World-famous classical violinist Joshua Bell -- perennial uber-cute Cyber crush of Jen-Luc Piquant -- travels all over the world performing, and his instrument of choice is a 300-year-old Stradivarius violin called Gibson ex Huberman. The violin dates back to 1713, when the famed Cremona violin-maker Antonio Stradivari was at the height of his prowess. It is valued at just under $4 million.

Intrepid acousticians all over the globe are still hot on the trail of "Stradivari's Secret": a.k.a., just what is it about a Stradivarius violin that makes it sound so much better than your average, run-of-the-mill instrument? It's been a topic of feverish investigation and much hot debate for over a decade, at least, and the latest offering comes from a Minnesota radiologist named Steven Sirr, who decided to run a 1704 Stradivarius violin known as "Betts" through a series of CT scans. The US Library of Congress helpfully loaned him the instrument for the experiment.

Why a CT scan? "CT is useful in measuring wood density, size and shapes, thickness graduation and volume measurements," according to Sirr, not to mention providing "detailed analysis of damage and repair."

A CT scan is simply a 3D version of your typical x-ray machine. Instead of just zapping an object with a single x-ray beam and recording the shadow image on special film, a CT scan features an X-ray beam that moves all around the object, taking a series of images from hundreds of different angles. A computer then compiles all those images into a single 3D image that enables an analyst to closely examine individual slices, one at a time.

That's what Sirr did with the Betts violin, collecting over a thousand "slices" (individual images from many different angles) and then compiling them into a 3D reproduction of the instrument. This gave him a sneak peek into the violin's inner workings. As Sirr explained:

"I assumed the instrument was merely a wooden shell surrounding air. I was totally wrong. There was a lot of anatomy inside the violin. Just like human beings, there is a wide range of normal variation among violins. When you are looking at an instrument that is hundreds of years old, you will see worm holes and cracks that have been repaired, as well as damage from being exposed to all kinds of conditions, from floods to wars."

Ah, but Sirr didn't stop there! He collaborated with two professional violin makers (luthiers) to recreate the perfect replica of Stradivari's exquisite instrument. It started back in 1989, when Sirr showed his first scans to luthier John Waddle. They spent the next 20 years scanning over 100 violins -- some common, others very rare and valuable -- as well as other stringed instruments to gain a better understanding of how they were made.

For the Betts scan, Sirr converted the CT images into stereolithographic files and fed them into something called a CNC machine. It's used for wood-working, among other applications, since it can use those files to carve out a real-world version of the imaged object. That's what Waddle and his partner, Steve Rossow did: they carved the front and back plates and scroll for the replica Betts violin, then assembled and varnished the new instrument by hand.

In Search of Stradivari's Secret

Ah, but the good Dr. Sirr is not the first to use CT scanning to study Stradivarius violins. Back in 2008, at a meeting of the Acoustics Society of America, Berend Stoel from the Leiden University Medical Center (LUMC) described his collaboration with a renowned luthier named Terry Borman. The two men put several Strads (and some modern instruments, for control purposes) into a CT scanner to study the materials properties of the wood out of which the violins had been made.

Why focus on the wood? Well, several theories about why Strads sound so good rest upon the notion that it's all about the wood used to make the instruments. For instance, some theorize that Stradivari used Alpine spruce that grew during a period of uncommonly cold weather, which caused the annual growth rings to be closer together, so the wood was abnormally dense.

The problem is that no two pieces of wood are exactly alike, so sculpting the wood -- delicately shaving the top and the back to get the best acoustical properties -- is critical during the violin-making process. A team of researchers from Mid Sweden University has been investigating computer models of violins for years, attempting to match in simulation that telltale Stradivari sound -- including simulating that sculpting process.

Another prevailing theory has to do with the varnish: namely, that Stradivari used an ingenious cocktail of honey, egg whites, and gum arabic from sub-Saharan trees, or perhaps salts or other chemicals. Joseph Nagyvary, a professor emeritus of biochemistry at Texas A&M University, made headlines in November 2006 when he claimed it was the chemicals used to treat the wood -- not necessarily the wood itself -- that was responsible for the unique sound of a Stradivarius violin.

Those chemicals included salts of copper, iron and chromium, all of which are excellent wood preservers but may also have altered the acoustical properties. He based his findings on studies using infrared and nuclear magnetic resonance spectroscopy to study the chemical properties of the backboards of several violins. (The backboard is the instrument's largest resonant component.)

So Stoel decided to study the wood yet again, using the CT scan. See, it's tough to study those woody properties without risking damage to this multimillion-dollar instruments. Stoel developed a computer program that non-invasively calculates lung densities in people suffering from emphysema, and adapted it to study wood densities from CT scans.

He found that while the average wood density of the classical and modern violins "did not differ significantly," according to the accompanying press release, "the differences in wood density between early and late growth were were significantly lower in the ancient violins. Since differentials in wood density impact vibrational efficiency and thereby the production of sound, it is possible that this discovery may explain the superiority of these violins."

Back in 2007, I sat down for a nice long chat with George Bissinger, a physicist at East Carolina University who also studies violin acoustics. Bissinger had the big Stradivari announcement in 2007 when he presented the results from his own investigations.

Using a 3D scanning laser, he achieved what he said were the most detailed and quantitative measurements to date of the acoustic properties of the Strad violins featured in the study as they vibrate -- essentially mapping out how they vibrate to produce those heavenly tones. The measurements are so quantitative, in fact, that it's possible to reconstruct the stiffness properties of the wood used to make the Strads, perhaps finally making it possible for modern instrument makers to replicate those unique acoustical attributes.

Bissinger is tall, slim, with cropped salt-and-pepper hair, and glasses, and while perfectly amiable, he's not really one for casual chitchat; he's more the quiet, deep-thinking sort. He doesn't exactly stand out in a room full of scientists -- until you get him talking about violin acoustics. Then he positively vibrates with intensity and becomes the most loquacious conversationalist on the planet.

That level of passion seems to be present in many who study Strad violins, never mind those who play them, like Joshua Bell (although a delusional Jen-Luc Piquant swears his passion for her trumps even his love for his Strad). The Cremona craftsman would no doubt find this quite gratifying.

Conducting this sort of experiment with bona fide Strads is a major logistics undertaking; Bissinger says it took him several years of careful "networking." First, he had to borrow two of the world-class instruments from private collectors -- no doubt having to pry the cases from the owners' panicked fingers on the train platform. Yes, I said "train platform," as in, Amtrak. The instruments were brought to the lab by train in plain cases. Sometimes being inconspicuous is the best security in the world.

A violin organization generously footed the bill to insure the instruments for the 2-1/2 days of the experiment -- you know, just in case the scientists dropped one or accidentally destroyed their tonal purity. Bissinger also brought in three other, lesser violins of varying quality for comparison purposes.

For the experiment, he hung each of the five violins by elastic bands, then struck the wood of the top plate with a little hammer, recording and measuring the vibrational modes with the 3D laser scanner. Bissinger specifically wanted to measure the in-plane and out-plane motion: the in-plane motion is the source of much of the sound energy, and this converts into out-plane motion, which produces the rich tonal sounds we associate with fine violins.

In addition, he hired a world-class violinist to play each of the violins used in the study for an hour so, to get the feel of the instruments, and then offer his subjective ratings for each one. The musician's subjective analysis was then compared to the objective acoustical data.

The Psycho in the Acoustics

Not surprisingly, Bissinger had a lot to say on the topic of what makes a Stradivarius violin so acoustically superior. "The big secret about Stradivari is that there is no one secret," he insisted -- no elusive key or magical formula that, once discovered, will magically make it possible to reproduce the sound quality of a Stradivarius instrument over and over again on a mass scale.

Bissinger believes it can never be reduced to blind routine, because there are so many different factors that go into making a world-class instrument. It's as much an art as it is science," he told me. "You wouldn't ask Leonardo da Vinci to reproduce the Mona Lisa en masse, perfectly, every time." For Bissinger, an instrument maker is just as much of an artist as da Vinci: "He is the bridge between the artist and the scientist, both of whom speak very different languages and have different concerns. The maker has to speak to both."

Certainly Stradivari was more than a simple craftsman: "He had some kind of conceptual understanding of the science behind what he was doing, even though physics technically wasn't around yet," said Bissinger. But he knew that doing one particular thing would have a desired effect, and he built on accumulated knowledge: each instrument was an improvement on the last, at least through Stradivari's Golden Period.

But while Stradivari's emphasis on geometry gave us the signature shape of a violin, Bissinger says there is little evidence this has anything to do with the famous "Stradivari sound." After all, Guarveri also produced exceptional instruments and wasn't nearly as fascinated by geometry.

Not every Stradivarius sounds alike, and frankly, says Bissinger, even a genuine Stradivarius violin isn't all it's cracked up to be sometimes. The passage of time can exact a devastating toll. Many of Stradivari's surviving instruments have deteriorated to the point where they are primarily collector's items. Play a violin too frequently, and the parts wear down and must be replaced, altering the sound; play it too little, and the sound deteriorates, too.

Most of the Strads still played today do not have all their original parts, although Joshua Bell prides himself on the fact that his Strad still boasts the original varnish. Still, even Bell adapts his playing to his instrument to get the sound he desires. Bissinger claims there is no "perfect" instrument, and Stradivari -- who devoted his life to the quest for perfection -- would probably agree.

As for the claimed acoustical superiority of the instruments, yes, they do sound lovely. However, "There's way too much psycho in the acoustics," according to Bissinger, referring to a subfield known as psychoacoustics. Basically, the very name Stradivari instills respect and awe, and this can't help but influence how people subjectively evaluate and/or respond to the instrument. "The truth is, there are many very fine world-class instrument makers today, producing violins that can hold their own against the Strads, but their names don't evoke the same awed reverence, and thus the perception is that they are not as good," Bissinger told me. In fact, more professional violinists play Guarveris than Strads, which have only become fashionable fairy recently.

Really, who wants mass-produced Stradivarius instruments, anyway? It's always been all about the craftsmanship.

Ironically, while he was still alive, Stradivari -- while hugely successful at his craft -- was not considered the best violin-maker in the world, although he certainly dominated the industry along with Amati and Guerneri. They were the Holy Triumvirate of the Golden Age of Violins, and after they died, the instrument entered into something of a acoustical Dark Age. Later instrument makers didn't share that all-consuming passion for improving the process to create ever-more-superior instruments: they just cranked out instruments the way it had always been done, with predictably pedestrian results.

Way back in 1819, physicist Felix Savard observed, "It is to be presumed that we have arrived at a time when the efforts of scientists and those of artists are going to unite to bring to perfection an art which for so long has been limited to blind routine." Here we are, almost 200 years later, still trying to map out all the details, still chasing down an elusive secret that might not even exist. Perhaps that ability to capture our imagination 300 years later is the true magic of Stradivari.

NOTE: Portions of this post first appeared on the old archived blog in July 2008.