Among the changes that Loar introduced was the f-hole instead of a round or oval sound-hole, another violin-family feature imported to the mandolin. He also "tuned" the tops of the instrument and the sound chamber (by removing bits of wood from sound bars and from the edges of the sound holes) so that the instrument's sound chamber was resonant to a particular note. Another change that Loar introduced to the Gibson line was a tone-producer, a circle of wood inside the instrument on the underside of the sound board that produced "overtones." His idea was to have a more complete set of these overtones with the carved top instruments. The result was an instrument that, like Stradivarius ’ violins, has presented challenges to duplicate. Luthier -researchers such as Roger Siminoff have worked to understand the fine details. Gibsons' and Loar's mandolins were instrumental in displacing the round-backed instrument from the American market and influenced mandolins worldwide.