Slight differences in the growth rings in wood from which violins are made might help determine sound quality, distinguishing a mellow-toned Stradivarius from an ordinary instrument.

Violins made by Antonio Stradivari in the early 1700s are perhaps the most famous musical instruments ever made. The sound quality of these instruments, as well as those built by fellow Italian Giuseppe Guarneri del Gesu, is the benchmark to which all modern instruments are compared, says Terry Borman, a violinmaker in Fayetteville, Ark.

Music lovers and researchers alike have long wondered what makes these instruments stand out from the rest. “Most big pieces of the puzzle have been looked at,” he says. But few answers have stood the test of time.

Now, Borman and colleague Berend Stoel of the Leiden University Medical Center in the Netherlands have analyzed five ancient violins — two by Stradivari and three by Guarneri del Gesu — and compared them with eight instruments built after 1995.

The researchers CT scanned the violins and measured the density of wood in various areas of the instrument. The scan was sensitive enough to detect paper labels attached to the violins.

Previously, some scientists speculated that the density of the wood used in Strads — taken from trees that grew in Europe during an extended cold spell called the Little Ice Age — was higher than those growing today, thereby lending a richer tone to the violin’s music. Borman and Stoel’s analysis suggests that notion is wrong: On the whole, the density of the wood in the ancient violins isn’t significantly different from those used in the modern instruments, which are made of European wood of the same species.

At the fine scale, however, Borman and Stoel found that, on average, density variations in the ancient wood — in particular, the difference in density between the spongy, light-colored portion of a growth ring and the dark wood next-door — were much smaller than those found in modern wood. This difference in the microstructure of wood may influence how vibrations traveling through the wood are damped, thereby affecting the overall stiffness of the wood, Borman says.

Studies suggest that the larger the vibrations in the faceplate of a Strad are, the better the instrument projects sound toward the audience (see Suggested Reading).

The density differences noted in these experiments “may contribute to the generally recognized superior sound production” of the instruments built by Stradivari and Guarneri del Gesu, the team contends in the July 2 PLoS ONE.

Not all researchers agree. The new findings “may have some forensic significance in how ancient violinmakers selected their wood,” says Jim Woodhouse, a mechanical engineer at the University of Cambridge in England who also studies violins. However, he notes, the density variations “are not things that would obviously make much difference in how a violin sounds.”

“There’s a lot of folklore” about Stradivarius violins, Woodhouse adds. Not all of the master’s instruments are blessed with the sound quality that the best are well known for, he notes, and the acoustical characteristics of some modern-made instruments are a lot closer to those of Strads than is usually imagined.

Developing a wood treatment that reproduces in modern wood the pattern of density variations found in the ancient violins could help today’s instrument makers replicate the tonal qualities of a Strad, Borman and Stoel contend.

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