The biscuit transferred the vibrations of the strings to the speaker cone, which would then resonate, thus amplifying the sound.
John, however, did not think that this single-cone design was well-suited for guitars, since they were so much larger than ukuleles.
More-advanced designs soon followed, including one which utilized a single aluminum cone to turn the entire guitar into a speaker cabinet.
The guitar strings sat on a thin wooden saddle (or “biscuit”) at the peak of the volcano-like aluminum cone.
As the acoustic guitar became popular in the early 20th century, players needed ways to increase the volume of their instruments so they could be heard when playing with other musicians in large performance spaces.
In the mid-1920s, John Dopyera developed an amplifying system, which he used when he founded the National String Instrument Corporation in August 1926 with his brothers Rudy, Robert, Louis, and Emil.
In addition to the cones, Dobro bodies featured sound holes between the resonator and the neck; in every model except Model 35, which featured segmented F-holes, these sound holes were circular and resembled the portholes of a ship.
Because of its overall design, Dobro resonator guitars had longer sustain than single-cone Nationals of the same period.
Their company would use Dopyera's system to produce “ampliphonic” or “self-amplifying” guitars, known today as resonator guitars.John received a patent for his design on December 31, 1929.In frustration, he resigned from National in February 1929 to found Dobro Manufacturing; he derived the name “Dobro” from the first letters of Dopyera Brothers.Dobro modified National’s biscuit design by flipping the cone upside-down and connecting the broad base to the strings with a four- or eight-armed “spider”; unlike the biscuit, the spider conducted the vibrations from the strings directly to the edges of the speaker cone, allowing the cone to vibrate freely.
In fact, the sound of these vintage Dobro guitars much more closely resembled that of the tri-cone or tri-plate National resonators, which, as their name suggests, had three resonator cones, rather than just one.
Each resonator cone was six inches across, and the three were arranged in a triangle with two on the bass side and one on the treble side of the body.