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Acoustic bass, elliptical shape

PostPosted: Mon Apr 23, 2012 9:12 pm
by Peter Wilcox
I meant to enter this challenge at the beginning, but early spring, garden, yard work and indolence have gotten in the way, and will continue to do so. So I will not finish this anywhere near in time, but for me the challenge is really just to get it started. Also, hopefully others here will be able to point out where I am going wrong with this and give constructive criticism in the little time that is left.

I never met an "acoustic bass" I liked, so I'm going to try to build one. They all sound very thin in the lower register, and I'm guessing it's likely due to inadequate air volume in the soundbox. A guitarron has a much better low end, though its lowest note is only A 55Hz. I estimate its volume at somewhat less than 2 cubic feet. A 3/4 double bass sounds a lot better, but with a larger box of around 4 cubic feet (my guess.) I'm going to try to make one of about 2 cu ft, and hopefully light enough and of a configuration that it can be played hanging from my shoulders somewhat in the manner of a guitar, though with a more vertical orientation. I haven't decided yet if it will have frets or not. The body shape is an ellipse - a compromise between a rectangular box which maximises volume but is ugly, and a pinched waist which is lovely but loses volume. It gets larger toward the tail to try to add more air volume.

Here's a general plan of the shape and dimensions, though the width and depth will have to be slightly smaller as my wood is not quite wide enough. The construction techniques will generally follow that of a steel string guitar, though I've never built one - :lol: .

Re: Acoustic bass, elliptical shape

PostPosted: Mon Apr 23, 2012 9:16 pm
by Peter Wilcox
The woods will be vertical grain hemlock for a 4 piece top, myrtle for the back and sides, and red alder for the neck. These are all found near northern California where I live.

Re: Acoustic bass, elliptical shape

PostPosted: Mon Apr 23, 2012 9:46 pm
by Jim McConkey
Except for the radically tapered back, it looks like a mandobass. You might be able to find some info on building those if you search.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:19 pm
by Peter Wilcox
Thanks, Jim - I checked out some mandobass threads in a few fora - people are not impressed with their sound, so I'll go a different direction - a fixed pinned bridge for one.

Here's the top hemlock resawn.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:21 pm
by Peter Wilcox
The grain is not completely vertical, but close enough for me.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:22 pm
by Peter Wilcox
Gluing the top.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:23 pm
by Peter Wilcox
A closer view.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:25 pm
by Peter Wilcox
Glued. It's 21 1/2 inches wide, so plenty big.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:32 pm
by Peter Wilcox
The back wood - myrtle. Unfortunately it's only 18" wide, so I'll have to add some wings.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:35 pm
by Peter Wilcox
The sides from the same myrtle. It's 10" wide at the near end and 9" at the distal end, so it's plenty wide enough - hopefully the straighter grained portion is long enough.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:42 pm
by Peter Wilcox
The neck is from alder. Here the scarf joint is being glued.

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:43 pm
by Peter Wilcox
Next, the stacked heel being glued.

While jointing a board to use as a caul during one of these glue-ups, I failed to note a staple in the board. So as an added benefit to this challenge, I was able to learn how to sharpen and set up jointer blades. :lol:

Re: Acoustic bass, elliptical shape

PostPosted: Sat Apr 28, 2012 7:45 pm
by Peter Wilcox
The rough neck.

Re: Acoustic bass, elliptical shape

PostPosted: Sun Apr 29, 2012 3:44 pm
by Steven Wilson
Wow. That's the most elaborate clamping system I've seen for a top. Looks good.
Steven

Re: Acoustic bass, elliptical shape

PostPosted: Sun Apr 29, 2012 4:19 pm
by Peter Wilcox
Steven Wilson wrote:Wow. That's the most elaborate clamping system I've seen for a top. Looks good.
Steven

:) Yes, I know it was huge overkill, but since it is a four piece top, and it's 31" X 21", and only 1/8" thick, I wanted to make sure it stayed flat against the bench, and that it wouldn't buckle.

Re: Acoustic bass, elliptical shape

PostPosted: Tue May 01, 2012 4:34 pm
by Peter Wilcox
I've decided to change the shape to try to get a little more air mass inside. I need to figure out the top bracing. I found this on Liutaio Mottola's site (its simplicity is very engaging):

Image

or I could go with a more conventional but robust X bracing:

Image

Any thoughts on the drawbacks of the A type bracing, or ways to improve it?

Re: Acoustic bass, elliptical shape

PostPosted: Tue May 01, 2012 5:19 pm
by Bob Gramann
Here's the top on the fretless acoustic bass that I made. It has a 17" lower bout and a 34" scale. Note that the bracing isn't nearly as beefy as what you propose. This bass works very well. I built it to accentuate the second harmonic since there's no way to get a strong fundamental on the low E string.

Re: Acoustic bass, elliptical shape

PostPosted: Tue May 01, 2012 8:56 pm
by Peter Wilcox
Thanks, Bob, that's a nice looking top. How do you brace it to accentuate any particular frequency?

Re: Acoustic bass, elliptical shape

PostPosted: Tue May 01, 2012 9:12 pm
by Bob Gramann
Getting the second harmonic strong involved two things: making the bridge/saddle combination tall enough that bridge rocked and torqued the top twice for every cycle of the string and making sure that the bridge was above the center of the lower bout so that when the bridge rocked the moving part above the bridge didn't cancel the moving part below. The bracing was done such that the top could move as much as possible but still support the string tension.

Re: Acoustic bass, elliptical shape

PostPosted: Wed May 02, 2012 1:49 am
by Peter Wilcox
How do you know how tall to make the saddle? Trial and error after the instrument is finished? How do you test it - by listening or by spectrogram?

Also, it looks like the A type bracing (as opposed to X bracing) that I drew on my plan above may dampen the top movement too much, though may be physically stronger to counteract string tension. ??