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Hi

Recently I was building a pair of 12 fret 000's.
One was Engelman/Padauk, the other Sitka/Bubinga.

Erwin speaks of how building 2 at a time gives you means
for comparison of tone while tap tuning etc. so when I had the
bodies closed and at the edge thinning stage, I noticed that although
both were at a similar frequency when tapped, the Bubinga had almost
zero sustain when tapped in the bridge location. This really puzzled me.

I use one of these vacuum thingies


during the edge thinning and binding
operations. It works on the venturi principle, when you lay the guitar body
on it and throw the switch, it seals to the surface, drawing a vacuum and
clamping the body. While the Padauk guitar, face-up, would clamp perfectly,
nothing I did would make it clamp the Bubinga. I also noticed that CA glue
would easily go right through the Bubinga.

I formulated a theory that this wood was so porous that it would not pull a vacuum
and that furthermore it was the cause of the lack of sustain. I really didn't have much
confidence in it though.
To test the theory I decided to fill the pores on the B&S's with Zpoxy and was surprised
to find I had a similar sustain to the Padauk.

Go figure!

Bob

Are you saying that you vacuum clamp the sides to the mold while tap tuning a plate that is glued to the rims?

If I'm understanding the question correctly then you are half correct. The porosity is the problem in that it's not allowing the vacuum to pull the sides tight to the mold.

But it's not the porosity of the wood that is causing lack of sustain. It's the fact that the sides are not tight in the mold. When tap tuning that way, if the sides are not allowed to vibrate, it kills everything. A spreader bar at the waist (without pore filling) would likely have gotten you the same results.

Good luck.

_________________
http://www.chassonguitars.com

I don't have any thing to add, but who is Erwin? Do you mean Ervin Somogyi?

It's the porous wood that causes the lack of sustain. Can happen on EIR, too. When making falcate braced guitars, the braces run between the bridge pins and I find it easier to drill the holes first then put the braces down. To get a good ring when boxed up and tapped you have to tape over the drilled bridge pin holes. Same porosity issue, slightly different scale.

BTW, if you tap test in a mould you get a different result from a tap test out of a mould. In the mould it's like adding huge side masses. Makes a big difference.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I misunderstood what you were doing. If in fact you are talking about Somogyi, he never mentioned voicing after the box was closed when I studied with him in 2007.

As for a mold adding mass while voicing, that's the point. The focus of his method, as I understand it, is to isolate the top while voicing it. Not saying that's good or bad but that's the idea.

But I'm skeptical that air escaping due to porosity has much effect on sustain of the tap tone. There aren't any typical guitar woods I know of more porous than Wenge and I've never had problems getting great tap sustain on a raw Wenge box. Even if air is escaping, the total area is small and the resistance in a tiny hole is huge, particularly compared to the pressure generated by tapping (as opposed to pulling a vacuum). A sound port or a slightly larger sound hole would allow way more air exchange and I've never seen that have a huge effect on sustain either. I need some convincing on that one.

_________________
http://www.chassonguitars.com

We'll send Luigi over to break a couple of fingers right away....

The Don

_________________
From Nacogdoches...the oldest town in Texas.

http://www.stephenkinnaird.com

Oops! Pardon the error. I did mean Ervin Somogyi.

The only point I was trying to illustrate here was the apparent effect of the porosity of
the Bubinga on the sustain. The clamp is only used as a holding fixture. The body is not
in a mold and the tapping was done while standing the body on it's tail.
As for your skepticism Kent, don't know what else to say.

Two identical boxes, other than material.
One exhibits normal sustain, the other doesn't.
Fill the B&S of #2 and now we have 2 boxes with similar sustain.

Bob

Have a read of this:
http://www.classicalguitardelcamp.com/viewtopic.php?f=11&t=66451#p731561

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Bob,

My experience would tell me that instead of the pores affecting the sustain (which I don't think is the reason) I think for the guitar with no sustain your top and back were at the same frequency and in phase with each other. I use that as an audible tool to know when my top and back are locked in tune when I am tuning the box and it happens exactly as you said, as I lower the frequency of the back the sustain gets better until they meet and then it is instantly gone and I only get a thud. I can imagine that the process of pore filling could raise (or lower, I'm not sure here) it enough to separate them so as to get the sustain back.

I would also be suspect of the CA being able to go directly through your bubinga. I would think that is a problem worse then large pores but I could be wrong of course!

_________________
Burton
http://www.legeytinstruments.com
Brookline, MA.

What Burton is talking about is an example of the most common guitar wolf note. With the 'main top' and 'main back' resonant pitches the same the 'main air' resonance gets really strong, and the energy is extracted from the string or the tap very quickly and turned into sound. You get twice the power for half the time. Since you're not nearly as sensitive to sound level as you are to sustain, all you notice is the lack of sustain.

Routing the binding rabbet around the edge makes a lot of little holes if you use normal kerfed liners. The flow through these holes reduces the pressure change in the box that is the 'main air' resonance, and lots of little holes have lots of drag which adds lots of damping, so the pitch gets spread out.

Obviously the pores in the bubinga are large enough and open enough to allow air flow through and break the suction. It doesn't seem to me as though they'd allow enough flow to kill the main air resonance, but anything can happen.

I tend to thing like Mr. LeGeyt.

Porosity may have somewhat effect, acoustically, but that much on the sustain, I don't think so. If you put a larger soundhole, it will affect the tone, no doubt, but won't affect the sustain THAT much.

I think it is really a matter of frequencies coupling. There's something wrong somewhere in the Trinity (Top frequency, Back frequency and Air frequency).

Maybe if you could get those three frequencies for us, I'm sure someone experimented will target at the problem.

_________________
Francis Richer, Montréal
Les Guitares F&M Guitars

Interesting read, this topic.
It always surprises how much coincidence there is in the world.
It's easy to make false assumptions on seemingly related events, I suppose.

As for the frequencies of the Trinity, have a look at my other posting
"Chladni Advice"
luthiersforum.com/forum/viewtopic.php?f=10101&t=35977
It has a picture of the back in question and details on frequencies.

Thanks for all the replies and opinions!

Bob

The flow through the pores doesn't kill the main air resonance, it just damps it heavily.

I've come across this perhaps half a dozen times. After ignoring it on the first couple of occasions, I did some investigations. The Q of the air resonance increases when the pores are filled and everything gets "livelier". Just as Bob describes. It doesn't matter whether it's a shellac pore fill or an epoxy pore fill (my experience) or a CA pore fill (Bob's experience). All my experiences have been on EIR and particularly on pieces with long grain run-out. With the usually reversing grain in EIR you can get pore path lengths not much longer than thickness of the board. Looking at the figure in the back of Bob's guitar, I see short pore path lengths.

With a main top resonance of 202Hz and a main back ~260Hz as Bob reported, it's not a case of over-coupling between top and back.

Yesterday, I laminated a EIR bridge blank with CF and WEST 105 epoxy. The long grain run-out was ~5 degrees. Under clamping pressure the epoxy was forced right through a 4mm lamination, making it look like it had been coated on the outside. If laminated linings are clamped to EIR sides, Titebond will also extrude right through the EIR pores. Make sure you keep your moulds well waxed!

Convinced yet?

Just had a thought...(dangerous, I know...). I plane most of my wood, so don't have sanding dust packed in the pores. Could explain why I come across this more frequently than others...

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

This would also be a reason why pore filling and laquering affects the tone, it would seem.

I'm making a couple of ziricote/lutz OM's right now

The Ziricote seems fairly heavy and stiff with little to no grain lines visible.

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Old growth, shmold growth!

Trevor wrote:
"The flow through the pores doesn't kill the main air resonance, it just damps it heavily. "

I thought of that after I posted. As far as I can see it's the only way porosity could have much effect. I wonder how this feeds into the old debate about interior finish level and sound?

I've never noticed porosity problems with epoxy laminated veneer back and side sets. And they seem to "ring" fairly well.