Computed tomography (CT) technology has undergone continuous improvement and evolution since its introduction in the 1970s, particularly with regard to temporal and spatial resolution. Clinical applications have increased, especially with the recent development of high-resolution multidetector-row spiral CT (MDCT) systems. However, non-clinical applications have also increased—including ones that are pertinent to the design and specification of high-end stringed instruments.
Last month, Berend Stoel (Dept of Radiology, Leiden University Medical Center) and Terry Borman (Borman Violins, Fayetteville, Arkansas, USA) published an exciting article in which they examined tonewood densities quantitatively by MDCT, in a number of Strads and other classical violins and in a number of modern instruments. No significant differences were found between the median densities of the modern and the classical violins. However, the density difference between tonewoods was statistically significantly smaller in the classical Cremonese violins compared with modern violins, in both the top (Spruce) and back (Maple) plates (p = 0.028 and 0.008, respectively).
Stoel and Borman used a multi-detector row CT scanner (Sensation Cardiac 64) from Siemens. They obtained 3-dimensional digital image data sets of approximately 1200 x 512 x 512 voxels for each violin. Edge-detection software was used to automatically detect the superior and inferior surface of the top and back plates in the image series from each instrument (or from each tonewood blank). From these surfaces, the local plate thickness, median wood density and density differential were calculated.
The MDCT densitometry settings were optimized for the highest sensitivity in distinguishing different wood densities. The image acquisition protocol included an 80 kVp beam, effective mAs of 53, 1 sec. rotation time, 512 x 512 image matrix, 0.6 mm slice thickness and 0.3 mm inter-slice increments. Software for wood densitometry was developed in MATLAB (The Mathworks, USA) image processing toolbox. The superior and inferior contours were detected in each axial slice by a ‘minimal-costs’ Sobel-type edge-detection algorithm. By stacking all contours, Stoel constructed curved multi-planar reformatted (MPR) images to enable completion of the tonewood density and thickness measurements.
The Siemens and Philips MDCT instruments are not the only modalities that are capable of performing quantitative densitometry of tonewoods. The General Electric LightSpeed series MDCT machines have the ability to visualize a volume with spans of up to 22 cm—adequate for an intact violin or viola; adequate for tonewood slabs for these instruments. Cellos and double-basses, not so much—too big for the aperture/gantry of the MDCT scanners.
Widths of the individual growth rings are another factor influencing wood density that Stoel and Borman studied. Previous studies are conflicting, finding both linear and nonlinear relationships between growth ring width (GRW) and tonewood density.
The published studies to-date suffer from relatively small experimental sample size and relatively limited GRW distributions, varying only from 0.5 mm to 2 mm. Giordano’s study showing relatively linear relationship between GRW and density covered the GRW range up to 4 mm (the maximum ring spacing usually found in violins is 2.5 mm to 3.0 mm; in violas 3.0 mm to 4.1 mm; and in cellos up to 5.0 mm) and, therefore, the Giordano paper is probably the best guide available at this time, for those who want to include GRW of the tonewood in the plates as a component of specification or evaluation for construction or purchase of a high-end instrument.
The density differentials found in Stoel’s and Borman’s study are thought to contribute to the superior sound production qualities of classical Cremonese violins.
Stoel and Borman discuss some of the ‘secrets’ of the Cremonese luthiers in relation to tonewood density—for example, the wood treatment referred to as ‘ponding’, where wood is submerged in water to alter the properties of the tonewood. Ponding alters wood properties by causing decomposition of various wood elements depending on the bacteria or fungi that penetrate the wood while submerged in water. Microbiological degradation results in lower tonewood densities.
Another technique involving ‘stewing’ or boiling tonewood in different solutions to achieve alterations of density is also briefly discussed. Stewing causes partial decomposition and loss of hemicellulose molecules in the tonewood, resulting in lower density and smaller density differentials. Fuming the tonewood blanks with nitric acid or ammonia are treatments that have been used throughout the years by luthiers. These chemical treatments would likewise decrease the tonewood density and the density differential.
Stoel’s and Borman’s results are uniquely useful insofar as they characterize specific, readily measurable property differences between the tonewoods used by the classical Cremonese and contemporary luthiers that are associated with their distinctive sounds. The measurements can be made non-destructively, on finished instruments and on raw tonewood blanks before they are incorporated into new instruments.
While Stoel and Borman modestly suggest that their findings may merely “facilitate replicating the tonal qualities of these ancient instruments”, in fact their findings may have much broader utility, I believe. For example, it is possible for violin dealers and elite performers to engage clinical radiographers and CT imaging centers with the appropriate facilities (the right CT machines and software; those having also the necessary interest and the necessary excess ‘exams-per-day’ capacity on their CT imaging equipment, unused capacity not already devoted to imaging human patients), to help them evaluate one or more instruments prior to purchase.
It is likewise possible for violin makers to engage clinical radiologists / radiographers to perform CT densitometry studies of specific resawn tonewood slabs, either (a) prior to purchase the slabs for a specific commissioned instrument or (b) after the instrument is completed, as a means of documenting the attributes of the finished instrument for its future owner(s).
I can readily envision that, for instruments worth more than $100,000 (or for tonewoods that are under consideration for possible use in such instruments), spending about $2,000 (less than 2% of the value of the instrument) to get a quantitative CT thickness and densitometric analysis of the top-plate and back-plate might become de rigeur—might in the future be a standard aid to a performer’s or a dealer’s decision-making.
The standard deviation of the differences was 7.5 kg/m3 (coefficient of variation CV = 1.8%) and 10.9 kg/m3 (CV = 4.8%) for the median density and density differential, respectively. This within-scanner/within-operator precision is high enough to ensure the practical replicability of the densitometric measurements—sufficient to enable the use of MDCT for the purposes suggested above, for instrument commissioning, specification, and characterization by elite performers, dealers, and luthiers.
Have a look at the Stoel-Borman paper (link below) to read more about MDCT and tonewoods. You may also like to find out whether any of the clinical CT facilities in your area might be willing and able to perform MDCT densitometry studies on your tonewoods or instruments. Busy acute-care hospital imaging facilities generally do not have unused MDCT instrument capacity since all of the capacity is consumed by performing exams on sick patients. However, free-standing private imaging facilities often do have unused instrument time, and the radiographers/radiologists are often classical music lovers. Besides the interest they may have in the incremental revenue that performing a few MDCT studies on elite string instruments or tonewoods would represent to their businesses, they may have considerable personal/aesthetic interest in doing such work for you.
- Stoel B, Borman T. Comparison of wood density between classical Cremonese and modern violins. PLoS ONE 2008; 3:e2554-60.
- Terry Borman Violins Inc
- Traditional Tonewood (Reinhard Zach)
- Northridge Hardwoods Inc
- Guild of American Luthiers
- Luthiers International
- Lehmann G, Lehmann M. Experiences and Observations on the Effectiveness of a Procedure for Vibration Treatment of String Instruments. Whitepaper, 2001.
- Radiological Society of North America [RSNA]
- European Society of Radiology [ESR]
- EuroRad (European Association of Radiology [EAR])
- DSM. Violin Physics, Chladni Patterns, Mysteries of Tonewoods. CMT blog, 13-MAY-2007. (Chladni patterns in violins)
- DSM. Some Resources for Timpani Chamber Music. CMT blog, 26-JUL-2008. (Chladni patterns in timpani)
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