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      It seems to me top plates are adequatly discused whenever guitar talk arises , what I am seeing less of is  back plate discusions. I am beginning my first solo build using  mahogany back and rims. I realize the action of the back is important in coupling the system and producing a proper  air pumping body ,I could use some ideas on back plate thicknessing and preparation , is there ideal thickness parameters?  Do any of you  experts  thin the perimeter of the back  as some do with the top plates ?does thickness vary with type of wood?or perhaps with the desired final sound objective? is quartered grain an important consideration ? any help is greatly appreciated.. thanks Jody

Some believe that back is semi passive and others that feel it is totaly passive. in any case if you make that back move like the top you end up with an air pump that has unwanted trubulance that will issues.

This is a personal opinion, so take it with a grain of salt.

I like for my back plates to be as rigid as possible but as thin as possible also. I make up for the thickness of the back by adding bracing. I use an x-brace for the lower bout and 2 ladder braces for the upper bout. The reason I want the plate to be thin is to reduce weight and for no other reason. I am in the camp of people that believe the back is a reflective body and I want it to refelct the sound of the top.

In general, I like for my back plate to be somewhere between .08 and .09

 

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Reguards,

Ken H

From a perspective of playing, doesn't the back plate rest against the body of most steel string players, and wouldn't that have a huge impact on how the back reacts, thick or thin, as a reflective plate, and doesn't that pretty much reduce it to a passive, reflective plate?

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Waddy

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Sound Clips of most of my guitars

i think thats the idea behind double back guitars.
ive seen it on some classicals...
what you have is a normail guitar BUT it has an extra back plate which is inside the body and is lifted just a bit from the "ouside back plate".
what you get is an "inside back" that is free of any "beer belly" dampening effects.
and basically, the "outside back" serves nothing but to act as a buffer zone between the player and the "inside back".
i have heard,seen and played more than a couple "double back" guitars, of different woods and different makers, and to me, while they all felt (obviously) heavy, they all had outstanding volume and projection.

Udi.

I spent many hours poring over every post I could find on the web by Alan Carruth related the coupling between backs and tops. His posts contain so much information it took me awhile to begin to understand what he was conveying. But the essence that I took from it was that with the box closed up, with the bridge on, you can get the back and top working somewhat together if they end up with the resonant frequency of the back about a semitone above that of the top. If they're too close in resonant frequency, you're asking for a wolf tone -- where the back and top work too well together causing a loud note heavy on the fundamental with no sustain -- too far apart and they don't work together.

Disclaimer: I was mainly concerned about resonant frequencies and did not pursue ideal glitter patterns as Alan does, and my experience with this is limited to one guitar.

On my second guitar, I used glitter to indicate the resonant frequencies of the top and back. In the process I found that an x-brace made it much easier to get the back at the desired resonant frequency, as mentioned by Alan. I got them close to the same before gluing them to the rims in anticipation of the bridge causing the top to drop by about a semitone, which is exactly what happened.
It was a fascinating process and very successful. The guitar, though not heavy on bass, is full in tone with good balance. Sort of a complete sound, without weak lower notes that I often hear in smaller body guitars. The lower notes are clear, even and separate. The overall process was essentially to voice the top, determine its resonant frequency with glitter patterns, then voice the back using glitter patterns so back would have the desired relationship with respect to the top.

As for damping of the back with the player's body, I notice some difference if I damp the upper bout of the back as if I were standing while playing, as is often the case with flatpicking. Damping the entire back made a big difference. But while sitting and playing fingerstyle, placing the bass part of the back against my scrawny chest had no effect that I could discern.

I intend to pursue this further, striving for more desirable glitter patterns.

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now known around here as Pat Foster
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http://www.patfosterguitars.com

Glad that worked out for you, Pat. Next time, try making the top a little stiffer: you don't have to lighten it up to get the bass, and a stiffer top gives you better trebles.

My feeling these days (and it could change at a moments notice) is that you want the 'main back' resonant pitch to be about a semitone higher than the 'main top' pitch, so that the two can work together to reinforce the bass range. Above that you want the back to be as stiff as possible to be a good reflector. Of course, this is not possible; you can't have a back that both moves and doesn't move.

The thing is that, as near as I can figure out, the lowest back resonance is the onlt one that can actually contribute tot he output of the guitar, and it's only really effectrive if it's 'tuned' to work with the top. Otherwise all of the higher-order back resonances seem to be 'losers', taking power out of the top and not putting it out into the room anywhere near as effectively as the top can.

What the back resonances do is give you some 'tone color'. Those dips in the spectrum make the sound more 'interesting', so you don't really want to eliminate them entirely: if you do, you get an Ovation.

Thus the idea seems to be to minimize the higher resonances of the back. You do that by making it reasonably heavy, so that even when it's moving it isn't moving much, and using a low damping wood, so that the back resonant peaks (and thus the 'dips' in the spectrum) are narrow. Thus the rational behind woods like Brazilian rosewood that are dense and have low damping.

This does not address the issue of whay mahogany sounds so good. I will say that some of the mahogany I've tested of late has had lower damping than I was lead to believe. Also, I think the lighter weight woods tyend to move more air and work more like a top in some ways.

The nice thing about studying guitar acoustics is the job security: you'll never figure it all out.

Infinite variables!

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

well said, Al!

_________________
Reguards,

Ken H

[QUOTE=Alan Carruth] Glad that worked out for you, Pat. Next time, try making the top a little stiffer: you don't have to lighten it up to get the bass, and a stiffer top gives you better trebles. [/QUOTE]

Alan,

I and many others are in your debt for the encouragement and the information you share with us. Many thanks!

Pat

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now known around here as Pat Foster
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http://www.patfosterguitars.com

[QUOTE=Hesh1956] [QUOTE=Alan Carruth]

This does not address the issue of whay mahogany sounds so good. I will say that some of the mahogany I've tested of late has had lower damping than I was lead to believe. Also, I think the lighter weight woods tyend to move more air and work more like a top in some ways.

[/QUOTE]

Al I always really appreciate your posts too - Thank You.

Based on your statement above why would one not wish to brace a light weight back, such as Mahogany or say Koa, much like a top would be braced with an X, tone bars, etc? I know some brace backs with an X pattern but they do so to support the dome shape not to make the thing ring.[/QUOTE]

I'm sure Al C will give his usual wise thougts.

The main difference that occurs to me is that the forces on the back are different than the top - no big thing called a bridge in the lower bout area with strings attached trying to pull it off The main forces from the strings are via the pull on the bottom of the neckblock and the air movement in the box. So like the top, you would have to go back to first principles to figure out what balance of structural versus tonal/performance bracing you would do.

I have generally seen two main reasons given by builders for X and hybrid X/ladder back bracing schemes. One is the doming control and the other is the easier "voicability" (specialy selected that one for a Howard K tilt - "Well shiver me timbers and krf me linings!").

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

krf being Pirate talk for kerf of course

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

The main reason you don't brace backs like tops (aside from tradition) is that softwoods and hardwoods tend to have different stiffness ratios. Softwoods that are particularly stiff across the grain tend to have a long/cross ratio of about 8:1, and the really floppy ones can get to 100:1! The Indian rosewood samples I've tested tend to be in the range of 4:1 to 8:1. The IRW back that is the floppiest across the grain has about the same ratio as the tops with the highest cross grain stiffness.

The stiffness raio and the brace angles work together to determine the shapes of the resonant modes of the top or back. If the wood has high cross grain stiffness you can run the braces more 'along' the grain and get the same sort of mode patterns. If it's floppy, you have to flatten out the brace angles a bit.

Of course, the back and top do different jobs. Also, as has been pointed out, the back doesn't have a bridge on it, and that's the heaviest brace of them all. Still, there is no reason why you could not try to 'tune' the back to have the same modes as the top. I tried that on a couple of guitars once, and it was an interesting exersize. It was a lot of work, but I know more now, and may try it again at some point.

  I appreciate everyone's opinions on this subject, seems like I have at least a few clues to go on  now !  I have  read several of Mr. Carruth's articles in some back issues of GAl publications. not sure I am getting it at this time .

   ( very,very)   basicly  ( i guess )  you assemble the back ( or top)plate with bracing ( ladder, cross, or otherwise). then you subject the plate to a range of  frequencies (sine tones?). by scattering glitter flakes on the plate , we are able to observe which frequencies create " closed " patterns on the plate by observing the glitter aligning in the nodal areas . If the " complete" patterns do not fall within the parameters of tones or semitones within the 440 A standard tuning(I might have gotten that from another tuning system) , we are then able to adjust or alter ( lowering ) the resonating frequency of the plate by tuning (  shaving braces and thicknessing the plates?) is this it ? ( basicly?)  LOL !   thanks Jody

The Chladni patterns in the 'free' plates tell you two things: the frequencies give you an idea of the stiffness/weight ratio of the plate with bracing, and the patterns tell you about the distribution of stiffness. As far as I can tell, the shapes of the patterns are far better predictors of the tone of the finished instrument than the frequencies. Remember, what counts in the end is what the _assembled_ parts do, and it's not easy to predict the pitches of the assembled modes from the free plate data. For one thing, the degree of 'closure' of the free plate modes alters the extent of the pitch changes on assembly.

Mark Blanchard has done a lot of work looking at the frequency relationships between different 'free' plate modes, and finding correlations with the tone of the assembled instrument. He gave a talk on that at the last H'burg show, and kindly sent me a copy, which I need to study a lot more before I can say much about it. Knowing Mark, it's important work.

If the tap tones of the _assembled_ box fall on or near pitches of the scale it will effect the sound of those notes for sure. One study (by Wright) avers that the overall timbre of the guitar is not effected much by even fairly large changes in these pitches, but it was based on a relatively simple computer model of the guitar (really the only way you could do it) and I'm not sure I agree with all of his conclusions. Again, the pitches of the free plate mode have only a loose general relationship with the assembled pitches.     

   o.k.  so  , if I am following you on this, the importance of         &nbs p;     " complete" chladni patterns is, they are an inidication of how well the plate resonates as an isolated  unit and how well it MAY transmit sound  from the strings to the body cavity .  so if the nodal lines are incomplete ,would you work on the bracing or the thickness of the plate in an attempt to connect the (glitter) dots?

                                                               Thanks  Jody

I work on the bracing.

This is always an interesting discussion and I enjoy reading and learning other's opinions on it. One of the things that I learned from Al was that a guitar is a system... no one thing will make much of a difference, but when you find a system that works for you, then stick to it.

Basically, this is what I have done. I now have a pretty good system for building guitars. Some of them I discuss openly and others I keep to myself as a trade secret type of thing. I like the sound that I am producing and I feel like my tops will stand the test of time and not develop too much of a belly over time. I am satisfied that the backs I produce are well suited to my system and whether it is a case of a better dome, or a stiffer back, I like the x/ladder combo I have developed. I have looked at others work who use similar bracing, and mine is slightly different, but to me it makes a significant differnece. The thing is, it might not work for your guitar if the rest of your guitar isnt exactly like mine.

I, too, appreciate everything Al has shared on his web site and in the forum. When I was a beginner, I read every page of it...studied some of it until I thought I had a good understanding of what he was sharing, and then used parts and pieces of it to develop my own style and system. It is a GREAT place to start looking for info on building guitars!

 

 

_________________
Reguards,

Ken H

Ken:
There are no 'secrets', just stuff you don't know yet.

 Alan , Thanks for taking the time to respond to my questions ! I think I am beginning to understand the qoals of free tuning . I also want to thank everyone who responded to this thread and shared thier hard earned knowledge , it truly is a learning experience ! thanks again Jody