Domestic Wood Species for Fine Guitars

"Traditional" Wood

We tend to build traditional instruments out of traditional wood species of course. A good question to ask is how certain species became the traditional woods of choice, because doing so provides us with some information about just how important (or not) these woods are to the performance of our instruments. For modern classic guitars, the most popular wood species are probably rosewood (preferably Brazilian) for the back, sides and bridge; European spruce for the top; and mahogany or Spanish cedar for the neck. These have been the preferred species since the time of Torres. But we don't have to look any farther back than the 19^th century to find European guitars built of all sorts of hardwoods for the back and sides, and all manner of European softwoods for the tops. It was generally the case then that the more expensive woods were used in the more expensive instruments, a trend which makes perfect economic sense and continues to this day. Readily available domestic (since here we are discussing European instruments, here the term domestic refers to European species) were certainly less expensive than imported tropical wood, and were thus used on less expensive instruments.

We can understand the use of certain woods in terms of fashion. Fashions come and fashions go, and some fashion trends last longer than others. Fashion trends are often set by those that can easily afford to set them, and followed by others. The fashion of using Brazilian rosewood for backs and sides of high priced guitars was well established in the 19^th century and continued into the 20^th century in both Europe and the USA. But price and availability of this wood were not stable, and at some point the price was high enough and the availability low enough that large guitar manufacturers such as C.F. Martin in the USA stopped using this species for production models, substituting more readily available east Indian rosewood.

What accounts for the fashion persistence of Brazilian rosewood for guitar backs and sides? A number of potential factors should be considered. One is the "golden age" perception of instruments made by Torres and his immediate successors, all of which used this wood in high priced models. Another is what can be called fashion inertia - fashions tend to persist without a new fashion to displace the old, and no new species gained serious fashion traction during this time. It is quite possible that the dwindling supply of Brazilian rosewood actually helped to keep its fashion prominence going strong, the price of guitars using this material staying high enough to maintain the fashion status of those instruments.

The fashion of Brazilian rosewood persisted long enough for a number of myths to develop about the wood, and none is more prominent and enduring than that instruments made of this species sound better than instruments made of other woods. Unfortunately all available data that supports this hypothesis is anecdotal - there have never been comprehensive blind listening studies to substantiate this claim. And speaking of anecdotal evidence, there are some intriguing facts which should give pause to those that would assert sonic superiority to instruments made of this wood. One of these is the generally accepted status of east Indian rosewood as the most reasonable second choice wood. East Indian rosewood certainly looks a lot like Brazilian rosewood, but there are many other species with density and stiffness properties closer to those of Brazilian than to those of east Indian rosewood. Curiously, one of those species with density and stiffness values very close to those of Brazilian rosewood grows in the USA. I'll talk a bit more about this in a subsequent section.

Can the superior tone asserted for instruments made of Brazilian rosewood be attributed to density and stiffness? This wood is quite dense and stiff. But there are folks that build instruments with backs ans sides of even denser and stiffer materials such as various ebonies, and these species have thus far not been mythologized to anywhere near the extent that Brazilian has. If, in the case of guitar tone, more density and stiffness is not really better, then maybe it is the case that the specific density and stiffness of Brazilian rosewood just happens to yield a superior sounding instrument. But if this is the case then species with similar density and stiffness values should also make superior sounding guitars.

"Alternative" Species

The automobile was popularized by Henry Ford, who's mass production techniques made that product available to the general population. As far as color choices for Ford's original models went, you could get any color you wanted, as long as it was black. For a long time you could get a classical guitar with any back and side wood species, as long as it was Brazilian rosewood. Things were a bit less conservative where Flamenco guitars were concerned - negra ("black") instruments used Brazilian rosewood, and blanca ("white") instruments used Spanish cypress for the backs and sides. In the same vein, steel string guitars could be had in either Brazilian rosewood or mahogany. These days you can get instruments with a wide variety of wood species used for the backs and sides. We've already seen that rising prices and decreased availability caused C.F. Martin to switch from Brazilian rosewood to Indian rosewood. But factors other than price and availability are also at play when it comes to the current more liberal view of acceptable back and side materials. It is certainly the case though that large manufacturers do not want to get caught in a situation where they have to rely on a single supplier or single country of origin for wood, or for that matter on a single species. Manufacturers in the USA were caught off guard during the 1970's when a world wide spruce "shortage", a result of Japanese manufacturers buying up much of the supply, caused prices of that wood to go up.

The shortage of Brazilian rosewood led to widespread use of east Indian rosewood for backs and sides, and the shortage of spruce led to use of red cedar as a material for guitar tops. And the possibility of shortages and price increases have led many guitar makers large and small to offer a wide variety of species for backs and sides, all to no obvious deleterious affect on instrument tone. This being the case, manufacturers and players are increasingly willing to consider a wider variety of wood species to be acceptable for use in guitarmaking, at least for steel strings. Even so, fashion inertia is strong, so we tend not to venture too far from some of the features of Brazilian rosewood when considering alternative back and side wood. We are very fond of other rosewoods, woods of the same genus (Dalbergia) as our beloved Brazilian. And we also like our back and side wood both dark and foreign, although some dark colored domestic wood species such as the walnuts are gaining traction for use in backs and sides.

Wood Species and Tone: the Physics

I've already mentioned that there are no conclusive studies which indicate instruments using traditional species sound better. This is not to say that this is not the case and as also mentioned there is anecdotal evidence on both sides of this subject. So what is a guitarmaker supposed to do if he or she wants to use alternative wood species, especially for classical guitars where conservative feelings run stronger?

A little understanding of the physics involved will help. Although it may not seem obvious, the vibrational characteristics of any vibrating system all come down to three qualities: density, stiffness, and damping. Two systems (and here, a guitar or component of a guitar is a system) with the same density, stiffness and damping will vibrate in the same way, no matter what material they are made of. So in theory, building an instrument out of any material with the same (or possibly similar - we don't really know how close we need to get for things to sound the same) density, stiffness and damping as, say, Brazilian rosewood should result in an instrument that sounds the same as one made of that wood.

Of these properties density is the easiest to understand and control. The density (specific gravity or g/cm²) of Brazilian rosewood is approximately 0.77. So, if we want to substitute another material, we can either chose another one with the same density (like east Indian rosewood for example, with a specific gravity of 0.70) or simply use more or less of a material with a different density. Black cherry for example, has a specific gravity of 0.5, so if we want to make a component out of that, we can make it about 50% thicker to get the weight to be about the same.

But things get difficult real fast when we also consider matching stiffness, especially where wood is concerned. Unlike, say, steel which is equally stiff when bent in any direction, wood has different bending stiffness depending on grain orientation. This means it is not all that easy to even describe the stiffness of wood, because it can't be done using one value as for density. But we can get at least some idea of relative stiffness using specifications for wood species collected for purposes of using wood as a building material. The measure of stiffness is called modulus of elasticity (MOE) and is often presented in units of mega Pascals (MPa). Brazilian rosewood has a MOE of about 13,000 MPa, and black cherry has a MOE of about 10,300. But here is where the problem associated with trying to duplicate the vibrationally important characteristics of one species becomes apparent. As mentioned above, we can match the weight of Brazilian rosewood by making a component out of black cherry that is 50% thicker, because weight is proportional to thickness. But, because stiffness varies to the cube of the thickness, increasing the thickness by 50% increases the stiffness dramatically.

I'm not going to go into much detail on the effects of damping here. The topic is even more complex than stiffness in wood, because it involves another variable, frequency of vibration. Damping is difficult to measure in any meaningful way (there are in fact a number of "standard" ways to measure it) except to draw the most general of conclusions. We don't even have reliable models of damping for relatively simple vibrating systems such as vibrating strings¹ so it makes little sense to consider attempting to model it for any practical purposes when selecting wood species for backs and sides at this time. As is the case with a number of luthiers, I will get a general idea of the damping qualities of a piece of wood by tapping it and listening to how long it rings. Samples of all of the domestic materials I use for guitar backs and sides will ring as long as samples of typical tropical species, and for some domestic species even longer.

Wood Species and Tone: the Psychophysics

All of the above may or may not be of interest, but the bottom line is (or at least should be) whether or not people can actually hear a difference between instruments with backs and sides made from traditional tropical species and those made from domestic species. A small well done experiment² by the advocacy organization The Leonardo Guitar Research Project provides some evidence that folks have no preference for instruments made of tropical species when they can't see what species the instruments are made of. An interesting aspect of this informal research is that it included instruments made of all sorts of domestic species. In this experiment the mass and stiffness of the wood used varied greatly yet listening subjects expressed no preference for instruments made with harder and denser back and side species. The conclusions of the LGRP were supported by a rigorous scientific study³ performed by a peerless group of researchers led by Samuel Carcagno. Not only did this study demonstrate that players did not prefer traditional back and side species over domestic species, but it also showed that most if not all players could not even detect a difference in a real musical context.

Using Alternative Species

Given the information in the sections above, is it even possible to make use of alternative wood species? The answer is yes, it is possible, and the main reason this is so is that the values we use when comparing wood for density and stiffness are averages. There is quite a lot of variation in both density and stiffness for each species, so much so that there is considerable overlap for these qualities among different species. This being the case, it is possible to identify species which are "close enough" in density and stiffness to traditional species, for those folks that consider doing so to likely be important to the tone of a finished instrument.

Which brings me back to the Cincia classical guitar and specifically to the version of it I build using only domestic wood species. Just to make the challenge a bit more difficult, the instrument shown on this page was built completely from species that grow in my home state of Massachusetts. Even so, the hardwoods used in this instrument are very close in average density and stiffness to traditional materials used in the classical guitar:

Even restricting wood choices to those grown in the northeastern part of the USA, it is still possible to come close in both density and stiffness to traditional materials. With less restriction on where the wood can come from one can even construct an instrument with a more traditional look.

A Word About Softwoods

Softwoods used for guitar tops are in good supply. This includes the European spruce generally preferred for classic guitars. Engelmann spruce is also commonly used and is very close in both density and stiffness to European spruce. Red cedar is a domestic species that is commonly used for steel string guitar tops. The Cincia classical guitar pictured uses red spruce for the top, a wood that is considerably stiffer than European spruce and that generally has a lot more cross grain stiffness than European spruce as well. Thinning the top a bit brought it within spec for stiffness along the grain, while using a top with very wide grain lines and that was cut considerably off the quarter put it in the range of European spruce for cross grain stiffness. Sometimes you have to think a little differently to make use of certain species, but by doing so you can end up with a finished instrument that compares favorably to those made from the most traditional species.

References

1. Christian, J. Parameter Estimation of Multiple Non-Linear Damping Sources in Guitar Strings. Savart Journal, North America, 1, jun. 2011. Available at: http://savartjournal.org/index.php/sj/article/view/14.

2. Authors unattributed. Leonardo Guitar Research Project: Research Report LGRP#1. Leonardo Guitar Research Project website, Available at: http://www.leonardo-guitar-research.com/research-report-lgrp.

3. Carcagno, S., Bucknall, R., Woodhouse, J., Fritz, C., and Plack, C. J. (2018). J. Acoust. Soc. Am., 144(6), 3533-3547. doi:10.1121/1.5084735 Available at: https://doi.org/10.1121/1.5084735.