MIMF

by **Bob Gramann** » Wed May 02, 2012 9:22 am

Having the strings half an inch or slightly more over the body top works for me. Much more would load the top just where I want it to move. We tested it with an FFT graph. On the low E string, the fundamental was very weak but the second harmonic was huge, We compared it to a string bass. The second harmonic was larger than the fundamental on the string bass but the fundamental was much larger and the harmonic much smaller than on on the guitar. The total power seemed to be about the same.

I don't have any experience with the A bracing. I designed mine to be firm around the bridge but loose at the edges to get as large a moving piston as I could. The X-brace seems to be a very good design to do that and does a good job of supporting the string tension.

by **Peter Wilcox** » Wed May 02, 2012 10:21 am

Thanks, Bob. I guess I've got to think this through some more.

by **Peter Wilcox** » Fri May 04, 2012 1:44 pm

Since this is a completely experimental instrument, I've come up with a bracing plan to hopefully let the top move to increase low frequency volume. The long vertical braces will have a long notch cut into them, spanning the lower half of the top, and glued for an inch or two at the end. Crossing underneath this will be a thin (~3/16"), wide (~3/4") transverse brace from one side to the other, reinforcing across the top grain (which seems quite weak in the hemlock), but flexible enough for movement in and out.

Any comments? Might this work, or are there cogent reasons it will probably be a disaster?

by **Peter Wilcox** » Mon May 07, 2012 9:16 pm

I cut out the top and back, and rough cut the braces for them. They're just laid out for fit. The back grafts will be continuous between the braces as usual.

by **Peter Wilcox** » Mon May 07, 2012 9:20 pm

The back.

by **Peter Wilcox** » Mon May 07, 2012 9:24 pm

A mock up to see the size of it - it's more ginormous than I thought (that's a 12" square.) It's 58" long. I was hoping to be able to play it slung on my shoulders, but that may not be possible.

by **Peter Wilcox** » Mon May 07, 2012 10:04 pm

If the tailblock and heelblock make a right angle with the top, then the back has to be slightly longer than the top, which is 30". By using either 5/{sin[arctan (5/30)]} or the Pythagorean theorum, it comes out to 30.414". I suppose the extra length should be stretched out over the whole back, but I just added it to the straight parts in the middle, keeping the upper and lower curves the same.

I don't know how to calculate the widths of the sides at different points, especially near where they meet the end blocks, so when the time comes I'll temporarily attach the top and back together with the head and heel blocks, and draw out a template.

by **Peter Wilcox** » Fri May 18, 2012 8:35 pm

Glued on the back braces.

by **Peter Wilcox** » Fri May 18, 2012 8:39 pm

Cut the sound hole, added a plywood brace around it, glued on the top braces and the bridge plate. The top has a radius of 30', the back 15'.

by **Peter Wilcox** » Fri May 18, 2012 8:43 pm

Made a truss rod, cut the slot and made a spline.

This is it for me in the challenge - got to go help some relatives move this weekend, but will be back in time for the eclipse - the midline passes within a mile of my house.

Good luck to all of you in the challenge - you're doing some beautiful work on some beautiful instruments.