In the 1920s, a Los Angeles-based banjo player, John Dopyera (1893-1988) found an ingenious solution to the perrenial problems of audibility suffered by onstage acoustic guitarist when he created a new, louder type of instrument for a local vaudeville performer, George Beauchamp.
It featured three aluminum resonators that gave extra projection to the vibrations picked up from it’s bridge, and in 1928, Dopyera, Beauchamp and other investors set up the National company to produce guitars, mandolins, and ukuleles incorporating these so-called “tri-cones”.
The first National guitars appeared in 1928. These were wooden. Steel single-cones like this “Spanish”-necked Duolian guitar date from the mid-1930s.
A maple “biscuit” under the string-guard transfers the vibrations to the single cone resonator, which amplifies the sound.