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Barry Young

CMSC

The Sweet Sound of Reverse Engineering

How do you measure violins worth millions of dollars? Carefully!

Published: Friday, June 27, 2014 - 09:36

What do you do when 20 ultra-rare violins from the 1730s show up in town, ready to be measured? Further, what exactly would you want to learn from these measurements? For many of us, the first thing that jumps to mind is, “Why do these particular instruments sound so much better than others of similar vintage and construction?” That’s the million- (or more accurately, the 2 million-) dollar question.

To answer that question, one needs consider many factors, including the type of wood used in construction, the wood grain and density, and the humidity at the time the violin was built. However, the most obvious and important factor is the exact shape and dimensions of the instrument, and it is here, in measuring and analyzing those features with incredible precision, that high-tech coordinate metrology comes into play.

Of course, getting those data is not easy. For one thing, you don't want to touch these violins with any type of mechanical instrument that might damage the finish. After all, these instruments are valued at $2 million each and up.

So how do you do it? An option that we employed was to laser scan the violins with a portable coordinate measuring machine (CMM) with an attached laser line probe (LLP). In this way, you can derive a point cloud of the exterior shape good to about .003 in., without having to touch the instrument at all. The total time to scan one violin is approximately 45 minutes, most of which was spent inspecting the point cloud for holes in the data, and then going back to fill them in. The actual time scanning the majority of the outside of the instrument was less than 15 minutes. A small bit of time was also taken to delete extra data collected from the jig on which the violins sat. The LLP is a line-of-sight instrument, so anything scanned with the laser will be picked up by the cameras and added to the cloud of points. Once we were satisfied with these data, they were saved to a file.

That took care of the collection of the external data points, but just as important for the purposes of reverse-engineering a replica is what the instrument looks like on the inside. Cutting it open was obviously not an option, but our local hospital had a magnetic resonance imaging (MRI) scanner that had the ability to provide interior point-cloud data very similar to those received from the LLP on the outside. The stated accuracy of the MRI is not as good as the LLP, but for this application it was more than adequate. To take the MRI scan, we sat each violin on a soft cushion on top of the table normally used for humans, and into the scanner they went. About 10 minutes later the MRI had completely mapped the inside of the violin. These data were also exported to a file for future use.

Now that the data were collected for both the inside and outside of the violin, it was time to bring the data together so that a complete model could be made. First, the LLP data were opened in the software that collected the LLP information, and then, via .igs, the MRI data file was imported as well. The MRI data also had information collected on the outer shape, so that is what was used to begin the alignment process between the two files. First, a point pairs-style alignment was completed to get us close. We then allowed the computer to do a best-fit analysis to really dial in the two clouds of data. Once the data were aligned, it was saved as one dataset. From this, the point cloud was analyzed and manipulated into what was eventually a 3D computer-aided design (CAD) model usable in virtually any CAD system.

Without focusing on too many further details, with this CAD file, the violins’ owners could now send these data to a computer-aided manufacturing (CAM) software company to write highly specific tool paths used by computer-numeric controlled (CNC) machines to cut both the top and bottom surfaces to the exact specifications of the old violins. Once all the pieces were made, the builder would then assemble the violin from these components parts and end up with an instrument that would sound as good as the violin they were trying to replicate in the first place... and for a lot less than $2 million.

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About The Author

Barry Young’s picture

Barry Young

Barry Young is a senior applications engineer at FARO Technologies, Inc. He has been with FARO for more than 15 years and is absolutely passionate about efficiency. Young enjoys working closely with customers to offer onsite support and application-specific consulting. When not working, he enjoys hiking with his wife.