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Yet another question on my hyrbrid AGB with "strut through" design. In case you have not read any of my other questions it will be similiar in size and apearance to the Taylor AB-1 but will incorporate strut portions of a laminated neck running to the tail block.

The top and back will be free floating.

Electronics will be piezo tranducers in the bridge and a floating Benedetto magnetic jazz guitar pickup by the neck.

The bridge will be like an arch top design but the strings will go through slots in the top and string throught the tail piece into slotted ferrules. I bought and read Benedatto's book on building jazz guitars for some reference.

Anyhoo on to my question.....

What characteristics, advantages and disadvantages does a carved top and back offer compared to a flat top?

I don't expect to get any real significant volume out of any AGB truly unplugged but it would be nice to be able to practice by myself or with a soft fingerstyle acoustic without an amp.

Q1 More specifically in very general terms; of the following characteristics how would you contrast the two?

Volume or projection
Tone or resonance
Strength
Bass specific frequency response.

Obviously an arch top would entail a great deal more time and sweat equity (as well as additional complexity)so I'm kind of curious what to expect before I decide and buy the wood.


What differences

Scooter B38913.5656712963

Save the archtop for your second one. I really don't know which would sound better, but for your first I recommend using proven designs and to keep it simple.

Al

Scooter B asked:
"Q1 More specifically in very general terms; of the following characteristics how would you contrast the two? "

I reply:

"Volume or projection" : Yes

"Tone or resonance" : Yes

"Strength" : Yes

"Bass specific frequency response." : Yes

Now, wasn't that helpful?

;)

Seriously, it's a hard set of questions to answer. One easy indication would be to listen to a Gibson L-5 and compare it to a J-200, which is the same size and shape. But, of course, that ignores a whole set of major differences, starting with round vs F holes. Round hole archtops sound much more like flat tops.

The basic idea behind the arch top was to take the torque load, which is what ultimately kills guitars, off the top. Arching the top allows for sufficient break angle at the bridge to stop the strings properly, and converts all the loads into compression. It does _not_ change the way the guitar works in and of itself except in relativelty small details. The change in string tension as it vibrates, for example, pushes down on the arch top, and torques the bridge on a flat top. Either way, it's a relatively small force that probably doesn't effect the output power a lot, although it can change the timbre.

The height and shape of the arch, and the thickness distribution in the top, work together to determine the distribution of stiffness and mass in much the same way that brace design does on flat tops. Becuase the loads are different you don't need the shoulder brace on an archtop, and more of the total area can vibrate to produce sound. That _should_ help the bass range. However, the stiffness you need to withstand the down load of the bridge means it's hard to get as much motion on an arched top as a flat one, so, in fact, it's usually more difficult to get a lot of acoustic volume out of an archtop. Note that arch height and thickness both determine the stiffness; making the arch high allows you to thin out more, but the total stiffness still has to be about the same to take the load. What changes is the 'center' of the frequency response, with a lighter top responding better to low frequencies. As usual, there seems to be an optimum balance, which is a little tricky to determine in advance: it's pretty common for most of us to say in the end: 'Darn, should have done it differently'.

Usually an archtop body can be a bit bigger than a flat top: back before good amps Stromberg made them as much as 19" across, which must have been hard to hold. Loud though.

So: acoustic volume is not often the archtops strong suit. Projection and 'punch', OTOH, can be.

Tone and resonance can be very much like a flat top, or very different: depends on how you do it. Good archtops are often said to sound 'almost as good as a flat top'.

Archtops are often very heavily built,and even when lightly built are, potentially at least, quite strong. Again, the devil is in the details, like arch shape and the way you make the recurve.

It's usually hard to get a 'full' or 'rich' bass out of an archtop, compared to a flat top. The jazz guys I talk to tell me you don't want a lot of bass for that style anyway.

Now, if you make the body about 48" long, and, say, 30" wide, with a top about 1/4" thick or a little more, with a soundpost to help support it and an arch height of around 1-1/4", and play it upright on a pin, you'll have some bass response. You'll also have a bass.    

Thank you both for an informative and entertaining answer.

I was leaning towards the flat top but wanted to do the jazz style bridge for the string through modification instead of a top termination at the bridge.

Now how to figure out the optimal angle to use coming off the bridge....

Any ideas?

I am beginning to feel like a curious three year old who respond to every answer with yet more questions.

Scooter,

I think that whichever way you go (flat or carved) you'll have enough resonance and volume in this design for solo, unamplified practice. Probably with a fingerstyle guitarist, too, but since you're incorporating non-traditional elements in your design it's difficult to say with certainty. I'll be interested in how it all turns out, so keep us informed!

Benedetto specifies a break angle over the bridge of 11 degrees, and that's enough to stop the strings properly. I've found that too much down pressure can kill the tone, and have tried to keep it to a minimum on my archtops. Arch height, bridge height, the 'overstand' of the neck; the height of the top of the fingerboard off the top at the upper edge, and the way the tailpiece pivots all help determine the break angle.

Thanks for the 11 degree tip that should at least give me a reference point.