=2006&as_maxm_ap=1&as_maxy_ap=2006&as_drrb_is=q&as_minm_is=1&as_miny_is=2006&as_maxm_is=1&as_maxy_is=2006