The Ionophone Project
Still in my pre-student years, I loved to attend lectures of the legendary Professor Paul Scherer at the Swiss Federal Institute of Technology (ETH). With his assistants he performed a real "Physics Circus" stuffed with amazing experiments. I was intrigued by a lecture demonstration: the "singing arc of light". He demonstrated a carbon-arc lamp similar to the ones used in the old cinema projectors. The electric current sustaining the arc discharge was then modulated with audio frequencies and the corresponding sound along with some noise could be slightly heard in the auditorium. The sound was produced by the instantaneously changing expansion and contraction of the ionised air between the two electrodes. There were no moving "heavy" membranes with all the problems you find in speakers or headphones.
As a student I was in quest of the ideal sound reproduction and I remembered this experiment. Together with my friend Guido Melliger (at that time studying Pharmacology) I developed the Ionophonic Headphone:
Two tiny arcs of light are produced, each between two platinum electrodes. The plasma is confined in small quartz tubes. The system is coupled to the auditory canal by a soft rubber piece. The acoustically relevant dimensions oft the tubes match the dimensions of the the human auditory canal in length and diameter. Thus the acoustic resonance of the combined system ear plus Ionophone matches the natural resonance of the auditory canal without Ionophone (remember the different resonant frequencies of open and closed organ pipes). Any excess of resonance amplitude is damped with additional measures.
While testing the Ionophone, we were astonished by the total lack of unwanted noise, the extremely high sound pressure attainable without audible distortion and the exceptional broad frequency range from practically DC to (measured) 60kHz! Never before we had heard a comparable quality of reproduced sound! Even the lifetime of the platinum electrodes was more than sufficient.
What happened to the Ionophone? Well, we learned that most good things have a side-effect: after prolonged music-listening we felt some slight irritation of the skin in the auditory canal affected by the partially ionized air. There would have been means to reduce this effect, but while studying Physics there are still other priorities left!
After my studies I experimented with big-sized, radio-frequency driven ionophonic loudspeakers for medium and high frequencies featuring omnidirectional sound distribution up to the highest frequencies. The produced gasses where exhausted by suction ducts directly above the ion-source and small fans and holes in the wall of our house! In the meantime general speaker technology has improved considerably, so there is less need for this exotic approach.