Faithful Reproducer

Electronic engineers loathe mechanical moving parts. One that has always bothered them is the light, vibrating diaphragm in the throat of a loudspeaker. Compared to the almost weightless electrons that flash through radio tubes, the loudspeaker membranes are sluggish. Their slow and clumsy response distorts the delicate signals brought to them by the electrons; the ordinary mechanical loudspeakers cannot reproduce the full range of music or the human voice. The ideal loudspeaker, the engineers have long believed, should have a diaphragm almost as weightless as the electrons themselves.

In the current issue of Radio-Electronics is a description of a French loud speaker that comes close to this ideal. It inventor, Paris-born Siegfried Klein, de cided that the vibrating parts of a loud speaker should be replaced by some device that would turn electrical signals directly into sound waves in the air. After many tries and failures, he developed his "lonophone," a complicated device whose basic principle is simple.

Many substances, including platinum give off ions (electrified particles) when heated to high temperature. At one end of the lonophone's quartz tube is a small quartz cylinder with a coating that contains fine particles of platinum. When the platinum is heated electrically to about 1,000DEG C., it fills the horn-shaped cavity above it with a cloud of rapidly zigzagging ions. The ion cloud responds almost instantly to changes in the strength of a high-frequency electric field around the little quartz cylinder, and the cloud's expansion and contraction set up sound waves. When a current carrying music or voice signals is fed to the apparatus, it turns into sound with almost no distortion.

To work at full efficiency, the lonophone requires a large horn, but even the table model is a remarkable improvement on conventional loudspeakers. It is sensitive, Klein says, to sound waves up to 400,000 cycles per second. (The average human ear can hear only about 16,000 cycles, and the average home loudspeaker does not work well above 10,000.)

Klein already has contracts with leading European manufacturers of loudspeakers and electrical equipment. The lonophone also has another talent, which should intrigue the military: it can be used as a microphone sensitive both to ordinary sounds and to ultrasonic vibrations. This should make it useful in submarine warfare, where ultrasonic ranging leads the hunters to their prey.

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