Sound is, in essence, our perception of the compression and rarefaction of air (pressure and vacuum, in crude terms). Yes, the compression and rarefaction do occur when a tree falls in a forest and there's no one to hear it, but the "perception" part does not.
When it comes to sound reproduction, an amazing variety of mechanisms have been used to recreate the peaks and troughs that make up sound-waves. The moving-coil driver (your basic cone-speaker) is the standard these days, but actually came upon the scene rather late, if you consider 1915 for Peter Jensen's Magnavox, "late". Jensen was preceded by 19th-century researchers like Siemens and Lodge, who devised speakers before the electrical era; there were also a variety of mechanisms with levers and fulcrums and even a few electrostats (which had a nasty tendency to short out spectacularly, due to a lack of suitable diaphragm materials).
Jensen's moving-magnet design was refined and defined (or "stolen", depending on your viewpoint) in a 1925 paper and subsequent patents by Rice and Kellogg, two researchers at General Electric. The magnetic field was still generated by a field-coil (electromagnetic) magnet assembly, as suitable permanent magnets didn't become commonly available until WWII and after. Many still find the sound of field-coils to be superior to those of permanent-magnet drivers; the last few years have seen field-coil drivers from Cogent, Shindo, Classic Audio, Lowther USA, and many other sources.
But I digress. Early electrical researchers noted the phenomenon of the "singing arc", in which the electric arc of carbon arc lighting created a constant, annoying humming sound. In 1899, English physicist/engineer William duBois Duddell was assigned the task of eliminating the hum, and discovered that by varying the voltage supplied to the arc, he could produce variable audio frequencies (as seen here
). Duddell installed a keyboard-controller across the arc, creating a steam-age synthesizer-- without amplifiers or loudspeakers! The device was viewed as a novelty, and no practical application resulted. I assume SOMEONE got rid of that hum, as I never noticed such a noise from old carbon arc projectors.
Other novelties included compressed-air sound systems utilized by the military, and a horrific flame speaker, with which Beethoven was reproduced by a McIntosh amp and an oxy-acetylene welding torch?!? Well-known transformer-maker UTC was responsible for that device, based upon a modulation phenomenon first seen in theater lights, in 1858 per this article
. Interesting, how old-time light-sources often produced sound; you don't hear compact fluorescent units playing Diana Krall, these days. It might make her more approachable.
Hugo Gernsback's Radio Electronics magazine ran, in November and December 1951, two articles awkwardly translated from the French magazine Toute la Radio ("All Radio(s)") concerning the work of a French physicist named Siegfried Klein. What? He doesn't SOUND French??
At any rate, Klein conceived and developed a non-mechanical loudspeaker he called the "Ionophone". Klein's device was essentially an ionization unit contained within a quartz horn, coupled to a large,standard exponential horn. A platinum wire seated in the quartz horn acts as a cathode, and a high-frequency, high-voltage field of approximately 10-12,000 volts at 400 kHz is established between the wire and a cylindrical shield which surrounds it. The field generates emissions of ions (charged particles), and thus heat, which is contained within the quartz horn-throat by an insulative vacuum enclosure which prevents both conductive heat-loss and loss of charge. While complex to describe, the device was fairly simple. As Duddell did with the singing arc, Klein modulated the high-frequency field at audio frequencies, and, c'est voila, SOUND. Also UV and heat.
The articles showed a frequency-response graph indicating a relatively flat response of +/- 5db from 25-10,000 Hz, and the text indicated he speaker could reach "much higher frequencies...however, its output diminishes in the supersonic range." The articles also painted a picture, in gushing prose reminiscent of old Popular Mechanics articles, of a future filled with massless loudspeakers directly coupled to radio-receivers without detector stages, and stated, "the (unidentified) biggest French manufacturer of loudspeakers is tooling up for mass production of Ionophones."
If we ask Dr. Phil's question of "how's that workin' for ya?", the answer would appear to be, "not well", seeing as we are NOT surrounded by massless ionic loudspeakers. I have no knowledge of any Ionophones ever made in France; however, there were some commercial speakers produced based upon Klein's work.
One of the Klein-based speakers was this guy here
, an Ionovac tweeter, made by the DuKane company in America, also licensed to Fane in England, as sold in the UK as the IonoFane. The 1962 Stereo/HiFi Directory (put out by Hi-Fi/Stereo Review, later Stereo Review, then Sound and Vision) lists this unit as costing $69.00 per pair. Response was listed as 3.5-20 kHz, with no indication as to the linearity. The DuKane unit was sold as shown, as an add-on unit; it was also sold as an add-on in a small enclosure, and as an element in four different full-range systems. The Ionofane unit was also utilized in an early Bowers & Wilkins (B&W) model.
Similar tweeters are made today by several German manufacturers, including Acapella and Lansche. Ironically, demonstrations indicate that the Germans often employ sizzly-sounding electronics to produce the spitty sibilance which is often heard as a national characteristic in German speakers--diminishing the ionic tweeter's greatest assets, its freedom from resonance and coloration!
There have been alternate takes in the world of massless speakers, most notably the Plasmatronics and a corona-wind variant built by Nelson Pass. More will be said about both, another day. While ionic/plasma speakers have so far been only an interesting novelty in the big wide world of audio, it may yet turn out that, in the words of Al Jolson, "you ain't heard NOTHIN' yet!"