Bent Bass Bars from xxx Journal
By Sam Compton, Natural Bridge, Station, VA
My dear friend Bill Fulton asked me to comment on my work concerning bent bass bars. This project is still “in the works”, but I’ll tell everyone what I’m looking at and why.
When I first began looking into Carleen Hutchin’s method of measuring the eigen-
I hope to prove in the near future (when I get around to it!) that a carved, chalked in modern bass bar and a bent baroque bass bar have exactly the same effect on the frequencies of a given top plate. In other words, I plan to make a top plate, tune it before the “F” holes are cut, and then cut the “F” holes and record the changes. Next I’ll fit a modern bar, and tune it until I get the same frequencies that were seen in the plate before the “F” holes were cut. Then I’ll remove the modern bar and put in one of my bent baroque style bars, and tune that bar until I get the same results seen with the modern bar.
Lastly, I’ll compare the relative size, shape and weight of the two different bars. Let me publicly state here for the record that I’m sure that this will work, I just want to be able to prove it! Let me also state that this is my opinion only, and as such is subject to change at any moment. 🙂
My technique: I finish the inside of my top plate, and get everything ready to receive the bass bar. The typical modern carved top plate must be graduated in such a way that there is extra thickness in the central area, which creates a problem with the simple method of fitting the bent bass bar that I will describe shortly. Because of this thicker area, the inside of the top plate must necessarily contain some transition areas between the thinner wood in the upper and lower bouts, and the wood in the central area. Of course there are no sudden lumps or steps in the graduation, but the inside surface of the plate will not show the smooth simple transitions that we see on the varnished surface.
On the other hand, if we start with a bent plate, we can create a top plate with what I call “diaphragmatic graduation”, meaning that the thickness of the top plate will be uniform all over (except for the edges around the f holes and a small circle located at the position of the sound post.) I’ve used a Hacklinger gauge to measure the plate thickness of several famous instruments. The fiddles that we love to listen to are generally graduated in this manner. In particular, I’m referring to Fritz Kreisler’s Guarneri del Gesu. The gluing surface of the underside of the top plate will be a smooth even curve in the case of a diaphragmatically graduated plate.
To continue with the technique of making a bent bar: I know exactly where my bar is going to go. I have a twelve inch long plastic contour gauge that is used to copy the shape of the inside of the plate exactly where the bar will be (a compass would work just as well) and I transfer this shape to a piece of half inch thick hardwood. I cut away the waste wood, and use a square to make sure that the concave surface just created is square and true with no undulations. At this point I have recreated the gluing surface of the underside of my top plate where the bar will be attached on the edge of this piece of one half inch thick hardwood. Now to the bass bar itself:
The bar starts out as a rectangular piece of bass bar stock, 5.5 mm thick uniformly over its length. It is 270 mm long, because when it is bent to match the contour of the inside of the top plate it will then be 266.7 mm from end to end. (I decided to try for a final length of 10.5 inches when I began this experiment…thus the 266.7 mm.) It is eight millimeters wide. That means that when glued in, the bar will be a maximum of 8 mm high in the center of its length. After tuning it will finish to slightly over 7 mm high. One longitudinal edge is planed perfectly straight, square and true. I then shape the other longitudinal edge (the one that hasn’t yet been planed) to a catenary curve, starting and ending at the ends of the planed edge and 8 mm high in the center. The ends feather out to nothing.
Now to bend the bar: I heat up a pan of water to 160 degrees F. The now flexible bar is clamped to the half inch thick hardwood form that was described earlier. The flat planed edge of the bass bar is the edge that will be glued to the underside of the top, so it goes against the concave surface of the form. I use two or three padded C clamps to lightly force it against the form for a perfect fit. The catenary edge of bar now looks almost straight, while the formerly straight edge is being clamped to a form that exactly duplicates the shape of the inside of the plate where the bar will go.
The form, including the bar and the three C clamps, is immersed in the hot water and allowed to remain until I’m satisfied that the bar is at 160 degrees also. Then the apparatus is removed and allowed to dry, still clamped, for several days. When the clamps are removed the bar retains its new shape, and is an effortless fit to the underside of the top. I prime the surfaces with thin hot hide glue, and then glue in the bar with three wooden cam clamps. One in the center, and the other two approximately two inches from the ends of the bar. There is no need for chalking or adjusting…it’s a perfect fit every time. Tuning the bar consists of rounding the sharp edges and little else, usually. The finished bar looks EXACTLY like the one taken from the Lady Blunt, as pictured in Beare’s book “The Stradivari Exposition of 1984”. If I used a chisel to bevel the ends slightly like Stradivari did, my bar would end up the same length as his.
To conclude: In my experience, this bent bar imparts the same stiffness as a modern bar, but is the same size and shape as a baroque bar. It’s also a lot simpler to fit! Fiddles that have been out there for several years show no deformations or problems at all.
Could the modern bar be a solution to a nonexistent problem?
Comments, anyone?
