Most loudspeaker drive units are of the moving coil variety. A coil of wire sits inside the field of a permanent magnet. The coil is attached to a cone or dome-shaped diaphragm. When a signal is passed through the coil, it moves forwards and backwards, thus causing the diaphragm to vibrate, creating a sound wave.
The shape of the diaphragm has always been a problem – how to get a light, stiff diaphragm that couples vibrations efficiently to the air? But pushing and pulling at the air isn't the only way to get it to move. What about squeezing the air, like shooting an orange pip by squeezing it between the fingertips?
The Heil air motion transformer, invented by Oscar Heil, works in exactly this way. The diaphragm is neither cone nor dome-shaped, nor flat for that matter. Instead it is folded like the bellows of an accordion. The magnetic field now does not push and pull in the direction of sound propagation. Instead it operates at right angles, squeezing and unsqueezing the folds of the diaphragm.
The result is a highly efficient drive unit because the velocity of the air particles squeezed out is much greater than the speed at which the diaphragm moves. The fact that it is called a 'transformer' is because the acoustic impedance of this unit is very similar to that of air. (An impedance mismatch means that sound doesn't transfer well from one medium to another).
The Heil air motion transformer has so far only found application at high frequencies. It is said to be crisp and responsive. But one additional good point is that it can be very efficient for its size. A tweeter should be small so that it radiates widely, and this has been one of the sticking points in conventional tweeter design, particularly for PA loudspeakers.
There is a small but growing number of loudspeakers featuring air motion transformers, both for studio and hi-fi applications. Whether this technology will really catch on and replace conventional tweeters remains to be seen. But it would be interesting, wouldn't it?