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Old 07-18-2017, 10:30 AM
Ibnrushd Ibnrushd is offline
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Default 16inch acoustic archtop design comparison

First off I would like to say I know absolutely nothing about guitar design and luthiery...But I thought this topic may interest some of you here and I am eager to learn what I can from those who understand more.

I've been reading about the early 16" L5 archtop, after listening to Julian Lage play this instrument so beautifully.

I notice that the modern "takes" on this design include variations that may be intended to have an effect on the sound of the instrument. I am curious about the intent and mechanism of these design changes, and how they may be combined in further variations. I understand this is a speculative/best guess thread topic, but I find the idea interesting and hope others may as well.

Bracing
I notice that the Collings AT16 and the Yanuziello use an X bracing rather than the original L5s ladder style. Is this intended to produce an acoustic sound more akin to a flat top? (Greater sustain, greater emphasis on harmonics over fundamentals?)

Wood
I've read that the back arch and material is important for generating the low end bass.

Eastman offers variations with mahogany back and sides, as opposed to the traditional maple. On paper, this seems like a promising idea since it might mitigate the harsh and trebly voice associated with small bodied acoustic archtops. Yet this seems to be an uncommon choice. Why is maple so ubiquitous for the back and sides? One suggestion I read is that mahogany increases warmth by emphasizing the mid range, but maple has a better low end bass response.

Is also seen Walnut and Koa backs.

Dimension

Depth
The Collings AT16 uses a shallow depth. According to their site, 2 7/8" as opposed to 3-3 1/4" used by the original and most variations. What purpose is achieved by reducing the depth?

Lower Bout
Several modern variations move to a 16.5" lower bout. Is this half inch increase intended to increase bass response and overall volume?

Shape
I notice the Collings AT6 has more rounded shoulder, whereas the original L5 and most variations feature a near perpendicular flat right angle between the body and the neck.

Additionally, based on a visual guesstimate, the Collings appears to alter the ratio between the upper and lower bout, reducing the volume of the upper and increasing that of the lower.

What is the purpose of these changes? Is it merely aesthetic? Or does they alter the sound as well?

F Holes
I have read that the size and the shape of the F hole must be matched to the dimensions and shape of the guitar, for reasons that I do not fully understand yet. However, I do notice certain variations.

Eastman (AR 905 and similar) models appear to enlarge the F hole rather dramatically. Why?

On the other end, Collings appear either the same or smaller than the L5.

Bridge
A couple variations (eg Yanuziello, Collings) use a bridge that makes contact with the body across its full length, as opposed to making contact only through two "feet" as in a violin bridge. Why? I assume this choice is significant since the bridge presumably plays a large role in transmitting the vibration of the string to the top.



That's all I've noticed so far. Can you think of other interesting design choices?

Last edited by Ibnrushd; 07-18-2017 at 10:39 AM.
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Old 07-19-2017, 02:16 PM
MC5C MC5C is offline
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I think that many people don't have the build experience with varied designs to be able to answer your questions. The physics of archtops is not well developed, and so many things go into a design that picking out specific details is not really very useful. With that said, here I go...

Bracing. Read this article: http://theartoflutherie.com/acoustic...bob-benedetto/ In it he comments on a few different bracing schemes but basically if it's well considered and well done, anything will work. Modern builders stress lightness over bulk, many instruments from the 1930's to the 1950's were significantly over-built and have that characteristic old archtop chunk that many people love, and I hate. I like light, airy, responsive, sustainy, loud instruments. You get that with light building, I find.

Size. There is a sweet spot for archtop sizing that has evolved to the 16" to 17" lower bout size. Similarly depth around 3". You can do more or less, but the balance of the voice seems to shift. Generally bigger is louder, brasher, smaller is more centered and refined. When you get too small, the instrument starts to lose it's acoustic voice, but people are developing smaller instruments.

Wood. Spruce is a good top wood, but people use many different straight grained, fine grained stiff, light softwoods. I really like redwood. Back wood is open to almost anything, people use maple, koa, mahogany, I've used spanish cedar. People tend to use maple because customers expect it, it's beautiful if highly figured, and it's very easily obtained. I don't think the type of wood affects the sound nearly as much as how the plate is carved and tuned. I think that tuning of the resonance and responsiveness of the back is critical to an archtop's voice. I don't find that type of wood has any impact on the voice of my instruments.

Shape, as long as the instrument has a waist, is aesthetic within reason. Great guitars are made with all sorts of shapes, in detail.

F-holes. The area, and hence how open they are, is not very important. I think that what is important is the length of the periphery of the edge of the hole. Violins evolved their f-hole shape over centuries, and got to a placement and size that further experimentation did not improve on. I personally think that Loar kind of "cut and pasted" violin family f-holes, they worked, he left them alone. Structurally they are kind of silly, they cut off much of the soundboard of the lower bout from vibrating at all, and the leave the center of the soundboard unsupported. The top is quite a heavily loaded structure (it bears the string tension load longitudinally from neck to tail block, and it bears the downward string load from the bridge, which can be 35 to 55 lbs). I quite like moving the sound holes to the upper bouts. I think the only less structurally effective style than f-holes is a round hole in the middle of the body, like a flat-top has.

Bridge. The physics of vibration transfer is tough, but surface area has little to do with it. A full length, full contact bridge looks nice, is hard to make and therefore impressive, but won't function much better, if at all, than a bridge with two feet. I favor very light bridges, I make them with two feet, non adjustable, and they seem to work just fine. If you consider that a typical adjustable bridge transfers the vibration through two 3/16" steel posts, with a surface area around .055 square inches (tiny, in other words) and they seem to work really quite well, I don't think a big surface area on the bridge feet makes any difference in sound. It does make a difference in spreading that 35 - 55 lbs of string load, so the feet have to be big enough to be stable and spread that load so the top surface is not crushed over time.

That's all I got, and it's all just my opinion...
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Brian Evans
Around 15 archtops, electrics, resonators, a lap steel, a uke, a mandolin, some I made, some I bought, some kinda showed up and wouldn't leave. Tatamagouche Nova Scotia.
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