Design / TONE
Over the years of working on mandolins, I have developed my own techniques and thoughts on how and why an arch-top instrument works the way it does. These may not necessarily be scientifically valid or the same as others may approach it, but for me, it is a way of working that makes some sense of the complex energy patterns involved and helps to consistently produce the best sounding instruments I can, batch to batch.
While the shape of the body and soundholes contribute to the tone of an instrument, it's the placement of the soundholes and bridge on the soundboard that fundamentally determines the difference in tone between f hole and oval hole instruments by defining the basic shape of the main vibrating area of the top.
On f hole instruments, like the violin family, this shape is basically a long narrow rectangular plate (portrait) running the full length of the body, bordered by the f holes either side. As wood is stronger along the grain (longitudinal) compared to across the width of the grain (radial), this produces a relatively stiffer plate on the f hole design that favours more mid-range frequencies, producing a brighter clearer tone that projects very efficiently. Fig 1
The shape, mass and placement of tonebars add finer adjustment to the tone. BRACING
This area on an oval hole instrument is shorter but wider (landscape) across the full radial width of the soundboard, resulting in a squarer, relatively more flexible shape plate that favours lower frequencies. This produces a mellower tone that does not project the same as an f hole soundboard, particularly when played with other louder instruments. Fig 2
The position of the soundholes also control the placement of the bridge, putting it in the centre of the main vibrating plate to maximize its acoustic efficiency, and ultimately determining the length of the neck - 15 frets on f hole models and 12 frets on oval hole models. The scale length is exactly the same on all mandolin models (13.9"), it is only the length of the neck and the bridge placement that changes. Midway between the tail and the neck joint with f holes, and approximately midway between the tail and the soundhole with an oval hole. The composite images in Fig 3 show the comparison between the two neck lengths as determined by their soundhole and bridge placement.
The single most important feature in the sound of the instrument is the spruce top. For maximum strength to weight ratio, the soundboard is always carved from book-matched quarter-cut spruce and graduated with the thickest point in the centre under the bridge and radiating outward to the thinnest just in from the edge. Fig 4 It's fundamental design, species and mass form a major part of its volume, tonal character and presence. As a softer piece of wood will vibrate at a lower frequency compared to a harder piece at the same thickness, each piece of spruce and maple selected for a pair of plates is tuned to a consistent frequency to help control the maximum response between both plates and between individual instruments. The maple back plate contributes to the tonal response according to its density and can be either quarter-cut, rift or slab-cut depending on the desired visual effect it has on the figure in the wood. As with the spruce, the harder it is, the brighter and clearer the tone will be as it reflects energy back to the soundboard. To help eliminate the dominance of any one frequency, the top and back plates are tuned a tone apart in a coupled relationship with the air chamber.
As air also has a resonant frequency and by adjusting the body depth, plate profiles and size of the soundholes, the air contained in the body of an instrument can also be controlled and tuned just like the plates to change the qualities of the tone. Deepening the rim and reducing the size of the soundholes will lower the resonant frequency of the body, while larger soundholes and a thinner rim will raise the frequency.
Linking the efficiency of the top, back and air are the soundholes that serve as breathing ports, allowing for the effect of compression and rarefaction of the air caused by the top as it pulsates in response to the frequencies transmitted down through the bridge. In effect, allowing the top to "breath" and vibrate as freely as it can to function as the speaker of the instrument.
All these subtle variations combine to produce an individual instrument's unique voice.
TONE - F hole models 5, 3 & 16
There can be many variables that determine the characteristic tone of an instrument, none the least being the natural variations found in the inherent acoustic qualities of the individual pieces of timber that make the top and back plates.
However, by designing and mixing different combinations of the two bracing patterns (X or parallel) with the softer engelmann spruce and red maple or with the harder red spruce and sugar maple, to a large extent the tone of the instrument can be adjusted and controlled. I like to think of it as a spectrum where, at one end of the scale, the softer wood and X brace emphasises more bass frequencies. At the other end is the harder species and parallel tone-bars that help boost midrange and projection, with subtle variations in between.
ES = engelmann spruce
RM = red maple
RS = red spruce
HM = hard sugar maple
TONE - Oval hole models 4 & 1
Because of their fundamental design, oval hole models have a natural abundance of bass response in their tone. To help boost the midrange frequencies, the hard species combination of red spruce and sugar maple is only used for these models. The option of "thickening" the tone even more with the softer species, as available for the f hole models, is not offered for oval hole models as the increased midrange response with the harder woods is a real benefit to their overall projection and power.