Smaller archtop bridge placement

[Christ Kacoyannakis]

I'd like to build some smaller archtops - 15" and 14" lower bouts, and am planning on using a 25" scale (at least for the 15"). I have seen some with the normal neck connection at the 14th fret, and some with a connection at the 15th fret. When guitars mostly had the neck join the body at the 12th fret, the bridge was closer to the widest part of the lower bout, but when they decided to join the neck at the 14th fret, they just moved the bridge up.

My question is, would there be some benefit to designing the guitar so that the bridge is at the widest part of the lower bout? If I do leave it at the further up location now used, how is that calculated (divide the length of the top into 3?), because the smaller bodies could have the neck join the body at the 14th or the 15th fret, or at some other location. Is there an ideal spot for the bridge on the body? Thanks all!

[Alan Carruth]

"My question is, would there be some benefit to designing the guitar so that the bridge is at the widest part of the lower bout?"
Briefly, no.

Even on flat top instruments there's really no need to get the bridge at the widest part of the bout. Even banjos aren't set up like that. Just as with a baseball bat, there is a 'sweet spot', and it's not always at the widest part of the bat. In an instrument I think of this as the 'acoustic center' of the top, the place where the strings are most effective at moving it, and it's more an area than a specific point. That area is established by the way the top is built. The shape contributes, of course, but you have to also take into account the way the mass and stiffness are distributed in the top. In a flat top this is pretty much determined by the brace setup. On an archtop you also have top thickness and arching.

The great exemplar of archtop construction is the violin, and the bridge is certainly not at the widest point of the lower bout there. In some part this has to do with bow access; The bridge has to be somewhere a bit below the waist to allow for free use of the outer
strings. For structural reasons the bridge on an arched plate should be at the highest point of the arch, and it would be awkward to move that to the wide part of the lower bout. It would be hard to produce an arch that would work structurally and acoustically and look reasonably good.

[Christ Kacoyannakis]

Thank you Alan for your response!

[Alan Carruth]

If you're interested there's an Android app called 'Luthier Lab' written by a friend of mine, you might find helpful. You can draw out a shape for a guitar with it, and then open that shape in another module, which will calculate 'curtate cycloid' cross arches, and print out a full sized contour map if you have access to a printer that can work with that size. There's lots more in the app, and, best of all, it's free for download.

[Christ Kacoyannakis]

Thanks Alan. Do you know if will calculate volume, so I can try to calculate the correct Helmholtz opening size, or is there a better way to do that?

[Barry Daniels]

I don't think you should do that. F-holes are never sized to the Helmholtz frequency, as far as I know.

[Christ Kacoyannakis]

Barry, thanks for that info. I was not aware of that. However, I am trying to do something different. I want to try an archtop with an oval sound hole in the upper left of the soundboard (so bass side upper corner) along with a side port. Obviously, there are no plans for such a guitar, and I am at a loss (other than pure experimentation) on how large or small to make the sound hole or side port. I am looking for somewhere to start, size-size, and looking for ideas.

[Alan Carruth]

Trying to calculate the 'main air' (Helmholtz' resonant pitch in advance is usually futile. The 'air'' mode couples with the motion of the top and back and all of the pitches get shifted depending on a range of factors.

Moving the sound hole up into the corner like that is usually done to drop the 'main air' resonant pitch. Adding in a side port will raise it. If you want to make the 'air' pitch low then make the hole and the port small, and plan to enlarge them a bit if you want a particular 'air' pitch. I'd use a normal size hole in the top, perhaps 3-1/2" diameter, and a small (1" or so) port in the wide part of the bout, so you can see into it.

I've made a few round hole archtops, with the holes in the 'normal' place, and find they work well. The timbre is closer to a flat top than the usual arch top, in particular due to the lower pitched 'air' mode. I also think it's a better place structurally.

[Christ Kacoyannakis]

Thanks Alan! That is exactly what I am aiming for - a sound more like a flat top than the woody muted sound of most archtops.