By Matt McGrath
Science correspondent, BBC News
Stradivarius violins have acquired a matchless reputation for tone and clarity
The unique sounds of a Stradivarius violin may come down to the density of the wood it is made from.
Scientists say the patterns of the grain are markedly different from modern instruments.
It is believed that the seasonal growth of trees in the early seventeenth century was affected by a mini-Ice Age.
Stradivarius had the benefit of wood that was produced in conditions that have not been repeated since then, the journal Plos One reports.
The work by a team from the Netherlands represents the latest finding in ongoing efforts to understand the sound quality of these violins.
The musical instruments created in Cremona, Italy, by Antonio Stradivari in the early 1700s have acquired a matchless reputation for tone and clarity down the centuries.
Around six hundred of the violins, violas, guitars and cellos made by the Italian master survive; on the rare occasion they come up for auction they sell for millions.
There have been several suggestions as to why these instruments sound so good and why the modern world has thoroughly failed to replicate their quality.
It was once argued that Stradivari and others used wood from ancient churches or that they added a mysterious ingredient to the wood or used techniques that have since been lost.
But modern technology first developed to help people suffering from emphysema may have unlocked the riddle of these fiddles.
Researchers at the Leiden University Medical Center in the Netherlands had developed a computer programme that analysed Computed Tomographic (CT) scans to see how effective certain treatments were in patients with emphysema.
One of the scientists involved was Dr Berend Stoel, a violinist with a keen interest in the secrets of the Stradivarius.
He adapted his program to work with violins and scanned five of the priceless instruments from Cremona as well as seven modern violins.
They show that while the overall density of the different instruments was similar, the 300-year-old instruments showed evidence of more even growth in the summer and winter. Dr Stoel explained its importance: "If you look at any piece of wood, as long as it's not tropical, you have these year rings.
"The differences between these rings are the density - the wood is more dense during the winter than it is during the faster growing period of the summer. That pattern is influencing the resonating quality of the wood."
The modern violins, according to Dr Stoel, show greater differences in their seasonal growth patterns. The older ones had more even grain, reflecting similar growth periods in winter and summer.
Since differentials in wood density impact factors such as "vibrational efficacy" and the production of sound, this discovery may explain the superiority of the violins produced by Stradivari and his contemporaries.
Other researchers who have studied the activity of the Sun have pointed to a mini-Ice Age that occurred in the early 1700s.
Experts say that this reduced solar activity, called the Maunder Minimum, could have hampered the regular growth of trees.
Temperatures in Western Europe dropped by between 0.5C to 2C. When trees grow in cold conditions like this their wood is denser.
However Dr Stoel is not entirely convinced that the magic of the Stradivarius is down to climatic conditions.
"We found these differences. But where do they come from? It could be a difference in climate when the trees were harvested, or it could also be that the masters used some secret treatment on the wood, or it could be that over the course of three hundred years the violins just gets better in tone," he explained.
"It's possible that you could use this CT technique to select different types of wood that would be more like the wood that Stradivarius used. But if you are a lousy violin maker and use the best wood, you will still end up with a very bad violin."