A 1709 La Pucelle Stradivarius owned by David Fulton is seen April 9, 2007, in Redmond, Wash. ((The Seattle Times, Greg Gilbert/Associated Press))

Growth rings in the wood used to make Stradivarius violins in the 1700s may hold the explanation for their unparalleled sound, say Dutch scientists.

Researchers at the Leiden University Medical Centre in the Netherlands, who put the instruments through a computed tomographic (CT) scanner, published their research Wednesday in the online journal PLoS ONE.

Many music lovers believe the classical violins made in Cremona, Italy, by famous masters such as Antonio Stradivari and Guarneri del Gesu, produce unique tonal expressiveness and projection. Despite three centuries of technological advancement, modern violin makers have been unable to duplicate the sound.

The scientists, who tweaked a computer program used to analyze scans measuring lung density in patients with emphysema, said that may be because of important differences in wood from the 1600s and wood today.

More than violins

Antonio Stradivari built more than 1,100 instruments in his lifetime, about 650 of which survive today.

In addition to the famous violins, there are 13 violas, 63 cellos, two 10-string guitars, two mandolins and an arpetta, a small ornamental harp. 

Lead author and violinist, Dr. Berend Stoel, put five of the ancient instruments and seven modern violins through the scanner.

Stoel found that the ancient violins had more even wood density, possibly the result of even growth rings in the summer and winter. The researchers speculated that this difference might influence how vibrations travel through the wood.

Other scientists have argued that a mini European ice age in the 1600s might have something to do with the quality of wood in the Stradivarius violins.

Trees — at least those outside the tropics — have growth rings of dense wood during the winter when the tree grows slowly, and less dense wood in the summer when they grow faster.

But cooler temperatures in Europe during the 1600s likely led trees to grow more evenly, producing even seasonal growth rings.

"Climate difference could explain part of it, but treatment of the wood could be another explanation. A third answer could simply be the aging of the wood over the past 300 years," Stoel told Reuters.

"There is no way of knowing from this data; we've just shown there are density differences."