New Archtop

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Re: New Archtop

Postby Alan Carruth » Wed May 23, 2018 12:54 pm

Acoustically there's not as much difference between arch tops and flat tops as you might think. The biggest one is related to the sound hole(s). Getting the arching and thickness of the plates in their proper relationship is also a factor, but in that respect it's just like getting the top thickness and bracing on a flat top balanced correctly, IMO. The string driving force at the bridge is very much a secondary consideration IMO.

In general, the holes on an arch top are effectively larger than the hole on a flat top. On my last one I used very narrow holes, much like the old Gibson I was basing things on. From the length and width of the holes they had a total area equivalent to about a 1-1/2"-2" round hole. The placement of the holes also makes a difference; they're closer to the center of the top area, which raises the pitch of the Helmholtz resonant mode for a given hole area. It ended up with a Helmholtz-type 'air' resonance at 115 Hz, just above the open A string. That would be on the high side for a flat top, although it's lower than most arch tops I've looked at, which tend to be around 125 Hz, near B, or higher. Many of the modern arch tops, with the really large holes inspired by the late D'Aquisto instruments must be even higher. A high 'main air' resonance tends to give a more 'forward' or 'open' sound, with less 'fullness'.

What flat tops do with bracing arch tops do with arch height and shape, more or less. The arch, of course, is used to resist the down bearing force of the string break angle over the bridge. You can get sufficient stiffness for that either from making the arch higher or the plate thicker, or some combination of the two. Making a tall arch with thin plates, to reduce weight, tends to produce a 'thin' and even 'harsh' sound, as I found on my first two arch Classicals. I subsequently learned that the trick there is to scale the arch height to the desired plate thickness (or vice versa), rather than to the length of the box. The 'art' is to find the balance point, where the thickness and mass of the plate are high enough to suppress the really high frequencies that produce a 'harsh' timbre, but not so high as to reduce overall power, always with an eye to sufficient structural stiffness to resist that pesky down load over the long term.

In that respect, arch shape also has a big role. I'm not a huge fan of wide recurves, particularly if they get too level too far in from the edge. I've seen some older boxes like that with incredibly tall bridges. I've been using the curtate cycloid cross arches for a while, with the low point just at the inside edge of the liners, so they're rising from the edge, and that seems to be pretty stable.

With the tailpiece taking up the string tension, there is no twice-per-cycle 'tension change' signal on the bridge. The driving force is pretty much the vertical component of the transverse string force. Even on a flat top that's the main force producing sound: driving the strings 'across' the top on a flat top produces about 20dB less power than driving them perpendicular to the top, so the 'tension' signal may be only about 1% of the actual power. This is consistent with what I saw in my 'string height and break angle' experiment on a Classical guitar. On a flat top, with a tall bridge, the 'tension' signal does enhance the second partial relative to the first, and maybe adds a little to the 4th partial as well, but when the energy of the pluck is controlled it doesn't add any measurable power.

Plucked strings also have a certain amount of energy in a longitudinal compression wave, up around the 7th or 8th partial, which drives the bridge in the same way as the 'tension' signal, by rocking it fore and aft. This adds a bit of 'spice' on a flat top with a tall bridge, since it's often dissonant, but it's missing in arch top sound.

The actual 'tap tones' of the top and back of an arch top can be surprisingly similar to those of a flat top in pitch. With arch tops there tends to be more vibration in the upper bouts, due to the lack of the upper transverse brace, so the areas in motion are larger. This is enhanced, of course, by the normally larger outline of an arch top as compared with most flat top guitars. This is why a good arch top can produce as much sound, or more, as a flat top: the stiffness and mass of the top are both greater, so it moves less, but the larger area makes up for it. The relatively reduced amplitude of the top motion in particular helps the strings to produce a 'better' tone, and reduces the chance of feedback in amplified situations.

There must be a lot of other details where the two kinds of guitars differ, of course. Two that I'd like to look at more closely are the 'upper cutoff frequency' of the holes, and the related 'coincidence frequency' of the plates, but these are tricky. Those have to do with mainly high frequency response, which has a lot to do with 'timbre'. Most of the actual power output in in the lower range, where the 'Helmholtz' mode and 'plate' resonances dominate, and these are actually quite similar for flat tops and arch tops, except as noted, so far as I can find out.
Alan Carruth
 
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Re: New Archtop

Postby Brian Evans » Wed May 23, 2018 1:21 pm

I have read, probably from you, that the perimeter length of the hole('s) is more important than the surface area of the holes. I've done two guitars with sound holes in the upper bout(s), to see what happens. One turned out fine, sounded quite good, the other is just getting polished so might be finished in a week or so. My idea is to have more of the lower bout in motion, I think that the traditional F-holes interrupt the sound producing real estate..
Brian Evans
 
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Re: New Archtop

Postby Alan Carruth » Thu May 24, 2018 2:01 pm

The claim of importance for perimeter length of holes came from some violin research. From what I recall I didn't find it totally convincing. IIRC, they showed a correlation but didn't provide a mechanism that explained it. If you look at enough pairs of data sets you can always find correlations: causation is harder.

I've made a few arch tops with round holes in the 'normal' position, and they sound less like 'standard' arch tops and more like flat tops. I've also made one guitar with a flat top and carved arched back, and helped a student make another. Those sound a bit different from 'normal' flat tops with flat, braced backs. Then there's the viol da gamba, with an arched top and flat back, which doesn't sound quite like a violin. Every change you make has some effect, but there's not a sharp dividing line.

I didn't want to hijack the thread. My point was to encourage folks to try making one themselves. If you've made a few flat tops you probably understand more about how arch tops work than you think you do, and the tool chops are not all that much different, or more challenging.
Alan Carruth
 
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