The Ionophone

Author: N.H. Crowhurst

A loudspeaker with no moving parts

With every kind of loudspeaker, there has always been the problem of mechanical parts which can add effects of their own to the sound radiated. It has long been a dream among idealists in high-fidelity circles to find a loudspeaker which had no moving parts, but converts electrical waveforms directly into sound waves. For this reason the ionophone has a strong appeal.

Its operation is based on the electric wind principle. When a high voltage is connected to a point, a discharge takes place from the point, in the form of wind. By having the high voltage vary in accordance with the audio waveform, the wind leaving the point will vary in velocity and produce a sound wave to correspond.

The velocity of the wind is not strictly proportional to the applied voltage. Quite a high voltage is needed before the wind starts, and then its velocity rises quickly. A way of avoiding the distortion this would cause is to use high voltage alternating at a radio-frequency and modulate it with the audio.

Its popularity has been delayed by the problem of getting enough wind to "generate" the lower frequencies. Many points could be used instead of one, but then there are apt to be acoustic problems as severe as the mechanical ones we want to avoid. Another acute problem is the wind itself. It is much more difficult to get a stream of air quiet enough not to obtrude into the quieter program passages than it is with an electron stream in an electronic tube or transistor.

One advantage to the thing, if these difficulties are solved, is that no detector is needed in handling radio signals. The radio signal input itself is simply amplified and fed directly to the ionophone.