July 2007

A Three-Dimensional Analysis Of A Stradivari & Guarneri
Plus Other Acoustic & Measurement Thoughts
Article By Steven R. Rochlin
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  Unless you have been living under a rock or enjoy burying your head in the sand, every audiophile realizes that room acoustics play an enormous role in audio. The acoustics of your listening room, your loudspeakers, and how you dampen first reflection points are of paramount importance to achieve the very best in music reproduction. Recording studios have specially engineered mixing suites that take advantage of various materials with solid strategies of controlling soundwaves and sound dampening. In America, new homes generally use very resonant sheetrock and wood construction while in Europe and Asia they use a more solid construction of brick. While each room environment has a unique sound signature, the same can be said for musical instruments and high-end audio gear.

The way sound is produced, travels through, and is projected from an instrument is what makes it sound different from other devices. Musical instruments may start out with a basic design, yet it is the details that make one violin become valued at $200 and another at $2,000, or $2,000,000! Naturally the quality and type of wood is of paramount importance, yet so is the smallest of factors including using special stain and lacquer. Every single part of a violin makes up the whole, just as each part within a high-end system makes up the whole, whose outcome could range from conflicting sounds that irritate the ear to a harmonious synergy that may be praised for many centuries. So what makes one violin so special while another being available at your local musician's store for less than a week's pay?

Who Is The ASA?

At the recent 153rd meeting of the Acoustical Society of America (ASA) Utah there were some very interesting topics that unfolded. For those not familiar, the ASA was organization in 1929 and today includes nearly 7000 men and women who work in the field acoustics. Many are scientists in various fields who carefully analyze the physics of soundwaves and acoustics. This can include mechanical, electrical, biological, physiology, psychological, both air and water propagation... and music of course.

"This diversity, along with the opportunities provided for the exchange of knowledge and points of view, has become one of the Society's unique and strongest assets. From the beginning, the Acoustical Society has sought to serve the widespread interests of its members and the acoustics community in all branches of acoustics, both theoretical and applied. The Society is primarily a voluntary organization and attracts the interest, commitment, and service of a large number of professionals. Their contributions in the formation, guidance, administration, and development of the ASA are largely responsible for its world-wide preeminence in the field of acoustics." says their website. 

Show Me The Data

With that said, let us get to the meat of this article. Perhaps anyone can make a Stradivarius violin, if you could reverse engineer the exact materials, the shape, their density, and resonant frequencies. Sounds simple, yet in reality George Bissinger and Danial Rowe of the East Carolina University came away with a vast amount of scientific data yet admit there are still some unknowns. Their recent findings were discussed at the 153rd meeting of the ASA and was perhaps the most ambitious endeavor in seeking to unlock the secrets of this legendary instrument.

According to the Peter Prier Violin Making School in Utah, which has taught nearly 500 luthiers over the years, to make a violin you start out with blocks of wood (Spruce, Poplar, or Willow are popular choices). Each piece of the instrument is carved from the various woods and may be placed in special molds to bend them into the desired shape. The precise size and shape of each piece makes up the whole, so any small difference will result in a distinctive sound. Everything from the top scroll size — where a deviation of a millimeter can completely change the feel of a violin — to the Ebony fingerboard is crafted and glued into place. There are so many variations from the material to the size of parts, how they are bent, and overall construction that it is nearly impossible for someone to make by hand two exact-sounding instruments. So the task at hand for George Bissinger and Danial Rowewould is to get a better understanding how this sample of a Stradivari measures using modern techniques, yet without taking apart the instrument. They needed to measure the final product without damaging this incredible valuable instrument.

They began their work by planning out a course of action. After acquiring a Stradivari, not an easy task mind you, the instrument was analyzed using a special tri-laser system that would record three-dimensional scans. Their paper goes on to say, "For two centuries scientists have been investigating violins to try to understand on a quantitative basis what determines a violin’s quality. To date there is no robust scientific descriptor of violin quality, and almost certainly there never will be just one quantity that could cover the wide variety of most desired violin properties. However in the last decade much entirely new vibration and sound information has become available."

And so began their scientific study and recording of data. Seen here is one of the many scans they acquired. You can see more detailed visualizations of the violin in motion by downloading a viewer (link) and then the 150 MB file (link). What you will see are the results from their scans and how sound propagates through this instrument. George Bissinger and Danial Rowewould of the ASA went a step further and took advantage of an anechoic chamber and used a staggering amount (266) microphone positions. The end result of all their hard work gave a surprising amount of data, yet still left some questions unanswered. Within their technical paper, they readily admit, "One of the really difficult aspects of trying to understand these old Italian violins is our ability to characterize the materials used in the construction. While CT scans give you the absolutely necessary density information, equally necessary to understand vibrations are the stiffness properties of the wood. Is 3 century old wood the same as modern violin wood in density and stiffness?" This last question is very justified as over the course of time the organic material (wood) has been subjected to varying humidity levels plus the act of playing the instrument also affects the instrument. 

They go on to say "Shells in general are a very difficult scientific problem even for relatively simple geometries. And the difficulty is compounded by the fact that when IP waves reach discontinuities like the ribs or bassbar they can be converted to OP vibrations that do radiate well. These first 3-D measurements have given us insights into violin motion at a level never previously attained. They have improved our understanding of how strong in-plane vibrations are for the violin vs. out-of-plane vibrations. The OP/IP ratio has given us an uncomplicated parameter to help gauge how some violins become more capable of directing sound out into the auditorium than others."

How Does This Pertain To Audio Equipment?

In many ways loudspeakers, perhaps the most important part of your sound system, can be considered a musical instrument. While some seek out to manufacturer the most rigid cabinet structure and kill any and all vibrations (Wilson Audio), others take advantage of wood resonances (older Snell loudspeakers). Another factor is the resonance frequencies of the components that make up the drivers and how they interact with one another. Many loudspeaker manufacturers include spikes as feet, so that various resonances are controlled. We could extend this controlling of resonances to electronics where metal spikes, Sorbothane, balls, bases, etc. are used as tweaks.

My earliest experience with an audio tweak was not as an audiophile but as a drummer. Drums are resonant by design and my goal was to achieve a certain sound. No matter what i tried, various drum heads, dampening techniques, etc., there were problems with certain tonal qualities until i discovered the Resonant Isolation Mounting System (R.I.M.S.). This is basically a resonance isolation (learn more by clicking here) that, like audiophile tweaks, allows for the device under test to achieve a higher isolation from its environment.

And The Point Of This Article Is...?

Even with the use of three-dimensional lasers and an anechoic chamber to study sounds from 266 microphone positions, scientists still have not been able to unravel the secrets of a Stradivari. With that said, using extremely crude (by comparison) techniques employed by many loudspeakers manufacturers and the press, it is no wonder that such measurements can lead to confusion. Worse still, it appears there is no 'control,' a scientifically accepted specific room or place where said measurement are taken. This means you can not scientifically reply upon the measured results from one manufacturer to another, or one magazine to another. One might use a anechoic chamber while the other just places a speaker outside somewhere(!) in an imprecise way.

So can we rely on measurements to tell the entire story? Of course not! If you could precisely measure, understand, and manufacture perfect copies of a Stradivari then you may indeed become the next legendary instrument manufacturer. Alas, as of this writing we are still far off from completely unraveling all the secrets of Antonio Stradivari's creations. Taking this a step further, our music reproduction system as a whole has many factors where resonances play a role to the final outcome. That leads me to firmly believe no two systems are alike and when we add in the room effect, a major role player and one not to be underestimated, perhaps resonances are a good thing on one hand and bad on another. Only each one of us can decide which resonances are the ones we prefer. Perhaps having a resonant system is not as bad as some people make it out to be?

Of course in the end what really matters is that we all....

Enjoy the Music (Itzhak Perlman right now),

Steven R. Rochlin

 

PS: To see a good selection of the technical papers at the 153rd meeting of the ASA click here.