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So as a critical step towards my own development I plan to build a "Frankenstein" guitar this summer through which I can experiment with different tops, bracing patterns etc. I'm look for some ideas from the rest of you as to how you would approach this in order to maximize one's ability to learn, develop and discover. I don't believe we have had a thread like this emerge for a while so it might be worthwhile for a lot of us to share our best ideas and practices

Here is my plan:
-simple walnut b/s
-using b grade tops
-I won't finish any elements of the guitar
-I won't bind the guitar but will rather leave 1/8" of the top proud of the sides, giving better ability to remove one top and attach another
-will use HHG
-will use placement pins at neck and tail block in order to line up tops and switch them in and out.
-Fingerboard extension will be a bolt down rather than glue down
-thinking of a variety of bracing patterns - tapered, scalloped, bracing extending to lining and falling short of lining, height of bracing, etc.

So, anyone have pics? Various bracing patterns to experiment with? Access port to adjust bracing in-situ? How to best measure various results?

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Doug

Don't let fear or common sense stop you from trying to build something

A key challenge will be controlling the variability in the tops themselves. I'd suggest using Trevor's techniques for that. Also brace wood properties.

B grade top cosmetics may be ok, but you'll need very tight control of them being well-quartered. So that will likely mean AA or better. Also buy direct from a resawer to get them from the same tree section, billet, and even sequential slices (even that is problematic, as on-quarter will vary, but at least it's similar wood density, damping, etc).

Other than the soundhole, another logical access port is the sides of the lower bout, near the tailblock.

I'd measure results first by ear (preferably at least 1 other player and critical listener than yourself), then by tap testing to find modal frequencies ('main air', 'main top', 'main back'...). The reverse order is very likely to produce bias.

Another difficult part will be 'design of experiment' issues and interpreting results. For example: Is bracing A better than B because it is 'inherently' better, or because its material props and amount of shaving happen to produce the modal frequencies of good sounding guitars? One answer to that is to shave each bracing pattern so they all produce the same 'main air' and 'main top', then compare tonality. But what if bracing A has its best tone at somewhat different modal frequencies than bracing B? There are also issues of replication, randomness, inter-rater reliability, and ear fatigue / time of day (for example, Andy Powers says he only listens critically for the first hour in the morning). The key point is that it's easy to draw the wrong conclusion when the experiment isn't sophisticated enough. That doesn't mean you won't develop a better sounding guitar, you probably will, but *why* it's better is a harder nut to crack.

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David Malicky

Joe Curtin uses CA to assemble experimental instruments. CA was made to stick to protein, and sticks about as well to hide glue as it does to fingers. He sizes all the glue surfaces, both sides, with hide glue, allows it to dry, and smooths off the surface fuzz, leaving a good layer of glue (I'd repeat if needed to ensure a continuous glue surface). Then he sticks the thing together with thin CA. This allows for quick assembly, and it's easy to get things apart with the knife in the usual way. When you've got it to where you want it you just use warm water to remove the hide glue, and the CA with it, and then glue it up in the normal way with hide glue. I'd try to avoid actually gluing brace ends into their pockets: if they're properly fitted they should not buzz and should have the correct stiffness, and it will be a lot easier to get the plates loose if the brace ends are not glued. You could use this process to swap out bracing on the same top, controlling a variable.

I make a lot of acoustic measurements. The most useful is the 'impulse' measure, but it can be a bit tricky to get it to be really repeatable in strength. I have a plastic bead, about 15mm diameter, that swings freely on a string, that I can use to tap the saddle. It's best at showing changes in frequencies of resonant modes, and does do pretty well at showing the relative strengths of peaks. A more repeatable one for levels is the 'step' test, which uses a wire break to activate the guitar. Loop a length of fine magnet wire (say, #44, from an old pickup) under a string behind the saddle, and pull up on it until the wire breaks. This gives a force that's repeatable within a dB or less. You can use the same technique to 'pluck' a string in a known location and direction. In theory, the 'impulse' test puts in equal amounts of energy at all frequencies up to a limit that's set by the contact time between the 'hammer' and the 'anvil': a light hard ball gives useful of energy up to 6-8 kHz. The 'step' test concentrates most of the energy in the low frequency band. Whatever you do, make sure the whole test protocol is repeatable. Small changes in things like the mic position can make larger changes in the output than you might think.

Chladni tests of the assembled guitar can show you what the wood is doing. I rigged up a small electret mic on the stick that I can put inside the box to read internal air resonances. In practice, of course, everything is coupled, and 'wood' and 'air' are always negotiating what they're going to do. Be prepared to be constantly confused for a while; say the next 15-20 years. After that you might only be slightly confused from time to time.

Any player/listener tests MUST be 'blind' to have any validity at all.

I have done some similar testing. I used newspaper and glued it to the rim and blocks. Then I glued the top and back to that. This made it pretty easy to remove the top and back. I would be careful about leaving the 1/8 around the perimeter. I have noticed a difference in tap between a trimmed and untrimmed top. O and the newspaper idea I believe came from a mandolin builder somewhere that was kind enough to post it on some forum somewhere. For my testing, I used the same side set for everything so I used less than perfect wood for that. As far as the neck goes, I did a butt joint bolt on neck and I didn't glue down the extension. This made swapping the neck simple. I did find that one of the tops I was really happy with eventually bellied pretty bad in a month or so. When carving and shaping them braces think about the longevity vs tone. Sorry if this is jumbled.

As I understand it, and even LMI says it on their grading page, is that tops are graded on cosmetics and not structure. Supposedly all of them are well quartered but who knows...

I've bought second grade tops from Stewmac and A grade tops from others, and don't really see the difference honestly, the builder makes the biggest difference. Naturally you want something free of runout and well quartered...

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Cat-gut strings are made from kitten guts, stretched out to near breaking point and then hardened with grue saliva. As a result these give a feeling of Pain and anguish whenever played, and often end up playing themselves backwards as part of satanic rituals.

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Interesting thread. I will be following. As a beginner builder all my guitars will be Frankenstein probably.


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I will also be interested in the results of this test. Being a new builder also it will be interesting to see how the different components effect the outcome.

As others have said I think you will have to have tight control over materials and processes along the way. I have a feeling that that will be the hardest part of doing something like this since it would seem even the slightest changes will have an impact on the sound.

I hope you keep us up to date of your progress and results.

Good luck and I look forward to watching..

Cheers,
Bob

Sorry double post... Never had that happen before...

Bob

My first two acoustic/electrics were frankenstiens that I built to learn how to make acoustics and to key in a particular sound which did work the second time around. They were a lot of fun. The second time around I experimented with creating a cutout on the bottom bout of the guitar to which a few people warned me about doing but it turned out to be a great sounding cool instrument. I loved it so much I had to build more. I don't much mess with my bracing on my tops or backs but I do want to get into Falcate bracing soon. Try different materials for top wood. I myself use curly maple, it looks good and has a nice tone to it. I'm still learning myself and will leave the technicals to the pros but I'd say be creative, don't be afraid to go unconventional and most of all have fun. I'm gonna keep this marked so I can follow.

_________________
In order to make an apple pie from scratch you must first invent the universe.
- Carl Sagan

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_________________
In order to make an apple pie from scratch you must first invent the universe.
- Carl Sagan

Check out F.O.F. Custom guitars on Facebook.

Www.fofcustomguitar.weebly.com

rusrob wrote:
"...it would seem even the slightest changes will have an impact on the sound."

Amen. I once made a 'matched pair' of Classicals where everything was controlled as tightly as I knew how at the time. All the parts were cut 'in flitch': redwood tops and mahogany B&S. All weights were controlled within a gram or two for plates, and the bridges were 'exact' within the limits of my ability to measure. Dimensions and resonant mode frequencies were matched as well as I could. They ended up sounding a little different: not much, but everybody preferred the same one by a little bit. On reviewing the data, I noticed that the 'free' top mode shapes were a little different, and attributed that to small variations in the grain direction in the fan bracing, which was sawn out rather than split. Next time....

I'm thinking about using one top. First try tapered braces, then remove and scallop, then remove and terminate bracing short of linings, etc. This should eliminate variance introduced by the top. Thoughts?

_________________
Doug

Don't let fear or common sense stop you from trying to build something

As with any valid experiment you need a control from which to start your data and then to add variables to a different piece to recieve different outcomes if possible or to produce a desired effect. So my question for you is in two parts.

1st what are using as a control?

2nd what is the eventual desired effect you wish to produce through experimentation?

_________________
In order to make an apple pie from scratch you must first invent the universe.
- Carl Sagan

Check out F.O.F. Custom guitars on Facebook.

Www.fofcustomguitar.weebly.com

Doug,
As has been stated so well by others above, there are infinite variables that affect the sound of a guitar:

Back braces:
Stiffness of the brace wood
Profile of the braces.
Shape of the braces.
Pattern of the braces.
Tucking vs. not tucking braces
Top Braces:
Stiffness of the brace wood
Profile of the braces.
Shape of the braces.
Pattern of the braces.
Tucking vs. not tucking the braces
Top:
Stiffness of the top.
Type of wood
Thickness of the top
Grain orientation of the wood
Back:
Stiffness of the back.
Type of wood
Thickness of the back
Grain orientation of the wood
Sides:
etc.
Neck:
etc.
Bridge
etc.
And on and on....

Each variable must be considered in connection with any change in each of the other variables. It is like looking for a needle, in one of the hay stacks, in several different fields full of hay stacks.

A lot of people build great guitars, but have not even scratched the surface of knowing all the answers on why they sound good. They have found enough of the answers to make significant improvement in the sound they are looking for.

I think removing the top and replacing it would change too many of the variables at once to nail down what is changing the sound. Your best bet, on an experimental model, may be to build one with a port that allows access through the back or side, to tweek the bracing profile and shape. You could screw or glue a temporary wood patch over the port while playing. Even this patch will add another variable to the equation, but this would help you shift through a small part of one of the hay stacks, in one of the fields.

James

How does finish (or not) affect the parameters that we evaluate (sound/tone, frequencies, modes)? Is a "white" guitar significantly different than finished?

A 'white' guitar can be significantly different from a finished one. Finish adds mass, and often alters damping. Adding mass cuts down on output at all frequencies, but generally more in the treble. Adding mass to the plates will also alter resonances. Since the top is the lighter plate, it's generally affected more by a given thickness of finish, with the result that the relationships between the two are altered. Some finishes, such as shellac and nitrocellulose, may have lower damping that the wood, while others (anything containing oil) will raise the damping. Damping usually cuts highs more than lows, and also reduces sustain. One reason French polish is so popular is that it adds the least mass because it goes on so thin, and the least damping.

The best situation involves making a change on a guitar that can be done quickly and quietly. For example, if you can open and close a port silently in a couple of seconds you can do blindfold A/B tests that will be pretty valid. Of course, altering the port configuration in itself causes a whole slew of changes to resonant pitches and strengths, and it might be hard to sort out which of the changes is the important one.

For something like 'spruce vs cedar' I can see no alternative but to go the statistical route, and that's probably out of the question for us. From what I can tell, you'd need to make around a hundred of each, under as good a level of control as you could muster, to be able to say anything valid. Not likely to happen. It's said that one of the talents of a good scientist is in distinguishing things that are testable from those that are not.