The Accordion That Sings: How a Forgotten Physics Trick Reinvented High-Fidelity Sound

A deep dive into the elegant science of Air Motion Transformers — the audio technology that doesn’t push sound, but squeezes it.

Every time you listen to music or watch a film, you are participating in a minor miracle of physics. The core challenge of any loudspeaker is deceptively simple: take an electrical signal and translate it into the physical movement of air, creating the waves that your ears interpret as sound. For over a century, the dominant solution to this problem has been the dynamic driver, which operates, in essence, like a tiny, high-speed piston. A cone or dome is pushed and pulled by a magnetic coil, bulldozing the air in front of it to create sound.

It’s a robust and effective method. It’s also a brute-force approach, bound by the fundamental laws of inertia. Every piston, no matter how light, has mass. It takes force to get it moving and force to stop it. This microscopic struggle against its own weight limits a speaker’s ability to reproduce the most delicate and rapid changes in music and sound effects — the crisp snap of a snare drum, the subtle decay of a cymbal, the intricate texture of a violin bow scraping across a string. This is the piston problem.

What if there was a better way? What if, instead of pushing the air, you could squeeze it?

A Lesson from an Accordion

In the late 1960s, a brilliant and iconoclastic German-American physicist named Dr. Oskar Heil addressed this very question. Heil, whose work spanned from microwave technology to solid-state physics, was dissatisfied with the performance of conventional speaker drivers. He recognized that the piston-like motion was the primary bottleneck for achieving truly lifelike transient response. His solution, born from a deep understanding of fluid dynamics, was as elegant as it was unconventional: the Air Motion Transformer, or AMT.

Forget the piston. Picture an accordion.

Heil designed a diaphragm made of a very thin, low-mass film (like Kapton), onto which a conductive circuit was etched. This diaphragm was then folded into a precise, concertina-like shape and suspended in a powerful magnetic field. When the audio signal flows through the circuit, the folds of the diaphragm contract and expand, moving only a tiny distance.

But here’s the magic. As the folds squeeze together, they force the air trapped within them out at an incredibly high velocity. The key insight is what Heil termed the “acoustic transmission ratio.” Due to the geometry of the folds, the air is accelerated at a ratio of roughly 5-to-1 relative to the movement of the diaphragm itself. The diaphragm moves one millimeter; the air moves five millimeters in the same amount of time.

This is the mechanical equivalent of a gearbox. It converts the low-speed, high-force movement of the diaphragm into a high-speed, low-force movement of air.

This principle single-handedly circumvents the piston problem. Because the diaphragm barely has to move, its own mass and inertia become almost negligible. It can start and stop on a dime, responding to the electrical signal with a speed and precision that a conventional dome tweeter struggles to match. The result is a sound that is astonishingly clear, detailed, and free from the blurring distortion that can plague lesser designs. It was, and still is, a masterclass in elegant physics.

The Science of a Disappearing Speaker

For decades, Heil’s invention remained a niche technology, beloved by audiophiles but largely outside the mainstream. Yet, its unique properties make it uncannily suited to solving one of the most persistent challenges in modern home theater: creating a truly convincing surround sound experience.

The goal of a surround channel is often misunderstood. Unlike your front left and right speakers, which are meant to create a precise, stable stereo image (a “point source”), the job of a surround speaker is frequently to do the exact opposite. It needs to create a diffuse, enveloping, and non-localized sound field. Think of the ambient noise in a dense forest, the echoing chaos of a battlefield, or the general murmur of a crowded room. You should feel inside this environment, not merely be aware of a speaker behind you making noise.

This is where the physics of sound dispersion becomes critical. A traditional dome tweeter acts as a point source, beaming high frequencies forward in a relatively narrow cone, like a sonic flashlight. This creates a small “sweet spot” where the sound is perfect, but move a few feet to the side, and the character of the sound changes dramatically. For surround sound, this is a fatal flaw.

This is where a modern embodiment of Heil’s principle, like the Folded Motion tweeter found in a speaker such as the MartinLogan EMFX2, demonstrates its inherent advantage. By using two AMT-style tweeters angled apart from each other, it leverages the technology’s properties to achieve a remarkably wide horizontal dispersion — a staggering 160 degrees.

This isn’t just a number on a spec sheet; it’s a solution rooted in decades of psychoacoustic research. Pioneering work by scientists like Dr. Floyd Toole at Canada’s National Research Council proved conclusively that listeners overwhelmingly prefer speakers with a wide and, crucially, even off-axis response. A speaker that sounds good not just directly in front of it, but all around it, creates a more believable and immersive experience. The wide dispersion of the Folded Motion array blankets the listening area in sound, preventing your brain from easily pinpointing the speaker’s location. The speaker begins to acoustically disappear, leaving only the sound field behind.

Simultaneously, the design cleverly limits the vertical dispersion to just 30 degrees. This is another piece of smart acoustic engineering, as it minimizes the amount of sound that splashes directly off the ceiling and floor, reducing muddying reflections that can confuse the ear and flatten the sense of immersion.

A Symphony of Systems

Of course, this remarkable tweeter doesn’t work in isolation. A loudspeaker is an integrated system, a symphony of carefully chosen components working in concert. The critical hand-off between the high and low frequencies is managed by a component called the crossover.

In this case, at precisely 2,300 Hz — a frequency where the human ear is exquisitely sensitive to imperfections — the crossover gently rolls off the signal going to the 6.5-inch mid-woofer and seamlessly passes it to the Folded Motion tweeters. This ensures a coherent, unified sound, where the powerful dynamics of the woofer blend perfectly with the lightning-fast detail of the tweeter.

Furthermore, the efficiency of the AMT principle is reflected in the speaker’s high sensitivity rating of 93 dB. In simple terms, this means it requires very little amplifier power to produce a robust volume level. It’s the acoustic equivalent of a lightweight sports car that can achieve high speeds with a modest engine — a direct result of a design that doesn’t waste energy fighting its own inertia.

Ultimately, the journey from a simple electrical signal to a rich, immersive soundscape is one of applied physics. It’s a story about overcoming limitations, like the inertia of a piston, through ingenious thinking. Dr. Oskar Heil’s concept of “squeezing” air was a radical departure, a clever trick of physics that solved a fundamental problem.

The best technology often becomes invisible, not by hiding, but by performing its function so perfectly that it removes itself from your perception. It allows you to forget the box, the wires, and the engineering, and simply connect with the story, the music, the emotion. And it all started with a simple, elegant idea: to build an accordion that could sing.