Official Luthiers Forum!

For an all around (some strumming, some medium picking) guitar, what
kind of tap tone are you looking for in a back? Strong ping with just a
little decay? A ping with lots of decay? I have martin style braces.

What size guitar are you making?
What are the woods?
How thick is your back ?

The less response from the back the better!
Make your top do the work!
Your back should be stiff and not impead the top to do it's work!
mike

_________________
Mike Collins

There appear to be a couple different schools of thought on all this. There's the stiff back camp, that Mike mentions, where the back is primarily a reflector, and there's the other camp which wants the back interacting with the top to support and augment it.

Whichever way you choose to go, you should know that people have had success both ways. Do which ever one makes the most sense for you.

Don Williams39026.8375

_________________
"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

Hey Don,
Now that makes me laugh...    
Thats a pretty "Libral" statement.
"can't we all just get along..."

Matching the top to the back creates more efficiency, however you don't want the pitch to be the same. I prefer to shoot for a Seven(7) semi-tone difference. But that's just me....

p.s. as someone I respect very much, recently said to me " this is one complex animal...so much to learn...so little time".

_________________
Dave Bland

remember...

"If it doesn't play in tune...it's just pretty wood"

I went the second route on no 2 and well, it sounds like a drum!

Nothing really matters. As long as there are no lumps and bumps to impede the flow of energy, it'll be a hoss.


..........?

    I'm typing with one hand, but wanted to chime in on this thread.

    All components should respond ad should exhibit sustain. The back
should be voiced to offer as lose to its potential resonance and sustain as
you can achieve while maintaining rigidity.
    
    We choose our back and side woods for their tone of we're buying or
building for tone. Most buyers, though, are choosing their wood for
appearance and little else.

   A rigid back/side assembly should suspend the top to allow it to work
freely, but also need to be able to be activated by the vibration
transferred by the top and its bracing as well as by the vibration of the air
in the body. Believe it or not, the back and sides are always major
contributors to tone of any guitar. I know its become kind of popular to
minimize their importance in comparison to that of the top, but I disagree
with much of it.

   Rosewood and Mahaogany guitars have distinctly and very obviously
different tones...even with identical top woods. The tonal color and voice
are largely determined by the back and side differences. I've always loved
that dry, immediate sound and response that is inherent to a nice D-18,
but also appreciate the more complex, and deeply colored tone of a
D-28. Both with Sitka tops and nearly identical bracing, but with different
back and side materials.

   Voice the back and document weight and dimensions of the bracing as
well as the final results in the finished guitar.

Regards,
Kevin Gallagher/Omega Guitars

Thanks everyone.

Kevin, I was thinking about you yesterday and wondering how your
shoulder/surgery progress has been. I read one of your posts in the
archives that your backs seemed quite a bit thicker except for the rim
which you make like the recurve on a carved top instrument. Why do you
keeps yours thicker?

I am in the camp that says the back should be active in reinforcing sound generated by the top.

Vintage Martins had parabolic back braces, FYI, and were much looser than todays models.

My "Sound is Round" model says that the back acts as two, nearly separate drum heads--upper and lower bout. Being larger in diameter, the lower bout finds resonance with low notes generated by the E, A and D strings, primarily. The upper bout can reinforce the D and G strings, if loose enough. The high E and B strings do not have enough energy to generate much reinforcement from the back.

In 2000, I was hired by Bob Taylor to visit his company in El Cajon to built some guitars using my bracing ideas. In one discussion, he said that he didn't want the back to add anything to the sound (outside of tone) so that holding the guitar to one's body wouldn't change the sound.

Interestingly enough, I recently did the set up on a local's Taylor dreadnought built in 2001, and noticed a very lively feeling back. Looking at the lower two braces on the back, I could see why. They were not the parabolic shapes I make, but were an incredibly good approximation. Normal Taylor back braces were about 7/8" side and 3/4" tall. The ones on this guitar had the normal, slightly curved top profile brace only in the middle 2 inches or so, and there was a straight, flat taper from there to the sides, where it was down to zero.

A photo on my web site shows the back of the Gibson I've written about on another forum topic recently. I've thinned the braces even more since assembly, but they basically look the same as in the photo. It is now a real cannon.

I'm know there are many good sounding guitars built by the tight back faction, but they can't vibe you back and feel alive in your arms, and they can't produce the full potential possible. This may just be a matter of preference.

However, in re-voicing guitars, I find that shaping the back braces an integral part of bringing the strings into even volume balance. The standard heavy lower braces that only scallop down to thin at the kerfing allow the low E string ring some reinforcement around the largest diameter of the lower bout, but do not allow much reinforcement of the A and D strings, which resonate with smaller rings dampened by the fat part of the braces.

Scott

Yes it may be a personal liking of one or the other ! but If you make the back vibrate to much you loose sustain and volume if the guitar is held against the body as most players do.
I've made guitars both ways and I find the stiff back will give a longer sustain and prescence to the notes!
Also they have incredible punch!
But that's what I like-my guitars are played in large halls where this is important and I find I like the sound in private situations also.

And how can you tune a plate before it's glued on ?
It will change after gluing on and binding!

Mike collins

_________________
Mike Collins

Very interesting. Scott, I'll need to find that photo. I'm not necessarily
looking for a canon. Ending up with something Lowden-esq would thrill
me. Do you think they're stiff or lively backed?

Mike,
    Martin, Taylor, Gibson and all other builders didn't just throw braces
cut to generic dimensions onto their backs and hope that they would
yield great tone and work well with their respective tops and sides. Their
dimensions were arrived at as the result of loads of trial and error,
research and development and close documentation of the changes that
occur when plates are assembled with their sides to form a closed body.

    There are so many builders who really don't understand the
effects of brace placement, shape, mass or fundamental resonant
frequency that they simply just use dimensions close to those used by
one of the major builders and throw the results to the wind and to the
mercy of the experience of those more established builders. Many haven't
taken the time or invested the effort to apply different techniques and
dimensional approaches to their own guitars so they haven't experienced
it....which is the only way to become skilled at predetermining what a
specifically tuned top and back will contribute to the guitar's final tone
and response characteristics.

     Having built nearly 500 acoustic guitars alone and by hand here in my
small Pennsylvania shop, I've tried just about all of it and have books full
of documentation and detailed notes from each guitar and the results of
the subtle changes that were applied from one to another. My guitars
have displayed an evolution both in design and tone over the years and
I've embraced and maintained some techniques and methods that have
been abondoned by other builders for various reasons.

     Some builders have moved to more generic techniques to save time
and to ultimately broaden their profit margin while settiling on a set of
default dimensions and methods that yield generally good tone that has
become signature to their instruments. Some have adopted very
production like tendencies as they simply pull the next top from their pile
and match it with the next batch of bracing cut from whatever stock is
closest at hand....all of high quality, but not particularly matched to
compliment each other as far as tone, balance and response are
concerned as goals.

     I still bend all of my sides by hand on an iron even though i own
several very nicely made Fox style bending machines and a dozen or so
heating blankets and thermostat units just because I enjoy the
experience. Are my sides any more or less accurately bent than those
bent on the machines?...No, but my machines were neatly tucked onto t a
shelf years ago when I decided that I missed feeling the wood relax in my
hands as I laid them over my iron. Sometimes our choice of method is
one made out of necessity and other times it's just a matter of personal
preference.

    For instance, I still separate and stock my woods....including tops,
back/side sets and bracing stock according to their fundamental resonant
frequencies in stockpiles labeled "low", "medium" and "high" so that they
can be matched to coordinate their respective tonal and response
contributions to present a completed guitar that is balanced and
responsive without overbearing bottom or thin top end notes. I oversee
the sawing of all of my bulk wood purchases in order to be sure that
quartersawing is achieved as closely as possible....even if it generates
slightly more waste than may be possible otherwise.

     As for the consideration of tuning the body through the assembly of
tuned components, I shoot for a target tone or note for the resonance of
the closed body prior to the joining of it to its neck and the application of
the string tension and its effects. My goal is to achieve a fundamental
resonance that provides the foundation for good intonation of the
fundamental note and harmonic blankets. If the two are aligned
improperly, ghost notes, standing frequencies and boominess can result.
The contribution of the neck to the final tone is also considered as it is
possible for it to present or create certain cancellation properties that can
give the impression of thin tone poor balance. All of these situations can
be addressed durint the building of the body by having experienced the
effects of many different scenarios in order to avoid those that present
less than the desired result and to repeat those that contribute in a
positive way.

     Check and document whatever you possibly can in every guitar that
you build and it is inevitable that you will become familiar with what
results you and your methods are going to provide. Side thickness, back
and top thickness, back and top weight, bracing weight, resonant
frequencies of all individual components as well as braced plates, weight
and density of neck blanks, truss rod weight and mass, headstock size
and thickness, distribution of the bracing weight across the plates and a
myriad of other dimensions can be documented on a guitar as it's built.

    Check and record how each contributes to response, tone, sustain,
balance, harmonic complexity and you will be able to more accurately
predict what you will achieve on your next guitar as you apply those same
methods of building and material selection. I've always used a Peterson
strobe tuner and chladnii technology to read subtle variations from guitar
to guitar while always allowing for a margin that is presented by the
organic nature and infinitely variable nuances that are present from one
piece of wood to another...even when cut sequentially from the very same
log or billet.

    It's relatively easy for me to predict what a specific tuning of a top or
back will cause to happen in a guitar since I've not only experienced what
a great number of them have in the past, but am also very familiar with
my proprietary designs and body shapes. Four hundred sixty or so guitars
is a lot to have pass across any builder's bench and i've enjoyed watching
each one take shape as they have over the years. I've spoken with other
builders who have built less than half as many guitars in the same
number of years that i've been building and they've never even considered
many of these factors as viable contributors to tone and I've spoken with
builders who have built many more than i have and have applied some or
all of them and have nspired and encouraged me to continue to do so.

    Detailed documentation is a bit tedious while building sometimes, but
is always interesting as it reveals very small variations from one guitsr to
another that may appear to be identical in many obvious ways.

Regards,
Kevin Gallagher/Omega GuitarsKevin Gallagher39027.8796875

Mario has the best answer!!

_________________
Mike Collins

[QUOTE=Mario] Nothing really matters. As long as there are no lumps and bumps to impede the flow of energy, it'll be a hoss.


..........?[/QUOTE]

Mario, is there a specific technique to make sure there are no are no lumps and bumps to impede the flow of energy? We want to make sure we get all our bases covered!

_________________
Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

Guys...

Mike,

but If you make the back vibrate to much you loose sustain and volume if the guitar is held against the body as most players do.

Agreed.



I find the stiff back will give a longer sustain and prescence to the notes

I can see how the sustain would be increased if you don't involve the back - since moving wood takes energy, you're not diverting energy to the back, and it takes longer to transfer the string's energy to the soundboard. Plus then the back is more immune to the muting effect of body contact. Interesting about the presence ... I have inadequate experience to comment on that.


And how can you tune a plate before it's glued on ? It will change after gluing on and binding!

Very true, but here's the thing. Builders who use tuning extensively, will tune the back to the top. Whatever resonance frequency changes occur on one plate when glued to the sides, will likely occur on the other plate as well, so once on the sides, while having different resonances than free plates, they still will work with each other at their new glued and altered resonance frequencies.

And that's basically the main benefit of tuning. To get the T1 and B1 (main top and back modes) working with each other to form a bass reflex couple which will improve the low end and perceived power of the instrument's response. So, to that end, you CAN tune a plate to some effect before it's glued!    Pete Licis39028.5086458333

Kevin's got the idea; keep records and find out what works for you!

Mario's beginning to sound like Scott there.....

;)

Mike wrote:
<<And how can you tune a plate before it's glued on ?
It will change after gluing on and binding! >>

Sure it changes, but after a while you learn how to correlate the 'free' plate behavior and the sound of the finished guitar. Again, keeping records....

My idea at the moment is that both the 'active back' and 'rigid back' models are right in some part of the range. As far as I can tell from my testing the only back resonant mode that is likely to add to the sound output of the guitar is the 'main back' one. The closer it is tuned to the pitch of the 'main top' mode when the box is all together, the more it is likely to contribute. If it's too close it can cause various sorts of 'wolf' notes, so you have to watch that. IMO it's best to have the 'main back' mode about a semitone higher than the 'main top' pitch on the finished and played-in guitar.

Higher order back modes do seem to be mostly 'wasters' of energy, so the idea is that you want them to waste as little as possible to get a lot of sound output. As far as I can tell, the back seems to get most of it's energy from the top through the air, rather than the sides. Using a low-damping wood, such as a rosewood, keeps the back resonances confined to narrow frequency bands, and it won't move much off resonance if it's heavy. Those back modes do have an acoustic effect, though; the dips they make in the output add 'color' to the tone, but you don't want the dips too deep.

I find all these models and theories interesting actually.

Scott's approach is very different from anything I've ever encountered to date, and I would certainly like to hear more of what he has to say. So Scott, if you're still following this thread, I'd love to hear more about what your theories of voicing are.

Interesting stuff all of this...and it seems that the smart way is to do as everyone suggests and take a lot of notes, and build a lot of guitars. It would seem that comparing the notes and results are the only real way of forming an educated opinion on all this.
If you want to know what something tastes like, I guess you just have to try it to find out.

I would personally appreciate the lack of heckling and sneering if people take the time to share their knowledge, no matter how much one might disagree with what is being said. This is an education process for us all, and nobody is so full of knowledge that they can't learn something occasionally.

Something I sort-of picked up from what Scott said reminds me of something from back in my days as a civil enginnering drafter. Engineers would use pipe for drainage that had those corregations in them. I wondered as to why they didn't use pipe with a smooth inside, but they said that the bumpy surface created turbulence, which slowed the flow down, but also allowed the water to move better in some cases. It didn't make a ton of sense to me, but if the shape of something can effect the ability of it to transmit energy, that would be useful information to understand and apply.

Nature and engineering both are full of examples of shape and structure effecting strength and energy transmission. It stands to reason that we can learn something from it. I remember someone attempting to use an I-beam as a post for a basketball backboard. The very shape of the beam created some serious issues with twisting. A round pole is able to resist twisting moments, whereas an I-beam twists all over the place. Needless to say, it was a total failure. Everytime the ball hit the backboard off-center, it would start twisting all over the place.
Don Williams39028.7769097222

_________________
"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

     I've always been of the belief that it all matters and is worth actually understanding rather than just assuming something because someone else says it works or has proven it to work generally. All of the big houses who have made millions of dollars while building thousands and thousands of guitars have spent millions of dollars on research and development as well....and for very good reason.

    Old C.F. Martin didn't just throw some bracing on a top or back and throw chance to the wind in hope that it would sound great. Instead, he began to try different designs and dimensions in an effort to reach conclusions that would yield guitars that would not only look and sound great consistently, but also last generations as they did. His application of violin construction technology was evident from the beginning of his guitar building efforts. The importance of graduating the top and back is paramount in any violin and the same is applied to guitars to a degree through the bracing, its dimensions, weight, resonance and placement as we strive to coax them as close to their tonal potential as we possibly can through voicing. The back's graduations on a violin are considered very important and the back of guitar bears similar burden in the overall tone producing system that the instrument is in my opinion. The back of a guitar is activated much more through air vibration than actual physical transduction throught eh sides from the top, though. A much different effect than that exhibitted in the violin.

    I've been in areas of the Martin factory, both the new and old buildings, where I was able to flip through boxes full of diagrams and notes taken by workers as long as nearly 100 years ago documenting changes in bracing placement and dimensions, plate thicknesses and all kinds of variations that were not necessarily standard to their models at the time that they were written. Nonetheless, time was taken and documentation was obviously encouraged among employees who were building the guitars. Some of the things that I was able to see and read had become what are now the industry standard for most guitars and others were abandoned and discarded as they were proven to be less than the best design alternatives.

    I just like seeing and being able to compare the results from guitar to guitar as I build, only because everything does make a difference and, in turn, does matter. You can....and many do...rely on the findings of other builders who take the time to actually find out what works as you throw guitars together with generally proven dimensions and designs and get generally good tone. But, in order to understand what it is that causes the very subtle variations in tone and response from one to another, you're going to have to pay attention....at least for a time or for a good number of instruments.

   You'll find that it won't be necessary to spend as much time on you 150th or even your 50th guitar when it comes to selecting the materials that will work together to produce a desired result. Since you'd done it on a good number before, your experience will enable you to recall past results and to achieve the same again and again.

    There is a sort of cockiness or arrogance that some builders have, but their success is sometimes the result of a lack of knowledge or discernment on the part of their customers rather than a depth of ability or knowledge on theirs'. I've had guitars that cost in excess of $12k brought here to my shop by their proud owners who bought because they heard or read that they were the best only to have them leave disappointed after they played a guitar that would have cost them less than half which was louder and more balanced and player better to boot. WE need to tolerate the very common status of ownership that is a huge motivator for many purchasers.

    A shiny smooth finish is enough to convince some players that a guitar is of good qualityor worth the price being asked for it. Tone in high end instruments is something that needs to be experienced, both in comparison to one another as well as in comparison to guitars of lesser qualityand price, in order to appreciate the difference and value of each. To many players, it just doesn't matter, but some will have developed an ear to tell the difference between when time is applied to certain areas of building and when it is not.

    I've been impressed on several occasions when players sit and play a group of instruments and are able to actually point out some of these subtle tonal and response differences in them. It comes from experience and paying close attention in their cases just as it does in the case of the builder.

   Take a few minutes to note and document things as you build and you'll grow to understand and appreciate the benefits with time and as the number of guitars you build increases.

Regards,
Kevin Gallagher/Omega GuitarsKevin Gallagher39028.7609722222

I agree very much. We need to engage beyond the visible surface of
things to understand bigger things that could play a role in shaping their
outcome. That's what I'm trying to do at this point. I know the guitars I
admire, and I'm trying to identify what could make them as they are. I'm
pretty sure I'll love my guitar, but I don't want it to be a complete accident
   

I was covering a kindergarten class this morning for an hour and
inbetween thinking, "this is not my gift or desire," I was thinking about an
experiment guitar. Imagine making a guitar to experiment with. It
wouldn't have finish and the bindings would be cheap b/c you'd rout
them away a few times. Two positioning pins in the tail block, two in the
headblock to align the top correctly each time you removed it and glued it
back on. Then you'd devote it to experimenting with brace positioning
and tap tuning/assessing.      

How do you know approximate the limits of the back's rigidness? What
are you feeling for?

Yes...well said Kevin and James both. This brings back to memory the "Beginner's Mind" philosphy that Bill Cumpiano espouses. Always consider yourself a student, with much to learn. That way you not only position yourself to be always getting better at the kraft, but you also keep yourself from getting arrogant.

_________________
"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

Thanks to everyone who has contributed thoughtful posts to this discussion. I'm sure I can speak for many of us on this forum in saying that your generous efforts to offer your experience, ideas, and knowledge for the educational benefit of all of us are greatly appreciated. I, for one, learn a lot from this, and consider myself very much indebted.

_________________
Todd Rose
Ithaca, NY

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

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

James and Don, and others,

I agree that the back is mainly energized by air conduction from the top's vibrations. However, the last two guitars I built used mahogany side support braces that were two kerfing pieces wide, and as deep as the kerfing, itself, about .200", to provide a continuous inside perimeter edge. I ran them from the top to the back, knotching the kerfing at each end, so that they butt directly against the top and back plates. Tapping over them gave a deep resonant tone compared with tapping over the perimeter in between. It seems that these braces transfer energy to the back, too.

I have never lost sustain thinning the back braces, and even though the volume does drop a little when held against the body, it is still louder than it would be with a rigid back. I have experienced this time and time again while re-voicing guitars.

I have no experience with a Lowden guitar, except that Greg Brown played one in a concert here 5 years ago. I introduced myself beforehand, and showed him one of mine, which he played for the first set, the Lowden, the second.   The real difference in amplified sound was that the individual strings in chords on mine stood out clearly to be heard amongst the others. The Lowden's chords had jangled strings, with little definition.

That is one thing that parabolic bracing gives to the guitar's sound--clarity. Individual notes can find their expression on the top without interference from uneven shapes of braces.

I think that sustain comes from the streamlining, too.    Having less brace mass, alone, leads to slower die out, and corners, ridges, peaks all absorb energy, instead of allowing it to flow through.

Another aspect of brace shaping that influences sustain is the shape of the arms of the X brace. If they are not curved/tapered/half parabolic all the way to the kerfing, the energy eminating from the bridge will be dampened, as the entire X seems to need to resonate with itself. I am still exploring some of the puzzling relationships of the X. Harmonics are greatly enhanced in volume and sustain, too, by careful shaping of the arms.

Back to the back, as far as looseness of the back braces, I thin them, keeping a parabolic profile in length and cross section, until I can feel each string's ring vibrating over the braces as well as over the spaces in between them. I described the layout of these rings earlier on this thread.

Scott

Engineers would use pipe for drainage that had those corregations in them. I wondered as to why they didn't use pipe with a smooth inside, but they said that the bumpy surface created turbulence, which slowed the flow down, but also allowed the water to move better in some cases.

Don, we consider this kind of stuff in underwater vehicles. Flow is an interesting thing, but not my line of expertise. I'm not sure how much a tubular vehicle moving through the water can be compared to a tubular slug of water moving through a tube, but if it compares at all, then I would agree that the bumpy surface may be preferable. When you have laminar flow across a body, some interesting things can happen. Sometimes vortexes and eddy shedding come into play, and a little bump or perturbation goes a long way to get flow moving evenly and better overall.