Below are the texts from three recent postings I did on Friends of the Mountain Dulcimer where I attempt to give a bit more detail about the design and construction of the nylon string instruments. In particular, I try to delineate the differences between my steel string dulcimers and the Aaron O'Rourke inspired nylon string dulcimer.
"Just got back from three weeks in China and I am trying to catch up with the interest Aaron is creating in the nylon string dulcimer. I will be back to work building Aaron's performance instrument now that I have returned. It will be very similar to the prototype he has been testing, but it will have some slight improvements. It will also have a bevel on the top to accommodate his style of play.
I will try to describe some of the major design elements that differentiate the nylon string dulcimer from my steel strings. These features were all derived from Aaron's description of the ergonomics and sound character he was looking for. I decided early on that the sound should be a blend of a dulcimer and a baritone ukulele with a bit of guitar thrown in. To achieve that, I wanted the top of the instrument to be the soundboard (unlike my steel strings where the soundboard is concealed inside). In order to obtain the volume and tone I was looking for I decided to raise the fretboard completely above the soundboard so that the entire soundboard could resonate. I could do this because I use a structural i-beam that runs the length of the dulcimer (photos at stephenslutherie.com) and the fretboard can be supported on four posts rising up from the i-beam. Like my steel strings, the fretboard is bolted on and can be removed at any time.
In chasing the sound of guitar family instruments, it seemed appropriate to taper the sound chest as is typical on those instruments. There is a slight taper so that the height of the rib is less at the head than the foot. The profile of the nylon string instrument is the same as my other dulcimers and it is built in the same mold.
The bridge is totally different from my normal bridge- designed for minimum mass and it has a unique string retention system that goes with it to provide for fast easy string changes and very direct application of the torque that excites the top. This feature is under review and will probably be enhanced to provide an easy means of string height adjustment as well as compensation.
The sound hole is located immediately below the point where the strings are picked to give maximum clearance to the soundboard with a relatively low fretboard height. The fretboard itself is tapered (lower at the bridge end) to provide the optimum combination of fretboard height without getting the string height at the saddle too high (which would require heavy bracing of the soundboard).
The bracing is loosely based on the asymmetrical bracing being used on some contemporary ukuleles. This has to be controlled with top thickness to achieve adequate volume without too much sustain. With Aaron's fast fingerstyle technique, we wanted to limit the sustain so that each note could be heard clearly.
The fretboard has Aaron's preferred 10.00" radius and the fretwire is also specified by him.
The tuners are ultra light GOTOH UPT with a 4:1 gear ratio that suites the nylon strings.
I am sure there are other differences, but at the end of the day there is not one part of the nylon string instrument that is common to my steel strings. It's been a great experience working with Aaron to bring his vision to life. Something tells me, this won't be the last dulcimer we collaborate on."
"I thought of a few additional differences between the nylon string and steel string instruments. While the use of the CNC router is valuable tool on the steel strings, I can't imagine trying to produce the nylon string version without it. The accuracy and repeatability of the CNC makes holding the very tight tolerances required possible. Aaron is very sensitive to even minute variations in the ergonomics of the instrument and a few thousandths of an inch can be critical. The CNC router can achieve the accuracy required, but it comes at a price. For example, the fretboard requires four separate setups and tools to produce. One setup to add registration holes, another to add the 10.00 radius, another to put in the position dots and a final one (with a 0.020 diameter bit) to do the fret slots. The mating neck requires individual setups for the carbon fiber slots, putting on the taper angle (with the part rotated 90 degrees) and adding the mounting hole counterbores relative to the taper. The two components are then glued together and put back on the router to cut the outer contour of the assembly. So in all, thats eight individual setups to produce the neck/fretboard assembly and that doesn't include several more for the headstock. Still, the accuracy achieved makes it all worthwhile.
Another process difference between the two types of dulcimers is that on the nylon string instrument, I use a fixture that clamps the top in place so I can hear the instrument prior to gluing it in place (see attached photo). This allows me to carve the bracing away in a slow and measured manner. Carve a little, listen to the result and either stop or carve some more. The removable neck/fretboard makes this possible. It's a slow, but worthwhile process that pretty much assures that the final result will be satisfactory.
As you can see, my goal is not to produce a high volume of instruments. There are many excellent dulcimer makers that can fill that need. My interest is in making a small contribution to the development of this unique instrument and to satisfy the requirements of a dedicated few players who want to push the limits of what can be done with it."
"Since my body still seems to be on China time, I had way too much time to think last night and I thought of another major difference between my nylon string instrument and my steel strings. Both instruments have soundboards that are free to vibrate for their entire length. The method used to transmit the string energy to the soundboard is completely different, however. On the steel strings the the string passes over the bridge and continues on to the tail block. The force of the strings on the saddle is pure compression that passes down to the concealed soundboard as a downward force. There is no torque applied to the soundboard- just a downforce of about 11 pounds. You can think of it as a flat top mandolin from the standpoint of forces and the bracing required to resist them. Simple transverse braces are all that is required. I use a combination of wood and carbon fiber to keep the vibrating mass low.
Since the goal on the nylon string instrument was to pick up some sound characteristics of the guitar family, I chose a bridge system that applies a torque (twisting) to the soundboard. This dictates a much more complicated system of bracing to resist the applied torque. It also provides the opportunity to create varying size vibration zones to generate a uniform sound profile across the full frequency range of the instrument. The bracing I use is an adaptation of an asymmetrical bracing scheme used by a ukulele maker. It seems to be working out well in terms of both it's structural requirements and it's flexibility in providing the desired voice to the dulcimer. I have a lot of options when I start to carve the braces and by testing along the way, can dial in the sound I desire. I guess that's the part where science yields to art, but that's the challenge and reward of instrument making. Since I have done a lot of measuring, recording and adjusting of each soundboard's longitudinal and transverse stiffness along the way, I am pretty confident that the right sound is in there. I just have to let it out."
If you haven't seen Aaron's video introducing the prototype, here it is.
Stay tuned, much more to follow.