Craig Bumgarner wrote:........ You might also take a look at my CB #03, last in the line. This guitar is one of my earliest builds. It was okay at the time and is probably better than a 3, but I wanted to put it at the bottom of the list because of the oddly high mobility number. It was so high that I redid the measurements, but this seemed okay. What you will notice is the particularly high deflection. That is quite so and is likely because the cedar top on this guitar is much thinner than most and this deflection is what is driving the high mobility number. Problem is, it doesn't sound like it. It just average in response, bass is strong as one would expect with the thin top, but FRCs are in the middle with all the rest. I'm perplexed. Is it possible that there is an upper limit to MM where going over gives no improvement?
Just to close the loop on this..... I have this CB#3 guitar back in my shop today. It is being sold and the buyer wanted me to look it over before fully committing to it. I redid the deflection and the top FRCs. The FRCs are the same as was tested last summer, but the deflection is half
of what I measured earlier.
My guess is I didn't have the load squarely seated on the bridge. Selmer guitars have archtop like bridges with very narrow edges where the strings go across. Unlike a fixed bridge, there is nowhere else to take the deflection measurement. I set a narrow block of plex across the strings, on the edge to land my dial indicator shaft. My guess is the block was off center and the deflection load was partially being taken by the strings. My bad! I need to cook up a little adapter that will self center on the edge.
I have been "calibrating" my thumb pressure on guitar tops and the first thing I did when I took this one out of the case was to push down on the top. .18mm deflection? No way!
The .09mm deflection measured today yields a mobility
of 13.1, much more in keeping with the way this guitar sounds. This confirms its position at the bottom of my list posted earlier, both in subjective assessment and in mobility
. Glad to have that little discrepancy cleared up.
I had mentioned the cedar top on this guitar is thin, very thin, 2.0mm and even a bit less in places which is why I initially thought the deflection number seemed reasonable, but in hindsight, it has a overbuilt bracing structure so the assembly is still overly stiff and heavier than need be.