Acoustic bass, elliptical shape

Here is where you put your entries.
User avatar
Bob Gramann
Posts: 1101
Joined: Fri Jan 06, 2012 11:08 am
Location: Fredericksburg, VA
Contact:

Re: Acoustic bass, elliptical shape

Post by Bob Gramann »

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.
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

Thanks, Bob. I guess I've got to think this through some more.
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

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?
Attachments
bracing1.jpg
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

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.
Attachments
topbraces1.jpg
topbraces2.jpg
topbraces3.jpg
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

The back.
Attachments
Back braces
Back braces
The outside of the back.
The outside of the back.
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

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.
Attachments
rough-mock-up.jpg
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

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.
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

Glued on the back braces.
Attachments
back-brace-glue.jpg
back-brace-glued.jpg
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

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'.
Attachments
bridge-plate-glue.jpg
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

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.
Attachments
neck_truss-rod.jpg
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Hans Bezemer
Posts: 373
Joined: Sat May 26, 2012 1:01 am
Location: The Netherlands
Contact:

Re: Acoustic bass, elliptical shape

Post by Hans Bezemer »

Peter,

How is this bass coming along?
I came to think of it, when you replied on my thread.
User avatar
Peter Wilcox
Posts: 1317
Joined: Sun Jan 08, 2012 1:31 am
Location: Northeastern California

Re: Acoustic bass, elliptical shape

Post by Peter Wilcox »

Hans Bezemer wrote:Peter,

How is this bass coming along?
I came to think of it, when you replied on my thread.
Hey Hans - I just saw your post here. I finished it over a year ago - here is the thread: http://www.mimf.com/phpbb/viewtopic.php?f=3&t=1830
Maybe I can't fix it, but I can fix it so no one can fix it
User avatar
Hans Bezemer
Posts: 373
Joined: Sat May 26, 2012 1:01 am
Location: The Netherlands
Contact:

Re: Acoustic bass, elliptical shape

Post by Hans Bezemer »

Thanks Peter. I've missed that thread: I like the outcome!
Post Reply

Return to “The Local Materials Challenge (Closed)”