That was a tube which ionized a volume of air and could be persuaded to produce a highly directional high frequency superaudio signal. The theory was that if one modulated both Ionovac's with, say, a 75 KHz "carrier" with one of them mixed with an audio signal, then the audio signal would emanate from the physical intersection of the quite directional audio signals from the two tubes. We convinced ourselves that these were too inefficient for our purposes.

Audio program, nowadays, is miked with many if not dozens of microphones, and mixed by people with many years' experience to get the sound that they want. Unfortunately, it's not always the sound of what was recorded. Even back in the '60's, when multi-miking was already popular, a few of us believed that "real" stereo reproduction was best achieved with two microphones in the recording process, and two sound sources in the reproduction studio (your living room, perhaps).

One could reproduce a voice-spectrum signal easily enough, as what passes through the telecom system. Low tones, however didn't have enough energy, due to the physics of the system and high frequencies were somewhat distorted, so we gave up after a few weeks. The effect, of course, was intended to have the sound emanate from the positions held by the two microphones with which the audio was recorded. It was pretty interesting having sound come from where there was no visible source, but we were concerned about what effects we were, perhaps, unable to observe, since the input/output power transfer ratio was so low. In order to have that "robust" bass, one has to move quite a lot of air. The energy required to move a lot of air by this method is very large. Mechanical speakers do it much more easily, if not faithfully.

RE