I was working hard and fast to get a guitar done before a show this last weekend. I got the strings on this guitar on Thursday for a show on Saturday. Just in time!
I was tuning it up to pitch, and heard some truly awful sounds come out of this beautiful guitar on the open A string. It created a resonant zithery wowowowowowowow noise that sounded absolutely dreadful. Humming into the soundhole showed that the main body resonance was directly on the open A pitch. Dismay set in. This is the first guitar I've made with a sound port, and the added aspiration from this port moved the resonance up right onto an open string.
I remembered this recent thread regarding a resonance on G and reviewed all of Al's suggestions there:
I just completed finishing on the guitar, so modifying the shape of the ports was undesired because I would have to refinish with only another couple of days until the show. So I scrolled down to the suggestion of loosening up the back braces. My back has 4 braces. The upper ones are about 1/2" tall in the center and the lower ones are pretty tall at 5/8", so there is no structural risk in removing material from them. I created a stack of about a dozen layers of blue masking tape 4" long and stuck this thick stack of tape to the third brace down (one of the 5/8" tall ones) and it moved the pitch down to just below the resonant A. From Alan... added mass is equivalent to lower stiffness so I know what brace to shave down.
I loosened the strings and took out my tiny brace plane and lowered that back brace down a bit. I have huge hands, but I was still able to plane this brace down a bit under control. Tightened the A string back up to pitch, and the resonance was reduced. Then lowered the pitch of the string a bit until I heard the awful warble. The warble was still a bit closer to the A than the G# pitch, so I planed a bit more off that brace until the resonant pitch was exactly halfway between the G# and A.
The guitar hasn't sold yet, but a couple of folks I met at this show are very interested. Thank you Alan for providing the great info to quickly salvage this instrument at the last minute!
I'll post up some pictures of the guitar to this thread soon...