I think it’s important to start out by saying that everything that I’m finding and the conclusions that I’m making are all within the parameters of the way I build in every other aspect of the process. I am finding things that do and do not work for me; and they may possibly work for you. But this is just based on the fact that there are far too many factors in the construction of a guitar which contribute to it’s resultant tone to say that what works for you will work for me. Not to mention that fact that we’re working with living, breathing pieces of nature that are inherently going to have their own influence on the outcome. Or, that we may want different things out of the guitar as a whole.
This article is less to say “this is how a guitar back should be braced”, but more to shed some light into the process of the luthier as he/she find’s their sound through experimentation and drawing conclusions- rinse, repeat. My hope is that if you’re a builder, you’re inspired; and if you’re a future or past client- that it illuminates a process that we luthiers go through. One of pain staking effort that has created the sound that suits you as an artist.
Now, to the good stuff.
As stated in more than one post previously, I’ve been fortunate to have a guitar that has stayed with me in the shop that has kind of become the guinea pig for a lot of my ideas and experiments, and has ultimately brought me to this point in my understanding of my guitars. You can read more about the Black Limba OM Project here and be sure to read Part I of the series on Carving Out My Sound.
As you now know, the OM has undergone extensive retrofitting of a new tail block that would allow me better access into the inside of the guitar for changes to be made, as well as a new lattice in the tone bar area. I suspected that I’d gone too far in carving the top in the first go around and in order to sufficiently prove that, the only way was to be able to get inside the guitar and replace the braces that I thought were too flimsy, without really disturbing much else. With the new tail block in place, I made a new recording of the guitar and took note of resonant frequencies as well as sat with the guitar for hours and hours to get a good sense of what it felt like to play it. With a good baseline or reference point, I removed the old lattice and replaced it with a new one at full height of what I would glue into a brand new top. This gave me an incredible opportunity to remove wood from the lattice and listen to how the response and tone of the guitar changed as I did so. But at a certain point, the sound stopped improving with working on the top alone. I had gotten to a point where some pieces of the puzzle were very present, but others were lacking in a big way. This is when I turned back to The Responsive Guitar- Chapter 14 to revisit the theories on how the back of the guitar contributes to the overall tone and presence of the guitar.
The Original Back
This is the original back for the Black Limba OM. As you can see, some sort of 3x ladder brace with some X-esque additional tone bars. Some sort of variation of the Gore back bracing method. The problem with this layout is that for the way I build, this leaves an extremely light back almost completely uncoupled from the rim in the lower bout quadrant, which is going to produce a very active and somewhat low pitched monopole mode. This is what I conclude contributed to the immediate, flappy response, but also possibly the nice lower register and low/mid overtones. The back frequency when tapped in the lower bout area produced 205Hz, and as you can see in this spectrum analysis, there is a nice fat peak right about there. (gray)
You can also see, in a very blatant way, exactly what I thought I was hearing.
What I was lacking
What the guitar lacked was an ability to produce a pleasing treble, accompanied with a punchy tone that happened instantaneously with little sustain of the upper registers, and a lack of volume. The bass was deep. The low overtones were haunting and beautiful. But what it lacked was a back that was working alongside the beautiful work the top was doing. My instinct is that the back is acting more like a catcher’s mitt than a participatory partner- absorbing the energy coming into it, rather than being driven by it. An interesting note is that while some volume was lost when the back was damped out against my body, the tone seemed to change very little. This was just one more indication that the back was not doing me any favors.
It should be mentioned that much of what I’m learning now deserves a big thanks to Michael Watts for his beautiful compositions. I learned to play two songs from his album Vetiver, specifically for the purposes of helping me to evaluate the OM, and it has helped immensely. La Tolita will be a familiar tune before we’re done with this OM project if you follow along ;).
The Responsive Guitar describes the back as being like a flywheel. It takes the energy put in from the top and it carries. It is heavier than the top(usually), and also higher pitched so therefore should inherently sustain longer once it is excited(again, providing that it is not so heavy and tight that it is functionally inert), adding richness and tone colour. And reading this for probably the 10th time now, just last week, was a revelation to me, as I’d finally heard and could put a sound to the descriptions of an ‘absorptive’ back vs. one that is either reflective or one that acts as a diffuser. Given my choice, I choose diffuser. One that contributes to the tone coloration, the breath between notes and sustain to help to fill out the sound for a solo performer. The other end of this would be the reflector, which should take the incoming energy from the top and blast it back out of the sound hole. For the OM, I don’t necessarily care if it doesn’t project like a Dreadnaught. A richness of sound is far more important to me in the voicing of this model.
And so work has begun to replace the back on the OM. It is braced and ready to begin voicing work. Stay tuned for the next update and more on why I decided to go with this skewed ladder layout.
Thanks for reading and feel free to drop me a line with your thoughts or comments.