The HEDD Air Motion
Transformer by Klaus Heinz
Of the many ideas on how to build a loudspeaker transducer, Oskar Heil’s (1908–1994) Air Motion Transformer is unique in its way of making the air move. In the course of the history of loudspeakers, many different transducer designs have been introduced. All of them feature piston-like diaphragms that move the air in a 1:1 ratio so that the diaphragm velocity equals the air velocity. Whether we talk about electrostatic and magneto-static speakers, ribbons, voice coil based dome and cone speakers or even a Blatthaller design – they all follow this particular design approach.
It was not until the early 1960s that German-American physicist Oskar Heil came up with a new idea and proposed a folded elastic diaphragm, where single folds open and close in an alternating pattern and thus „breathe“ air in and out, with the special charm that the air driven through the folds is accelerated to a ratio four times as fast as the diaphragm itself. In the early 1990s, Klaus Heinz was so intrigued by Heil’s idea that he used it to build a compact, reliable, and superior sounding tweeter.
Heinz later went on to found ADAM Audio in Berlin, a company that manufactures loudspeakers based on Heil’s invention. The X-ART tweeter designed by Heinz during his time at ADAM was widely praised and gradually revolutionized the speaker landscape, as similar designs emerged on both the HiFi and the studio monitor market. In 2020 HEDD has introduced another evolutionary step in the development of the AMT: The HEDDphone® is not just the first full-range AMT headphone, it also incorporates VVT® technology. This means that the HEDDphone® is based on a new diaphragm geometry that expands the AMT principle to a linear full range (10Hz–40kHz) headphone transducer. It replaces the fixed geometric structure of conventional AMT drivers as the folds vary both in width and depth.
Design and Principle
The Air Motion Transformer is an electromagnetic driver, as it is based on the Lorentz force that moves the air in the single folds.The diaphragm itself has an aluminum circuit printed on it (violet arrows) and is surrounded by a strong magnetic field. The graphics in and around the small circles show the motion of the individual foils producing a sinusoidal waveform: from where it starts (black circle) through the positive (green circle) and negative (red circle) half-waves. The resulting air flow (blue arrows) is four (!) times faster than speed in which the individual folds move, which is a big advantage when it comes to reproducing music signals with fast transients (cymbals, plugged guitar strings, etc.).
Klaus Heinz’ early encounters
with Oskar Heil in Silicon Valley
Klaus Heinz and Oskar Heil’s first meeting took place in the 1980s. It is a moment that Klaus Heinz remembers vividly:
“It was back in 1985 when I got to know the Air Motion Transformer and its inventor Oskar Heil. I was intrigued by the idea and the sonic results of Heil`s ESS loudspeakers, which used the Air Motion Transformer in a rather quirky way and definitely included some questionable details. And yet, the accurate and brilliant sound removed a heavy curtain from all the recordings I liked and had previously listened to in what I had until then considered a rather decent quality. I was curious enough to visit Heil in his home in San Mateo, south of San Francisco, an area known as the Silicon Valley. I listened to his life story, we discussed electrons (his favorite „animals“) and, of course, loudspeakers. I was very impressed when granted the chance to visit his laboratory in Belmont. Here, I found myself in the most disorderly, chaotic room I had ever been in, and yet Heil definitively knew where to find what. He showed me quite a number of new transducer approaches that were different from the boring voice coil / piston-like diaphragm combinations that we were, at the time, familiar with, be it in the form of cones or domes.”
Klaus Heinz recalls how Heil presented some of his highly original and often quirky inventions to him – and how fascinated he was by the physicist’s imaginary reservoirs:
“What fascinated me most when discussing loudspeakers with him was the fact that he really thought originally. He constantly looked for new ways to create sound out of alternating current. He did not first ask which voice coil parameter or which resonance frequency would play what role, as a typical engineer in the Research & Development community would have done, but rather focused on the physical interdependencies between current and force in magnetic or electrical fields. Heil also constantly thought about ways to develop a better flaming ionic tweeter than Alan Hill’s Plasma driver, a fascinating device that excited air molecules electrically in order to reproduce frequencies between around 700hz and the ultrasonic range. While Hill’s invention was probably great for listening to single songs, a Mahler Symphony would probably have caused lethal lung cancer or similar illnesses with all the ozone produced by the music.”
During one of their first meetings, Oskar Heil surprised Klaus Heinz by telling him that it had actually been him who invented the dome back in the 1930s.
“How was that possible? From everything I knew, Wolfgang Seikritt invented domes in the 1960s, and in Germany. Yet here was Oskar Heil who led me through the almost absurd process of his invention: “What I did,” he explained, “was that I took my mother’s silk stockings, then added a darning mushroom, drew the stocking over the mushroom, applied egg white as a thin coating, let it dry out, glued it to a voice coil – and there it was: the first dome tweeter made of wood, stockings, and eggs.”
Although quite a few other audio-related inventions can be traced back to Heil’s ideas (such as the ‘traveling tube,’ the so-called ‘Wanderfeldröhre,’ which sensationally allowed to amplify frequencies up to above 1 GHz), Klaus Heinz’ favorite discovery in Heil’s lab was the Air Motion Transformer, which should later become the signature design of the many loudspeakers that Heinz developed from the 1990s onwards and that he remains dedicated to until this very day.
Variable Velocity Transformer:
The Evolution of the Air Motion Transformer
In the development phase of the HEDDphone® it became necessary to rethink one of the key properties of the original AMT driver geometry: that the folds in the driver share the exact similar shape. A new diaphragm with folds varying both in width and depth had to be engineered to reproduce the full audio spectrum from 10 to 40 kHz. We call this evolutionary step of the Air Motion Transformer VVT - Variable Velocity Transform.