Air Motion Transformer

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 aluminium struts positioned in a high intensity magnetic field.

How it works

The diaphragm pushes back and forward from itself in a similar physical motion pattern to what is observed when an accordion is squeezed in and 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, caused both by the extremely low mass of the polyester driver, and by the far smaller motion range it undergoes on each “swing” compared to a dynamic driver. In this technical respect, it shares characteristics with the electrostatic driver.

The discernible motion of each of these diaphragm flexes 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 one-inch-wide AMT strip has a functional driver area which is comparable to a circle-shaped dynamic cone with a diameter of eight inches. The folded driver design, combined with the small motive range, means that the AMT acts like a point source version of a larger driver, inherently resulting in lower distortion in sound reproduction. As a result of its motion pattern, the AMT "spits" the air out in a way which is compatible with what happens when, although you apply only a small amount of pressure to a slice of fruit, the pip inside shoots out at high speed. The motion of the air as it leaves the diaphragm is in fact around five times faster than the motion 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 only been used by the TakeT for their headphone and tweeter products.

Application/Use

In the tweeter market the AMT must compete 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 lower down in the speaker market. This is mainly as a corollary of their dipole sound radiation, which makes enclosure in traditional speaker cabinets difficult without sacrificing sound quality or employing sound reflex baffles. The AMT can also reproduce sound right down into the low midrange fully competently, making design decisions regarding crossover points difficult (depending on the model ESS was using a cut of frequency of 1 kHz, 1.5 kHz or 2.5 kHz). The last difficulty to design a loudspeaker with an AMT (midrange-tweeter) is to find a woofer that will be fast enough to be a good match to this very fast transducer.

Consumer products

The AMT was first used in 1970 by ESS (ElectroStatic Sound) a Californian 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 Tower was a speaker design with 1 AMT and a woofer and line transmission. 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 70s, ESS and Dr. Heil introduced the ESS Transar using one AMT for the high frequency and one low frequency AMT.

Until 2006 it was still possible to get ESS speakers from Sacramento on demand. 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). In Germany it is also possible to get ESS speakers (new designs or classic ones) as well as the ESS AMT drivers (e.g., Great Heil).

Recently the low frequency AMT technology was rediscovered by a company called Tymphany. Using what appears to be one of the embodiment of Dr. Heil patent US#4,039,044, the company tries to produce compact woofers and sub-woofers. Tymphany names this technology LAT ( Linear Array Transducer). Precide also use an AMT driver for their top of the line headphone.

Dr. Oskar Heil's "Air Motion Transformer" (AMT) US Patents

;US#3,636,278 issued Jan 18, 1972.

:(Describes the Heil AMTs variations some of which resemble the ELAC JET Transducer) [http://www.google.com/patents?id=Yjw1AAAAEBAJ&printsec=abstract&zoom=4#PPP1,M1&as_drrb_
]

;US#3,832,499 issued Aug. 2, 1974.

:(Describes the Heil AMT embodiment used in the ESS AMT series) [http://www.google.com/patents?vid=USPAT3832499&id=ibI8AAAAEBAJ&printsec=abstract&zoom=4&dq=ininventor:oskar-heil&as_drrb_
]

;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_
] ;US#4,039,044 issued Aug 2, 1977.

:(Low Frequency AMT. The patent also describes the principle used by Tymphany for their LAT transducer including the embodiment type) [http://www.google.com/patents?id=IpQTAAAAEBAJ&printsec=abstract&zoom=4#PPP1,M1&as_drrb_
]

;US#4,107,479 Aug 15, 1978.

:(Further refinements of the Low Frequency AMT) [http://www.google.com/patents?id=km8zAAAAEBAJ&printsec=abstract&zoom=4&dq=ininventor:oskar-heil&as_drrb_
]

;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_
]

Links

*ESS Speakers
*ESS AMT Professional 6
* [http://web.archive.org/web/20000614002208/http://www.essspeakers.com/ ESS Speakers Home Page Web Archive]