Official Luthiers Forum!

In a thread over on the AGF, Matt Mustapick posed a very good question
regarding this phenomenon:

After a guitar has been bound, there is often a very noticeable increase in
the volume and resonance of the guitar body. There is a difference even
before the binding ledge has been routed. This was my explanation,
which didn't seem to jive with what Matt has experienced. So I am
wondering if anyone has some insight into the matter.

Simply put, my explanation is that the binding acts to "seal" the guitar
body. And that even before you route the binding ledge, you still have
various airpockets around the perimeter of the guitar - headblock, brace
ends, and heelblock. And then when you route the binding ledge, you
often will have quite a few more gaps in the kerfing - depending on how
thick your binding is. I have noticed that once the binding ledge has
been routed, that the guitar body has much less resonance compared to
the bound body.

I have also noticed that since I've switched to double sides, that my
binding ledge never goes thru to the kerfing and that this operation
doesn't decrease the resonance of my soundboxes as much as it once did
when I used thinner sides and had gaps a inbetween the "kerfing blocks".

I am just wondering if that explanation is correct, partially correct, or
entirely off the mark. Also, how does a soundport affect the resonance of
a guitar body - because this seems to conflict slightly with my
explanation. Unless, the "position" and "size" of the soundport has a big
effect - which I am thinking is the case.


So fire away if you have any good ideas on the subject.


Thanks,
Simon

I'm not sure the kerfs alone are the culprit, but in any case I have to agree to the importance of binding in the system. 

I have actually strung up a guitar with the ledge cut but no binding and it sounded quite poor.  After I've glued one half, it sounded better, and better again when it was complete.  The ledge was quite narrow, it was not exposing kerfing gaps.

The resonance of various bits within a guitar is given by their stiffness, weight, and damping. AND the effect of what is glued to them. A soundport can't have much effect on the stiffness and weight of a side, less so on the more important top and back, but it affects the air inside the guitar.

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The most recent jumbo I finished (H'burg attendees may have seen the one with the large rope purfling) was bound with carbon fiber. I laminated three .021" x .250" strips around the guitar with superglue, and I also lined the reverse kerfing with one strip of the .021" x .250" CF. The reverse kerfing it itself glued onto a side doubler that is about 1 1/4" wide laminated to the sides all the way around. This has made for an incredibly stiff edge/rim around the top of the guitar.   I think it's helped the already good sustain and also enhances the headroom capacity of the guitar. It's a loud sucker and the volume is only limited by how high I can stand to have the action.

I've measured the resonances on a few guitars at different stages; closed box, binding rabbet routed, and binding on. On my guitars the closed box is pretty well closed; there aren't any extraneous holes to speak of.

Usually what happens is that the 'main air' mode is totally messed up by the routing. The pitch rises and it becomes very weak, owing, no doubt, to all the leakage. The 'main top' resonant mode drops in pitch by a fair amount, although I can't tell you off the top of my head how much. It's probably something like a few semitones. The higher order top modes also drop a little, but less.

When I glue on the bindings the air mode pretty well goes back to where it was with the box freshly closed. The top modes also recover somewhat, but are not quite as high in pitch as they were before the binding rabbets were cut.

Part of the drop in top pitch with the rabbets cut is, no doubt, from the loss of stiffness around the edges. However, part of it is also from the weakness of the 'main air' mode. Remember, the 'main top' pitch is the result of a couple between the air and top, which pushes the pitch of the 'real' Helmholtz mode down and the 'real main top' mode up to the values that we see on the completed guitar. With less air pressure change in the box, the rise in the top mode pitch is lessened,

Putting the bindings on certainly seems to get the air mode back to it's old robust self. It also probably adds some stiffness to the edges, but it's hard to say how much.

The bottom line, then, is that routing for the binding has some acoustic effect, but it's going to be hard to sort it all out.    

Great info, thanks for sharing...

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It's interesting that on the guitar I work on now, the binding did not change the main top mode. With the ledge cut, the half tones above and below the main mode were very strong and was hard to decide that the center one is the strongest.  Bindings made the outer peaks weaker, or maybe enhanced the center tone.  

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