A while back I posted pics of a top that I just couldnt close thr ring+ mode on.
http://luthiersforum.com/forum/viewtopic.php?f=10101&t=39676
The lower arms of both x braces just kept getting smaller and smaller to the point where I knew the top was done.
So I went back to the drawing board on it. I scraped all the braces off and started over. I also discovered and refined my understanding of chladni tuning a top along the way.
My theory is this; when tuning a top using chladni patterns, the top reaches its optimum resonance and efficiency when the stiffness of the top is equal both with and across the grain.
When that happens, the ring plus mode will close.
When I originally braced this top, I wanted to use a double x brace. I knew that the ring+ mode would close at the bottom if I removed all material from the other x legs below where the line of intersection with the edge of the lower bout with the main x legs. This was a trick I picked up from Mark Blanchard in a post.
So I tightened the X up to pish that intersection line as low on the body as possible so the other x legs could be as long as possible and offer more structure and stability to the top. It seemed like a good idea at the time.
But what I did was take a piece of cedar that was already stiff along the grain, and reinforce that stiffness with the x brace. There was no possible way that chladni mode was ever going to close.
With that revelation in mind, i re-braced the top.
I think I am on the right track because this is what I found with the first test.
The ring was closed without any tuning. I was shooting for 250hz and that is exactly where the mode was found.
I wasn't real big on all that unbraced real estate below the x braces, but I was reminded by Al Carruth in his video that there isn't much that needs bracing from a strength and structure standpoint down there anyhow, so I am going for it.
Hope my nightmare and lesson helps others on the journey.
Dave