Heil Air-Motion Speakers
Last Edited: Dec 26, 2023
What Are Heil Air-Motion Speakers?
People often confuse Air-Motion Speakers with ribbon loudspeakers when they see the folded diaphragms set in the shorthorn. But they are pretty different. A ribbon radiates the sound by the whole diaphragm moving backward and forward uniformly, with the plates never changing their angles. What's more, the air-motion transformer, quite differently, moves its diaphragm in a concertina movement. It draws the air into the folds as they expand and expels the air as they contract. The German company A.D.A.M. Audio recently featured this technology in how S.L.S. used ribbons. Dr. Oscar Heil designed the air motion transformer.
Construction
In the original design of DR. Heil's air motion speaker, the amt diaphragm consists of a mylar ribbon bonded with conductive aluminum strips. It is equivalent to a conventional seven-inch cone-type mid-range driver in surface area. However, it is accordion-folded down to a compact one-and-a-half-inch grouping for point-source dispersion. The low-mass diaphragm is suspended within a dipole magnetic field. It's concentrating an intense field around the said diaphragm. When signal current passes through the aluminum strips, the ensuing bellows-like motion of the folded pleats moves air five times faster than a conventional cone driver. This rapid acceleration of air motion provides enhanced sound reproduction. It includes a high dynamic range and a broad frequency range.
Current Flow
The current flows through the flat conducting track in more recent Heil air-motion-based loudspeakers. Because of the tracks and diaphragm connection, the conductive strip is parallel to the adjacent fold. When the current flows through one direction over the entire circuit, it travels in different directions in the conductors of adjacent folds. So, the nearby permanent magnets either attract or repel the magnetic field. When the current in the circuit reverses, the open folds close, and vice versa. Hence the name because there is a ratio of about four to one between the air particle velocity in and out of the folds relative to the velocity of movement of the plated diaphragm.
Large Magnet
The magnet structures need to be very large because the entire plate diaphragm must fit in the gap between the poles. The diaphragms are made from plastics such as p.t.f.e. Or polyethylene, which has good damping. Current units can work from about 500 Hz to 20 kHz. Some models have been shown to produce quite high levels of second harmonic above about 5kHz. Still, the subjective audibility does not seem significant as the distortion products are all above 10kHz and about 30 dB down relative to the signal.
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