The impact of the different sound hole types
The size and shape of the sound hole has an impact on the Helmholtz frequency and therefore on the sound character of an instrument. The positioning of the sound hole on both sides of the fingerboard or omitting the sound hole entirely makes it possible to extend the fingerboard and the playing range to two octaves and also to fundamentally change the bracing and shaping of the instrument top. The impact on the sound will be described in the following paragraphs.
History of the double sound hole:
The most famous instrument with a double sound hole is probably the guitar made around 1929 by Francisco Simplicio (http://www.themomi.org/museum/mfa/popular_guitars/1929-30_F_Simplicio.html, as of the 25th of June, 2008). Simplicio is credited to have seen 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). The positioning of the sound hole on both sides of the fingerboard also permits an extension of the playable range to two octaves.
Options for the sound hole:
The current options for the sound hole are: Traditional circular sound hole, double sound hole beside the fingerboard, leaving out the sound hole entirely (this creates the need for a sideport though).
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 can be lowered significantly, which in turn has an impact on the tonal character of the instrument.