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 quite different. A ribbon radiates the sound by the whole diaphragm moving backward and forwards in a uniform manner, 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 expelling the air as they contract. The German company A.D.A.M. Audio recently featured this technology in the way that SLS made big use of ribbons. Dr. Oscar Heil designed the air motion transformer.
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 in surface area to a conventional seven-inch cone type mid-range driver. 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 over a broad frequency range.
The current, in more recent Heil air-motion based loudspeakers, flows through the flat conducting track. Because of the tracks and diaphragms connection, the conductive strip lays parallel to itself on 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 attracte 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.
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, but subjective audibility of this does not seem to be significant as the distortion products are all above 10kHz and about 30 dB down relative to the signal.