Aaron has been working very hard to finish two beautiful classical guitars that he has been building, side by side. One has a Western Red Cedar top with Cocobolo sides and back. It has Bocote trim and a Spanish Cedar neck. The other guitar has a German Spruce top with Indian Rosewood sides and back. It has Bloodwood trim and a Spanish Cedar neck. He has been working on these two guitars for over two months now.
Although much of this time was spent in crafting the two instruments from their component pieces of wood, a significant amount of time was spent performing extensive tests at every level of construction. The goal in doing such thorough testing is to have a clear scientific understanding of every step in the process of building each classical guitar he creates.
Although this testing and record keeping takes a lot of time and work, we believes it is well worth it for three reasons.
We want to be absolutely sure that each Wood Ring Guitar is built to last a lifetime. This is done by applying both theoretical and empirical data to our building approach in order to make sure that the most vulnerable parts of the guitar are strong enough to endure decades of playing (strength testing).
We want to be able to reliably reproduce the qualities that contribute to a great sounding guitar (acoustic testing).
We want to continually improve the building process and the finished product from both an acoustic and artistic point of view (record keeping, feedback, and analysis).
Our aim is lofty but we feel strongly about it. We are committed to creating instruments that will gain in value over time because of their unique artistic beauty, their outstanding sound qualities, and a look and feel that gives each owner that special feeling that only a select few instruments throughout the world can bestow upon them.
Today, I want to discuss the process we use to test the strength of our soundboards. This testing process is very important because:
It allows us to fine tune the soundboard such that we are absolutely sure that each guitar is strong and built to last.
It gives us feedback in fine tuning the top and bracing system such that we can achieve the optimum sound characteristics.
It backs up our commitment that the guitar will sound as good or better 10, 20 or 30 years in the future as it does the day it is purchased.
Sound Board Strength Testing
It is the soundboard that is responsible for most of the sound that eminates from a guitar. When the player plucks or strums the strings, they start vibrating. This vibration is transferred through the bridge and into the soundboard. For standard guitar tuning, frequencies from 82.407 Hz (Open 6th string - E2) to 880 Hz (17th fret on 1st string - A5) are generated. To achieve this, a lot of tension (both static and dynamic) is exerted by the strings onto the bridge and the soundboard. The tension exerted by each string depends on several variables including (a) active string length, (b) frequency of the string, and (c) mass per unit length of the string. For a classical guitar the total tension exerted by all six strings is approximately 85 - 95 lbs. of force. Due to the way the strings are attached to the bridge, the force of this string tension applies a rotational torque on the bridge and soundboard with the front of the bridge pushing down on the area between the bridge and the soundhole and pulling up on the area between the bridge and the bottom edge of the guitar.
As a general rule of thumb, the less mass there is in the soundboard and bracing, the louder and more responsive the instrument will be. Of course there is a limit to this and in reality a luthier must find a balance between the soundboard strength and the amount of mass that the soundboard will have. As mass is removed from the thickness of the soundboard plate and the thickness and height of the braces, the more the soundboard will deflect in response to the tension placed on the bridge by the strings. This is good to a point and every classical and acoustic guitar top deflects a small amount downward between the bridge and soundhole and upward between the bridge and bottom of the guitar. As long as this deflection is kept within some well established design parameter guidelines this results in great sound and a very durable instrument. If this deflection exceeds these guidelines, then over time, the top will start to deform and cracks may appear around the bridge. Ultimately the soundboard may collapse. This effect can be magnified if the guitar is exposed to extreme conditions of varying temperature and humidity.
Testing Apparatus and Procedure
As part of our testing process, we use a method of soundboard stiffness testing and deflection compliance that was first suggested by David Hurd in his book, "Left-Brain Lutherie - Using Physics and Engineering Concepts for Building Guitar Family Instruments". This is an outstanding book on the application of science and engineering to the craft of Lutherie. In this book, David proposes that deflection measurements of the soundboard be taken at the point in the building process where the soundboard has just been attached to the sides and the back has not been attached yet. This allows the lutheir to make both soundboard thickness adjustments and bracing height and thickness adjustments based on results of the deflection tests and Chladni pattern tests.
The apparatus that we use for deflection measurement was hand made and is based on David's Luthiers Forum series of articles which outline how to build the apparatus and how to use it. The apparatus is simple and is cheap to build. It is a well thought out apparatus for measuring guitar top deflections. After we built it and calibrated it, we performed extensive testing to establish repeatability and error values and we found that it can reliably measure deflections of a top within +/- 0.001" which is very acceptable.
There are several stages in the guitar construction process where it makes sense to perform deflection tests involving the soundboard. The first test is done during the selection of the tone wood material as part of the criteria in making sure that we start with an optimal piece of tonewood. The next is done before the braces are added to the soundboard in order to determine an optimal starting thickness for the top. The next is done during the final thicknessing and brace shaving step of soundboard tuning (which is what this blog article is about). The next one is done after the bridge is added to the soundboard. Finally, as the instrument approaches completion, the deflection caused by the tension of the strings is measured above and below the bridge and recorded.
Generally, the procedure we use for measuring and recording deflections is as follows.
Before we start the deflection tests for the final thicknessing and brace carving stage of construction, we create a very thin paper template with a 1" x 1" grid on it which we place over the soundboard so that it is protected from scratches and dents. We use a very thin paper which we tested to confirm that it does not affect the deflection values. Initially we perform a full set of measurements before the bridge is attached to the soundboard. Once the deflection measurements are taken, we normalize them based on a standard force value of 2 lbs. This is done so that data can be compared with values taken by other luthiers (as proposed by David Hurd).
Next we use a contouring program to map the deflection values for the entire soundboard. This map is then reproduced on the full size Grid template.
This sheet is initially used as a guide for where to take the deflection values and after the data is contoured, it is used as a guideline along with Chladni patterns in tuning the soundboard by adjusting the plate thickness and by adjusting the height and thickness of the braces. This is an iterative process. We carefully carve and adjust thicknesses and then take deflection values and perform Chladni tests again.
This pain staking process is repeated until we reach the pre-established guidelines we have worked out regarding the deflection pattern that we want in the final product. It is these guidelines that each luthier must develop and refine as part of their artistic contribution to this process. This allows us to achieve the ultimate balance between strength and the tonal quality we are wanting to achieve for each guitar. There is a lot of work and time involved in doing this but the payoff in a masterfully constructed hand crafted classical guitar is well worth it. I wanted to write this article so that our customers know how much care is taken to be sure the value of these guitars far exceeds the cost and that each Wood Ring guitar is truly an investment that is built to give them a lifetime of enjoyment.