Photo below shows model of bass resonators for natural bars (C2 - A2). Model resonators were fabricated with poster board.
The bass resonators create a design challenge for a 5-octave instrument because the required tube length is greater than the distance from the bar to the floor. It is desirable to design the resonator with a straight tube, and in the past, a few bass marimbas were manufactured with the keyboard very high off the floor. However, this requires the player to stand on a platform, which is not desirable. The common instrument design solves the problem with a mitered tube. Typically, the bass resonators are mitered into a J-shape (five sections, each cut at 22.5 degrees, which results in a 45 degree angle at each miter). I have not been able to find any design guidance for mitering marimba resonators, but some literature on pipe organs suggests the following: 1) keep the number of miters to the minimum possible, 2) the first miter should be no less than 2/3 the total length from the open end. It seemed to me that it might be possible to make resonators in an L-shape (2 miters) rather than a J-shape (4 miters), in keeping with suggestion number one. And the model shown below demonstrates that it is possible to design a set of resonators with an L-shape for the natural bars. Suggestion number two cannot be met in all of the bass resonators because of the limiting height of the keyboard. Therefore, the best accommodation for suggestion number two is to run the tube down close to the floor.
Photo below shows model of bass resonators for sharp bars (C#2 - A#2). Designing the set of sharp resonators proved to be more complex. One of the criteria that I set for the design is that the tubes should not protrude outside the area defined by the frame (we would not want to have someone trip over a resonator while walking past the instrument - and in any case, the instrument already has a large footprint). The protrusion of the natural bar resonators over to the sharp side of the instrument limits the space available for fitting the sharp resonators. The most difficult problem was fitting the resonators for C#2 and D#2 (the two on the right in the photo). In order to keep these resonators within the boundary of the frame, and to keep them out of the way of the remaining 3 resonators, I found it necessary to create a tube with 3 miters. And as you can see, the first miter comes substantially before the 2/3 limit, but this can't be helped. In any case, the total design appears to be good for 10 of the 12 bass resonators. I must admit the the appearance of the sharp resonators is a bit funky, but I believe function should prevail over appearance.
Currently, the three tube sections for the E3 resonator are cut and ready for brazing (joining). A brazing rod formulated for aluminum will be used. Brazing of this resonator should be done this evening (2/23/04). Then I will test the resonator in place on the instrument to confirm the proper formula for tube length in an L design. Then I will continue fabricating the remaining 10 resonators.
Jig used to hold bass resonator sections while brazing (photo below). Brazing rod used was HTS-2000. While the information supplied with the brazing rod suggested that a propane torch would work, I found that it did not produce enough heat for this application. A MAAP gas torch was obtained and found to work well. The first step is to bevel the tube edges at 45 degrees, which creates a v-groove at the joint. The groove provides a space for the brazing material to accumulate. The tube edges are "tinned" with brazing rod, rubbing the area with a stainless steel brush while the material is molten. This process promotes bonding of the brazing material to the aluminum. Then the tube sections are assembled into the jig and brazed together (but it is not as easy as it sounds). I have never been very good at welding or brazing, but after some practice, I managed to join the tubes in what seems to be a structurally sound (although not air tight) result. I had to learn how to keep the brazing material from draining through the joint. The trick is to heat on both sides of the joint, constantly moving back and forth just until the rod starts to melt when touched to the joint, then work along the joint quickly so that the brazing material stays put. Another problem was cracking of the joint. You have to develop a technique that does not result in quick changes in temperature of the tubing. I tried to make sure that a large area of the tubing was preheated with the torch, and to keep heating a little in the area of the joint after brazing to promote a slower decline in temp. Then examine the joint and if there is a crack, just do it over again.
Finished resonator for E2 in photo below. After several frustrating tries to get a brazed joint without any pin holes, I decided to seal the holes by using JB Weld (epoxy for metal). The epoxy is a darker color than the polished aluminum, so the joints were painted with a chrome colored paint. The resonator was put in place on the instrument and an adjustable end stop was put in place. Then the stop was adjusted for maximum resonance. We (Katy and I) found that it was difficult to judge exactly where the optimum stop position should be. In any case, we finally selected a specific position and made a measurement of the stop. It was found to be 3/16th of an inch from the position I had calculated prior to the test. The calculation was done assuming that the tube length should be that of the median position (i.e., the length is measured along the center of the tube). The test confirmed that this is essentially correct. Therefore, I could proceed with cutting additional tubing for the remaining resonators. As of 3/2/04, two resonators have been brazed (E2 and F2, both L-shaped). Tubing has been cut for another 3 bass resonators (F#2, G2, G#2). There are a total of 11 bass resonators that must be mitered. So I hope to have these finished in about 2 weeks (ha ha ha).
Three sections of D2 resonator, mating ends cut at 22.5 degrees. These tubes have been polished, which is done prior to brazing to eliminate the difficulty of working with the buffer on an assembled resonator.
Resonator sections for C2, C#2, and D#2. These, along with the D2 resonator are 4 inch diameter tubes, the largest on the instrument. These tubes are not polished yet. Only three resonators (A2, A#2, and B2) remain to be cut as of 3/9/04.
I discovered that the MAAP gas (44% methyacetylene-propadiene 56% liquefied petroleum gas) torch can melt the aluminum tube if you leave the flame in one location for too long. You need to keep the flame moving to avoid this. Photo below also shows what a section looks like after "tinning" with the brazing rod.
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