Timber selection and its Acoustic properties
Timber is a natural substance and as such is subject to great variation,even within the same piece of wood. This variation occurs not only between species but also within the species. It is this characteristic that makes timber selection a very important part of building musical instuments. One part of the tree may produce timber that is very acceptable, but timber from another part of the same tree may be of no use at all to the luthier. There are many stories around about how how the tree must come from the Norht side of the hill and grow above a certain height and be cut down at a certain time of year, for the timber to be suitable for instrument making.
Each of these statements, even though they are considered folklore, will have some merit. Timber on the northern side of the hill will get less direct sunlight (in the northern hemisphere), especially in winter. Timber grown above a certain height will have less variation in temperature from summer to winter, and trees at certain times of the year have more sap present. But, these are not facts that can really be substantiated especially when viewing already cut wood in a timber yard. So at this point it becomes more important to be able to select wood with certain physical and acoustic characteristics. If it is possible to go into the forest and harvest your own trees, it is possilbe to select just the right tree, but otherwise it is left up to the luthier to be able to select the timber they are after from the logs, flitches or boards that have already been harvested.
A great deal of acoustic and physical testing has been done on different timbers. This testing includes physical properties such as weight, density and strength (along and across the grain) and also acoustic properties such a the rate at which certain species transmit sound, internal friction etc. This testing will probably be of use some day, but at the moment it is still too much in its infancy to be widely used by mainstream luthiers. Although there has been a great deal of testing and quantitising of various species of timber, there is very little evidence around that actually relates wood properties with the production of a better instrument. It is no guarantee that you will build a better instrument if you have the better timber, this is still pretty much up to the skill of the individual luthier.
I select timber using a variety of methods. Each luthier has their own personal preferences and these preferences and building skills are what make their instruments individual. I use the following methods for selection timber.They are not listed in any particular order of importance.
- Appearance
- Density & Weight
- Strength along and across the grain
- Sound
- How the timber is cut or split
Timber for the top must have a suitable appearance or it is not really fitting to use it on high quality instruments. This is mainly from an aesthetic point of view and is nothing to do with the acoustic properties. I personally do not like "Bear Claw" or "Hazelficht" as a top wood as I find it inconsistant in its behaviour. It is certainly attractive, but I leave it to those who like it. The timber for the top of instruments should be straight grained and the grain should be even in appearance or gradually growing wider to the one edge. I prefer timber that has a close even grain across its width and that does not really increase across the board. This is a personal preference and not mandatory, but it does indicate that the wood is from older larger trees where the growth rings are more similar. The prominence of the grain is also important to me. I prefer if there is not much distinction between the summer and winter growth. Again this is a personal preference and many makers prefer to be able to see variation in the grain. The dark grain is the wood that grows in the warmer months, the cells are more plentiful and smaller and closer together. The paler softer growth is the winter growth. Some makers prefer to see good distinction here. It is a personal preferance.
Medullary
rays are a good indication of how well quarter sawn the timber
is. The wood must be well quarted and preferably has initially
been split to ensure the maximum length of fibres and minimisation
of runout. Runout can be seen easily, especially in Spruce. If
the top has marked runout, one half of the top will appear darker
than the other side, and if you turn the guitar upside down,
the colour variation will change from side to side. This colour
variation is caused by the light hitting the end of the wood
fibres. The tree is usually initially cut into lengths of about
600mm and then split in wedges or flitches, similar to cutting
up a pie into small pieces. From these small wedges, about 100mm-150mm
thick at the wide end, are sawn the bookmatched top pairs. This
is not the most economical way of cutting timber for instruments,
but it is the preferred method. Sawing the log into long planks
about 1 inch thick and then cutting them to length and then cutting
them into the tops, is not the best method. as runout if most
likely to be present because trees usually grow with a twist.
Different species show differing amounts of medullary rays. They are usually more obvious in Spruce than in Cedar. Timber that has been split has little runout (hopefully). Runout is most obvious when you view the finished guitar and one half of the top appears to be darker than the other. This is not a good starting point for a fine instrument. It means the grain is short and this is not optimal.
The weight of a piece of wood is directly related to the density. The greater the density of a given piece of wood, the more it will weigh, given the same dimensions. The weight of a piece of wood directly effects the performance of a piece of wood in acoustic terms. If a piece of wood has a natural frequency upon being struck, then this frequency will rise as the weight or density of the timber rises. Denser woods are often stronger and can be made a little thinner to compensate, but this is not always the case. Some lighter timbers are naturally stronger than some heavier timbers.
When choosing wood for the top of the guitar, some makers prefer lighter, less dense material. Less dense materials, like Cedar are very easily damaged and this can be an issue, especially for guitars that need to be robust, like flamenco guitars. The lighter the top, the easier it is for the strings to set the top in motion and the result of that is more volume, and a faster attack. This may not be required, it just depends on the sound the maker is after. Heavier tops tend to sound quieter, but they also tend to sustain for longer. Every aspect of instrument making is a trade off of some kind, and the maker must maker a qualitative decision based on his experience and skill when selecting the most appropriate timbers for an instrument.
The timber for the back and sides is important, and the weight of these parts, while effecting the final sound of the instrument, are more importantly going to effect the final weight of the instrument as well as the balance. If a light weight timber like Cypress is used for the back and sides, it then becomes important what wood is used for the neck. Mahogany, though not often used on classical guitar necks, is not really suitable with lightweight backs and sides. Spanish or Australian Red Cedar is a much better alternative.
The strength of a piece of wood is important to both the sound and the structural integrity of the guitar. If a piece of wood is too weak, it will not be possible to make it thin enough to vigrate properly as a guitar top. If the timber is also too weak, it may collapse under the string tension. This will effect the lifespan of the guitar as well as things like the action. Many makers measure the deflection of the top once it has be thinned and assembled. This if done properly will assure the maker that the top is strong enough to withstand the string tension and not deform.
When choosing wood for the top of the guitar, it is important to take into account the strength of the wood both along and across the grain. This is important from an acoustic point of view more than structural. Timber is an anisotropic material, in that it is stronger in one direction than the other. Part of the luthiers task is to blend and use this difference to control the sound of the guitar. A top which is stronger across the grain, will tend to make guitars that are brighter. This aspect can be controlled by the luthier by the angle and stregth of the relative braces, as well as timber selection.
Each and every piece of timber will have its own characteristic resonances occuring at varying frequencies. This will have a marked effect on the sound that these pieces of wood produce as a final instrument. When selecting timber the luthier can be seen tapping the various pieces, listening to the sounds that emanate from the piece of wood. Some of the characteristics the luthier is listening for are sustain, clarity, high pitch, low pitch and volume. It is hard at this point to determine that the finished top will actually respond in the same manner, but it is a good starting point in the selection process. I have many times started with a top that sounded very good in all aspects, only to find that when it was thinned to the required thickness, many of the desired characteristics had diminished completely. This has made these tops unsuitable at this point.
Another sound that I listen for when selection a piece of timber is that sound the wood produces when I just pick it up and handle it by rubbing it between my fingers and thumbs. I can feel the vibrations and hear the high frequencies from good pieces of timber. The timber can actually sound "noisy". Some pieces are quiet and these pieces I discard or overlook. This particular aspect of timber selection is probably related to the speed of sound as it is transmitted through the timber. A faster more resonant timber, will produce a better guitar, but it will need to be controlled properly or it can be unruly and produce a guitar that does not have a good sound.
Ideally the luthier is looking for timber that is "quarter" sawn. When timber is quarter sawn, if it is viewed from the end of the piece of wood, the grain will be seen to be vertical. The great the deviation from being quartered, the weaker the timber will get, especially across the grain. The timber is at its weakest when the grain is roughly at a 45 degree angle. Because the way timber is cut or split has a marked effect on the strength of the timber, both along and across the grain, this in turn will effect the sound of the timber and thus the sound of the instrument. Timber that is split is ideal because the wood fibres are longer, and this makes the wood stronger.
Quarter sawn timber can be a little boring to look at, especially for the back and sides and it is also the least economical way of producing timber from a sawyers point of view. Timber that is back sawn is often more attractive, but it is also less stable and this is important to the luthier as it can cause buckling of the thin timbers due to changes in humidity.
