The Thickness of an Instrument’s Top & Back Is Measured in Tenths of Millimeters, but There Is No Perfect Formula

By James N. McKean

Twenty years ago or so my friend Guy Rabut and I were in one of the small tryout rooms at Jacques Francais’ shop on 54th Street, huddled over a violin. It was one of the rarest and most famous Golden-Period Strads, in fact, which just happened to be in pieces for a restoration. Jacques, who was perhaps the most generous person I’ve ever known to young makers, thought we might want to record it—up to a point, at any rate. Tracing the outline and f-holes, measuring the arch and the purfling, the ribs—that’s what he had in mind. But when he happened to walk by as I had the micrometer on the back and was reading out the graduations, he stopped, came directly in and took the paper out from under Rabut’s pen. “Never do that,” he said, as he tore the chart in half.

The fiddle business can be as much a matter of killing a sale as making it, especially in New York. And the fastest way to kill a violin is to say it’s too thin. At the time I thought that was being a bit too extreme (that particular violin, by the way, was well within the bounds of normal), but I was faced with the same situation myself ten years later. A client of mine had agreed to lend his cello—again, one of the rarest and best of that maker—for an exhibition. Just in passing, my colleague organizing the show mentioned that they were going to do charts of all the thicknesses. I immediately called my client and advised him to pull the cello.

Why? In this case, the back was indeed thin. Only by a couple of millimeters in the chest, but that’s the difference between a cello and a viola. No matter that it is hands down the best cello I’ve ever heard. And it’s made it for a couple centuries without even distorting, much less cracking. It’s made from a stunning piece of Bosnian maple, with grain so tight you can’t even see it, which makes it immensely strong. But even so, if word got around that it was thin, it would materially damage its value.

This doesn’t mean that thicknesses don’t matter. Quite the opposite; they’re critical for the stability and the resonance of the instrument. The trick is finding just the right balance. And that’s quite a challenge. There’s a basic paradox in building a violin. It has to be strong and rigid enough to withstand the enormous pressure of the strings and the constant vagaries of the weather, yet at the same time, it has to be flexible enough to amplify the vibrations of those strings so that they can be audible in the back of the largest concert halls. The trick is knowing where to leave the wood thick, and where you can thin it down.

Adding to the challenge is that you’re working with two radically different types of wood. The top is carved from spruce, a very light softwood. The back is a hardwood—always maple in violins and violas, and most often in cellos, too. And they vibrate in completely different ways; the top rocks back and forth and up and down under the bridge. The back moves in and out, with the soundpost acting as a plunger.


Wood, to put it mildly, is infinitely variable, even within the same log—especially spruce. The top and back arches are quite different to account for the different ways they move, but making an arch higher or fuller, say, will change the way it vibrates. The depth of the channel—the reverse curve over the purfling, before it rises to the edge—will affect the vibration of the top, as will the shape and thickness of the edge itself.

Once the arching is complete, you then cut the f-holes in the top. This will have just as decided an effect on the sound as the shape of the arch. Increasing the distance between the upper eyes or across the belly at the notches even a couple of millimeters, for example, will stiffen the top. The total width of the f-hole—from the inside of the upper eyes to the outside of the lower—also plays a role, as do the length and width of the f-hole itself. And this can be counterintuitive; one of the most powerful violins I’ve ever heard, the 1714 “Soil” Stradivari played by Itzhak Perlman, has the closest set f-holes of the Golden Period violins I’ve measured.

Photo by James N. McKean

So all of these choices begin to shape the character of the instrument before you get to the next step, which is graduating the plates. But while these previous choices will determine its edge and power, the graduations are where the maker can do the most to determine its richness. The top and back are far from uniform in thickness, although the differences in the top are measured in tenths of a millimeter. The top is left thicker around the soundholes, where the cut wood makes it weaker, and under the soundpost, where it comes under the most pressure.

The top is also left thicker at the ends, where it meets the blocks—there’s quite a serious compression on each end of the box, from the neck and the endpin. Adding to the complexity for the maker is that this is precisely where the spruce is the weakest, and therefore most unstable, because of the way the arch cuts the wood.

When I’m graduating the top, I take it down tenth by tenth, using brass thumb planes. I’m constantly flexing the top and twisting it. I also hold it loosely and tap it. I’m listening to the way it rings. As I reduce the thickness, the overtones begin to emerge. And when do I stop? How do I know I’ve reached the optimal thickness? Well, that’s the mystery.


Back when I was working for Nigo (Vahakn Nigogosian) I would stand by his side and watch as he worked. When he had reached the point when he thought it was done—a soundpost patch, a top thickness—he would hand it to me and say, “Like this.”

While the top is set in motion by the movement of the bridge, it’s the soundpost that gets the back going. So it’s graduated entirely differently. We use a bell pattern, although the thickest point is not under the soundpost, but above it. This maximizes the flexibility of the back while ensuring the greatest acoustical and structural strength. While the center of the back is almost twice the thickness of the top, the flanks are the same.

How do you know when your finger is in exactly the right spot on the fingerboard for a note to blossom and come alive, or the exact distance the bow should be from the bridge, or the angle of it, or the speed and pressure? Graduating the plates is exactly the same: You just know. You feel it in your fingers, you hear it in the ring when you tap it. Some makers try to achieve a certain tap tone in each plate. They’re fundamentally different, due to the basic densities of spruce and maple; it’s one of the main elements contributing to the richness of the overtones. Other makers resort to scientific instruments, influenced by research that reveals certain patterns, called eigenmodes, in the vibration of the plates.

I was recently going through my book of spec sheets, and was reminded that I used to record the tap tones of the top and the back. I stopped at some point back in the early ’90s. It just seemed irrelevant. Violins are like the larger world: There can be too much information, a deluge of data. The problem is finding a correlation that matters, and while all of this was sort of interesting, it didn’t make a scintilla of difference in the finished violin. Also, the tapping—and all of this measuring—is done with an unvarnished free plate. The way it vibrates is completely different once glued to the ribs and varnished, to say nothing of putting a bassbar in the top.


And, at least in the case of violins, the perfect is most definitely the enemy of the good. Norman Pickering, the great acoustician of the violin, once remarked to me that he was more and more convinced that the inexactness of the graduations of the older violins—especially the great ones—contributed to the richness of their sound.

Still, the exact graduations are critical to the sound. A violin can indeed be too thick or too thin. But too thin on one cello is just right for another. The same is true of the wood—every set of maple or spruce has its own proper thickness that allows the overtones to blossom. The difference can be measured but not quantified. If an instrument is wolfy, strident, lacking in overtones, and the neck goes up and down like a yo-yo, then chances are it’s too thin. But the micrometer is not the way to find out.

The violin is made up of 120 separate pieces of wood. Each and every one plays a critical role, structurally and acoustically. Think of it as a miniature orchestra. And whether it’s a violin or an orchestra, the pieces all need to work. Some are better than others; the best can be transcendent. But not always—even the best can have an off night. You know it when it happens, whether you’re sitting at a music stand or out in the auditorium.

The graduations are just one of the 120 people on the stage, all working together to transform the notes into music. What makes it come together is ineffable. But that’s what makes music. Or a beautiful violin.