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I was thinking the same thing - I've built three banduras, two with Engleman tops and one with what i think is some sort of Cedar. I was planning on using Sitka for the next one but now I'm wondering....why does everything have to be so complicated!

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I couldn't find the Rick Turner project with the Al and Rick discussion but I did find this from your topic:

"As always, there's no one 'best' way to get a 'good' sound out of a guitar, just a lot of good ways. Light backs and heavy backs can both work if you get the rest of the instrument to do what you want. And, as always, I'll say that the whole thing is probably a lot more complicated in detail, so that there will be times when this sort of simple explanation falls down. It keeps things interesting." - Al

I also recall from another forum where I think it was David Schramm decreed that "the upper bout does not add to tone production in a guitar" to which Al answered something along the lines of, sure, if that's how you build them (David builds Smallman style guits).

I also recall where Mark Swanson brought a guitar to I think GAL with an access port and learned how to improve the performance of the guit (don't remember if it was Volume or Loudness though - hopefully he'll chime in) by loosening the back by shaving the braces.

Finally, I've got the example of my latest instrument with a very soft back that's not braced very well producing more Volume and loudness than my first instrument which has a 1/4" birch plywood back and a new top installed about a year prior to the completion of my latest instrument.

My thought or question being that perhaps stiff backs can produce a guitar that "projects" or is loud but does building with a looser back preclude having a loud instrument? Does building with a stiff back and sides automatically mean your guit will "project" (as you seem to be implying).

p.s. could you try and find the thread you were referring to?

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Todd Rose wrote:
"Just to clarify, when you said, "If you make two tops of different density wood, but make them to the same thickness, you'll find that the one with the lower density will be a bit lighter in weight" ...you meant stiffness, not thickness, right?"

Right, missed that mistake, it's two tops of different DENSITY and the same STIFFNESS.

Also, I didn't say you should look for FLOPPY tops, I said look for LIGHT (low density) ones. I, too, try to avoid tops with too much cross grain flex, if only because they're harder to tune.

Back weight:

In general, you would not expect the back to be as good a sound producer as the top. After all, it's usually heavier than the top, it's not being driven directly by the strings, and it's facing into your soft pudgy belly, where the sound can't go very far anyway.

As far as I can tell, the back only contributes to power output in the 'bass reflex' range, where air is being pumped through the sound hole most actively. If the 'main back' tap tone is low enough for it to couple with the 'main top' tap tone, it will help the top pump air through the hole, and also act as another added mass (along with the top) that will drop the 'main air' pitch. Usualy this works best if the back is about a semitone above the top pitch.

A light back seems to be more effective at working with the top in the bass reflex range. Surprisingly, in some ways, since the 'bass reflex' is the low range, this often seems to make for a 'bright' sound. I suspect that this has something to do with the faster 'attack' of a light back: the sound is more 'edgy'. It's also likely that another mechanism that was noticed by Wright in his 1996 PhD modeling study of guitar acoustics comes into play.

We all know that any guitar top or back will have a number of resonant pitches, where it is easy to drive to a high amplitude. These are analogous to the 'overtones' of a string, but not at 'harmonic' pitches, because of the complicated structure. It turns out that the 'main top' and 'main back' modes are much more effective at producing sound than any of the other modes. This is simply because all of the others consist of a number of areas that are out of phase with each other, and thus cancel out, more or less. Wright noticed that the 'main top' mode is so much better at producing sound that it dominates the output of the guitar all the way from the 'mair air' pitch, an octave or so below it's own natural frequency, all the way up to about 1000 Hz; the fundamental of the highest note you can play. Thus, making the 'main top' mode stronger can actually improve the output of the high frequencies. This seems weird, but remember, resos pretty much only have a 'main top' type of motion, and they've got lots of high end.

Above the 'main back' frequency any energy that is fed into the back is energy that can't be driving the more effective top. Thus it seems that limiting the amount the back moves at higher frequencies would probably be helpful. There are a couple of ways to do that.

One is to give it a high 'impedance'. That's just a measurement of how hard it is to drive something at a particular frequency; physically it's the Force/Velocity at that frequency. Since (again, acording to what I can find out so far) most of the energy that moves the back comes from the air in the box, and air has a low impedance. If you make the back heavy or stiff most of the energy of the air just bounces off, and doesn't cause the back to move.

Another is to limit the 'damping' of the back: decrease the amount of energy dissipated as it moves. Not only does this cost less power per cycle, but it also limits the range of frequiecies over which the back can be easily driven. This is because low damping oscillators have a narrow 'band width' (or 'high Q values'): they can only be easily driven near the resonant pitch. A familiar example is a kid on a swing; you can only get them going high if you match your pushes to the natural rythm of the swing. On the guitar you could think of the top as the person doing the pushing, and the back as the swing. If you look at the output spectrum of a guitar, it will often be the case that the back resonances will be seen as dips in the output. A high Q back will tend to have narrower dips. You don't always want to _eliminate_ these: they seem to add to 'tone color', but you want to minmise the amount of energy you lose.

There's another aspect to low daming/high Q that's interesting. One thing that Q value means is the proportion of energy that's being lost per cycle. A piece of rosewood with a Q value of 100 is losing 1% of the energy of vibration for every round trip. It's also hard to feed energy into the system much faster than that. Again, you don't usually try to get the kid on the swing up to full height with one push. It takes a while to get a high-Q back going, and this, too, probably limits the losses to it.

In light of this it's probably no coincidence that we like rosewoods, which are dense and have low damping. It's also no surprise that either light or heavy backs can work well, although they do tend to sound different, of course.

Andy, I'm not sure you understand what I was originally trying to say. We all come to the bench with different goals in mind as far as our instruments go. As Brock said, we might want the guitar to create an envelope of sound around the player, or we might want them to project outward to the audience (the latter is why we have archtops, or in other words, guitars engineered to have extremely stiff tops and backs). They both work, as you put it, but the sound is projecting in different ways (projection is not the same as volume).

viewtopic.php?f=10102&t=14318&p=203531&hilit=backs+thin#p203531

Another interesting post (look for Rick's input towards the bottom):

viewtopic.php?f=10102&t=12773&p=178623

The keyword in the first link is di-pole. We have a few threads in the archive that go into mono-pole and di-pole. They were quite educational for me and will help if you're not familiar with those terms and what they mean when used to describe a back.

Got it, Al. You were talking about the correlation between density and long grain stiffness, not cross grain. I'll have to do some weighing and long grain flexing of my tops and see what I can discover.

Perhaps the ideal back would be light and stiff, and have low damping? If it were both light and stiff, though, it might be tricky to get it's pitch down to a semitone or so above the top's.

Thinking of a light and stiff back with low damping, a wood like palo escrito comes to mind. My only first hand experience with that wood was at the GAL convention. LMI had sets there, and I was struck by how light it was, while having the very lively, glassy ring of other, far denser rosewoods. Can one interpret tap tones like that to mean that it has low damping? I don't recall exactly how stiff it seemed, actually; I just remember that in my hands it felt like a rosewood, only without the weight, and it sure had the ring. Do you have first hand experience with palo escrito, Al?

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

No ones mentioned parabolic bracing yet.

Certainly louder in the mids and top end.

Dave and I build with scalloped braces most of the time
but the parabolic braced guitars we've done were plenty loud -
probably louder than the scalloped.

Bob

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Bob Connor
Geelong, Australia

<edit> Sorry, wrong thread.

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Rian Gitar og Mandolin

One of things I'm planning to do on the guitar bouzouki I'm designing, for both the loudness and tone my customer wants, is use bracing profiles that are sort of parabolic... more like elongated bell curves, actually. The bracing layout will combine elements of traditional X bracing with double X and lattice. The main X will be fairly tall and narrow with a more or less triangular cross section.

Making a Nomex sandwich top would be another option, but I'm not going to go that way, mostly because I have concerns about repairability of those tops (I know there will be others who disagree with me on the repairability issue with Nomex sandwich tops, and I don't mean to open that can of worms... I'm just stating my reason for not going that route at this time).

This instrument will also have a cantilevered, elevated fretboard, with a relatively high "negative" neck angle. It will also have a relatively lightweight bridge (and lightweight bridge plate, too) made of a low-damping wood.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Another thing is high string tensions - long scale and heavier gauge strings (this also means the strings are more massive, so there's more oomph on the bridge). Along with that, a stiff enough top (incl. bracing) to both withstand the higher tension and provide plenty of headroom. Stiff enough, but still lightweight.

I think there are a number of things like these that can add more power to an instrument whether it's designed to project or envelop.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Al,
Nice post with lots of great info.

I prefer a smaller guitar for my personal playing situations. A 00 size guitar like a 00-18 or a 00-28
is my favorite simply because of the punch and surprising volume that can be achieved by them.

I was convinced back in the late 90s when it appeared that most builders were adding more and more
depth to their smallish bodied guitars in a futile effort to achieve more volume or more bass...or both
that the people who ended up with the guitars would be missing out on the very best characteristics of
a small body.

I agree and have done enough research to conclude that a more shallow body (within reason) will result
in greater volume and a surprisingly more full, balanced tone.

Being from north New Jersey, I used to spend lots of time with Fred Dickens and was able to play his
7 1/2 inch deep classic guitar when he first built it and then several times during the reduction in its
depth in 1/2 inch increments. When it was deep, it wasn't any louder than it was when it was more shallow.
In fact, it was boomy at times, had no articulation or note separation and was not powerful at all in the
frequency slices that give a guitar its projection capabilities.

When he got the depth down to 4 1/2 inches, it started to offer some power and the very last time that
I was able to play it, the depth at the neck block was about 3 1/4 inches and it sounded great....much louder
then when it was twice that depth and much more balanced and focused and separated as well. It finally
sounded like a nice classic guitar.

Fred went on to take that guitar down to about 2 inches in depth and he later told me that it lost that great
tone and most of its volume as he did.

He was a fun guy to be around and would call me often to bring guitars out to Whippany so we could throw
tone to them and just read what they did in response. Fred had the same view as I always have as far as the
comparison of guitars to Helmholtz resonators goes. Even though the guitar is a resonant chamber, the
activation being created at a different point than the opening through which the air escapes from the cavity
makes it very different than the application and occurrence that allows an actual Helmholtz resonator to
be activated. It is a stretch at best, but I understand the need to parallel the application and results for a lack
of a better reference alternative.

I've never bought into the "deeper is better" mentality simply because so many incredibly loud and projective
small bodied guitars have proven otherwise for so long.

I also agree that the soundhole being moved toward the neck helps a guitar to produce more volume. I did
just this back in the early 90s while developing my MJ and GC model shapes and building about 25 prototypes
for each of them. I still like to keep 20 frets available to the player, but moved my soundhole as close to the
upper transverse brace as possible. I don't mind being able to spot that brace through the soundhole, but do
appreciate the difference in tone and volume that having the hole closer to the neck brings.

Finally, As far as soundhole diameter goes, we need to be careful. I've always been a strong believer in the
fact that the back and side woods make a major contribution to the voice of the guitar. The top, of coarse, is
the chief determining factor in the response and much of the guitar's tone, but I feel that many builders are
not giving enough consideration to the contribution of backs and sides.

With that said, I believe that the contribution of the back and sides is generated more by physical vibration
caused by both the contact with the air that is excited inside a guitar during playing and the commuication or
transduction that takes place between the major components of the guitar. Meaning that, as the top is activated
by the striking of the strings, its vibration is communicated into the sides through the solid and integral joints
created by the kerfed linings and binding/purflings. Then the sides act as a sort of bridge between the top and
back as they communicate that vibration from the top to the back allowing it and the sides to present their
tonal contribution coloring the overall tone.

The back and sides, with their surfaces being interrupted by bracing and the curvature that is common in both
are not ideal reflective surfaces so the claims that reflection being the chief function of them makes no real sense.
I'm sure there is a certain level of acoustic reflection that takes place, but not enough to constitute the tonal
differences between guitars with various back and side woods.

Since the soundhole determines the speed with which the air inside the guitar can escape and present the guitar's
tone to the player and listeners it needs to be considered when arriving at a final diameter. If the air is allowed to
escape too quickly, it doesn't have time to do its part in the activation or vibration of the back and sides. If it is held
inside for too long, there is a loss of volume and projection and a guitar can be boomy and lack note separation. I've
built prototypes with various soundhole sizes and locations and the effects that they have on tone, volume and balance
are very interesting and sometimes surprising.

Mine ended up slightly larger than what is common and slightly closer to the neck than is most common.

Thanks,
Kevin Gallagher/Omega Guitars

Thanks for weighing in, Kevin!

I didn't get to spend much time with Fred: just some evenings when I was studying with Carleen, but I treasured every minute of it. I'm still working on stuff he suggested. If it hadn't been for him, for example, I'd have never thought to look at the torsion modes of the string in my experiments. I wish I could tell him that he was right.

"I prefer a smaller guitar for my personal playing situations. A 00 size guitar like a 00-18 or a 00-28 is my favorite simply because of the punch and surprising volume that can be achieved by them."

One of the most interesting things that came out of Wright's work was a point that his thesis advisor, Richardson, made in a subsequent article in the (late, lamented) Catgut 'Journal'. Wright found that the most audible change he could make in his model was to alter the ratio of area to mass of the top modes. Richardson pointed out that, since a small top can be made thinner and have lighter bracing than a larger one, it will tend to have a higher A/m ratio, and put out more power. Also, since the smaller guitar will usually be more 'treble balanced' I find that they tend to project better. Those big Dreads and Jumbos fill a function, but if you really want to be heard across the room, get an OM or 00.

"I was convinced back in the late 90s when it appeared that most builders were adding more and more depth to their smallish bodied guitars in a futile effort to achieve more volume or more bass...or both that the people who ended up with the guitars would be missing out on the very best characteristics of a small body."

More depth tends to make the 'main air' resonant peak in the spectrum lower, and effectively wider. The guitar has a more 'even' sound in the low range, but less 'punch' and 'power'. This can be useful to know, say if you're trying to tame a 'forward' wood combination like walnut with Red spruce. OTOH, I made a little 00 size box in cherry/redwood for the Montreal show that has tons of power, and plenty of bass too, and it's about the same depth overall as my classicals.

"Meaning that, as the top is activated by the striking of the strings, its vibration is communicated into the sides through the solid and integral joints
created by the kerfed linings and binding/purflings. Then the sides act as a sort of bridge between the top and back as they communicate that vibration from the top to the back allowing it and the sides to present their tonal contribution coloring the overall tone."

When I was doing all the measurements on the 'corker' (a couple of thick file folder's worth), it seemed to me that very little of the energy from the strings gets transmitted to the back through the sides. As I opened more ports the sound pressure level in the air inside the guitar went down, and so did the activity of the back, in general. I don't think that removing the corks altered the way the sides transmitted sound very much, but they sure did change the way the air in the box worked. Again, this was an overall result: in some specific instances at some frequencies, the back activity did increase, but that was usually attributable to a particularly 'nice' alignment of frequencies. Opening ports changes air mode pitches, of course. I will say that, as usual, more research would be nice, and if somebody shows that I'm wrong with better data I'll be the first applaud. I've got to go with the data I have, though.

One thing I want to look into is the 'upper cuttoff frequency' of the soundhole. This could be a major player, so to speak, but it will take some work to nail it down. I suspect that, like everything else on the guitar, it will end up being really complicated....