During the period between the outbreak of World War I and the end of the 1920s electrical sound generation instrument research was closely linked with the development of radio technology and public address systems. From 1904 inventors directed their efforts toward improving loudspeaker-receivers, sound reproduction systems, and to achieving amplification and generation of sound loud enough to be heard by more than just a few people. A major breakthrough was enabled by the invention of the vacuum tube or triode by Lee De Forest, "the father of radio," in 1908.
De Forest first patented the 'Audion valve' in 1906, a sensitive refinement of John Ambrose Fleming's oscillation or diode valve of 1904. The Audion valve reworked and further refined by De Forest into a tri-electrode device or triode, became the first relatively reliable electronic amplification device and was a key advance in the emerging radio technology, long-distance telephone communications, and radar.
Public radio broadcasts only became feasible because of the vacuum triode. Prior to the triode, radio receivers could only produce very low volume signals and required listeners to use headphones. The first successful demonstration of an experimental broadcast included a live performance of Tosca on January 10, 1910, followed the next day by a performance which included Italian tenor Enrico Caruso from the stage of Metropolitan Opera House in New York. Very quickly improved vacuum triodes could reliably amplify the signal to any desired level, revolutionizing radio communication and leading to the development of proper loudspeaker systems, which, in turn, facilitated the development of the next generation of electro-acoustic and electrical instruments.
The first application of the triode in musical instruments was the invention of the Audion Piano by De Forest in 1915. The Audion Piano was a simple keyboard instrument that exploited the heterodyning beat frequency technique - a way of creating sounds by combining two high frequency signals produced by a valve in order to create a lower composite frequency within the audible range. The Audion Piano used a simple valve per octave controlled by a set of keys allowing one monophonic note to be played per octave. The output of the instrument was transmitted via a set of loudspeakers installed around the room giving a sound a novel dimension. In his autobiography De Forest wrote that the Audion Piano could produce "sounds resembling violins, cello, woodwind, muted brass and other sounds unlike anything ever heard from an orchestra or by the human ear up to that time – of the sort now often heard in nerve racking maniacal cacophonies of a lunatic swing band."
The other immediate and most lasting application of De Forest's vacuum valve and heterodyning effect was created by Lev Termen (a.k.a. León Theremin, pictured) with his Theremin series of instruments. Termen's machine of 1917, further redesigned in 1920, named Theremin or Aetherophone (sound from the 'ether') was played by moving the hands around a metal loop for volume and around an antennae for pitch. The output was a continuous tone modulated by a performer with a timbre which resembled a violin string sound and human voice. This sound was produced directly by the heterodyning combination of two radio-frequency oscillators, one operating at a fixed frequency of 170,000 Hz, and the other with a variable frequency between 168,000 and 170,000 Hz. The proximity of the musician's hand to the second antennae for pitch determined the variable frequency. The difference of the fixed and variable frequencies produced an audible beat frequency between 0 and 2,000 Hz.
The De Forest's triode not only became the primary mechanism of audio synthesis until the invention of the integrated circuit in the 1960s but also was the key to the development of sound amplification systems first applied to radio, then to phonographs, talking pictures, and musical instruments. Without the development of a reliable valve amplifier around 1925, other experiments in developing electro-acoustic instruments exploiting pickup technology in electric violins, cellos, pianos, guitars and other struck, plucked, or bowed string and reed instruments could not have been successful. The triode amplifier also freed musical instruments from having to use phonograph horns, the telephone, and a radio set as means of amplifying the signal.
By the end of 1920s, the vacuum tube brought the dream of Edward Bellamy of portrayed in Looking Backwards in 1888 close to a complete realization. The vacuum tube allowed radio to become an affordable device that provided everybody with music in their homes, and enable the creation of new devices that provided new ways to create music and introduced novel types of musical sound.
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