Sound hole size & location question

[Derry]

have often wondered how the exact SIZE and LOCATION of the sound hole is determined,, is there just a standard size and location that has been accepted to offer the best possible sound,, ???

when we see some guitars with D shaped holes and holes on the edge of the upper bout sound board does this really help or do anything for the sound or just a change in general appearence,,

appreciate any comments on actual data or hearing such instruments,,

thanks,,

[MarkJ]

The location is easy to determine if the 19th fret is to be split. The fretboard edge of the hole is typically 2 mm past the 19th fret location - where that is relative to the 12th fret at the edge of the body depends on scale length. The center of the hole would then be 1/2 the diameter of the hole minus 2mm from the 19th fret.

Te diameter of the hole depends on the box volume. Some people call the combined effect of internal volume and hole diameter the Helmholtz resonance frequency. That is the tone you get fron blowing air across the hole. In general, smaller shallower bodys have smaller diameter holes - Hauser used 84 mm, other similar-sized instruments are 84 to 86 or so. Larger bodied instruments may have a 89mm hole. Adjusting the hole size affects the bass and treble balance for a given cavity volume.

[Azalais]

I was curious about this too, and actually put a garbage bag inside the body of one of my oddly shaped projects and filled it with water to see what the actual volume was... using the Helmholtz fomula, it turned out that it needed a bigger diameter soundhole because I had added depth by arching the back. Increasing the diameter is one of the final ways you can use to "tune the assembly" after it is all glued up.

[Pepe Vergara]

I make the soundhole tangent to a horizontal line conneting the points where a vertical is tangent to the curves of the waist. That is what reserarchs have discovered Antonio de Torres did with his guitars. Any deviation from it would make the guitar look "different". In a regular 650 mm scale, the 19th fret would have to be split pass certain distance. I have guitars where the 19th fret is not split. The diameter of the soundhole depend on each maker. However, aything from 86 mm to 90 mm would be considered normal for me. Bigger or smaller than that may affect the sound too much. I am speaking of the Spanish guitar, not the steel string or the other Contreras with the soundhole way above "normal."

[Azalais]

Speaking of looking "different"... I became very curious about this question because I had collected so many photos of vihuelas, renaissance, baroque and early romantic guitars... and well known "modern" CGs. I started with a measuring grid in photshop and adjusted them all to have a common 650mm scale between nut and bridge... I overlayed transparent layers of maybe thirty different instruments (it's a huge file)... there was an amazing amount of difference in body volumes (some had deep rounded backs), where the neck joined the body and where the bridges fell, the relative diameters, shapes and placement of the sound holes (often due to the bracing and shape of the waist and sides)... even the differences among Torres/Fleta/Hauser/Ramirez were rather striking... I guess that the only conclusion that I could draw was that I couldn't really draw any conclusions! I couldn't decern any hidden unbendable rules in the geometry... and certainly nothing "sacred"... so, does function follow form, or does form follow function? (The eternal chicken vs. egg question from first year architecture school will continue to haunt me )