http://andalusianguitars.com/catalogue-homeAG.html
References to the history of the double sound hole on the top
Guitar made around 1929 by Francisco Simplicio http://www.themomi.org/museum/mfa/popular_guitars/1929-30_F_Simplicio.html
Simplicio noticed a disadvantage of the traditional sound hole position in the weakening of the guitar top in a crucial area and also in a reduction of the sound emitting surface of the top (Huber, J.: The Development of the Modern Guitar, Westport, 1994, p.27). The increased sound emitting surface should also improve the sound emitting in the bass range (Stark, D.: Diplomarbeit – Entwurf und Bau einer Konzertgitarre mit doppelter Decke in Sandwichbauweise, Weimar, 2006, p.6)

Impact on the sound:

Omitting the sound hole or positioning it on both sides of the fingerboard leads to an enlarged vibrating surface of the top. Cue to the ratio between top length or width and top thickness, it is possible to:

– attain the same degree of flexibility of the top as in a traditional guitar, but obtaining a higher mass of the top (this leads to a better sustain without the loss of attack and responsiveness)

– attain a more flexible top than on a traditional guitar, but maintaining the same mass of the top (this leads to increased loudness and responsiveness without the loss of sustain).

In addition, the Helmholtz frequency http://polettipiano.com/Pages/airresengpaul.html
can be lowered significantly, which in turn has an impact on the tonal character of the instrument.

See: 21 features out of the box on AG Simplicio
https://youtu.be/LaZ4mcFKtVQ?t=2m

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