A "Scientific" Approach to Soundboard Consistency

by Don Chesser

Chris Jenkins has built his shop in what was formally the attic space of his home. The shop area is completely finished with heating and air and a couple of skylights that bring in a tremendous amount of light. Although small, Chris ’ shop is very well organized and accommodates an amazing amount of machinery. He has a bathroom/storage area/spray booth that is separate from the work area and exhausts fumes (natural or lacquer) via an explosion proof fan.

The topic for Chris’ demonstration was selection of soundboard material. Chris attended a class taught by Ervin Somogyi back in the fall and many of the methods he is now using are a result of that class. Chris’ goal is to make a soundboard which is as light and thin as possible, which conducts sound well, and is strong enough to be structurally sound. Chris is using a fairly simple but scientific method to determine both the wood he uses for his soundboards as well as the thickness that particular soundboard should be.

First, in an effort to select the best soundboard material, Chris uses a tops density in grams per cubic centimeter in addition to its tap tone, tightness of grain and visual appearance. He has a gram scale the he uses to weight the piece and then uses simple geometry to determine the gram/cm. The beauty of this method is that he can compare rough stock regardless of dimension. (Apples to Apples). Chris is looking for Sitka spruce in the 6 to 7 grams/cm range. Once he selects a top, the next thing he wants to do is to thin the top, not to a specific thickness but to a specific stiffness. He does this by placing a top onto a fixture and measuring the deflection created by applying a know weight. The fixture consist of a piece of MDF (medium density fiberboard) that has two 1” dowel rods spaced 18” apart. The top is centered on these dowel rods and a dial indicator is placed in the exact center of the top. Chris then places a five pound dumbbell 3” away from the center and measures the deflection. He then thins the top until the deflection is .280”.

Soundboard Stiffness Testing Device

Chris does not claim that this method insures a great sounding top, but it does eliminate some of the guesswork involved at the beginning of the construction process. Chris believes that if he begins with a higher level of consistency in the materials, then the tonal differences between tops are more a result of the differences in the bracing patterns and brace dimensions. As with any discussion like this, the issue came up about trying to scientifically quantify the acoustic properties of wood versus intuitive building.

Although I couldn't (and wouldn't) try to resolve that discussion, we did come to somewhat of a consensus. That consensus was that regardless of the method you use (scientific vs. intuitive), our goal as builders is the same, to build consistently higher quality instruments than are available from manufacturers. The choice of how you built instruments is still your own. I will add my own little bit on Chris’ demonstration (Those who know me are not surprised). Although I lean more toward the intuitive side of this argument, I will be making my own stiffness tester to check my soundboards.