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Im just wondering what basic pinciples any of you might use for doing this.

I know its not quite so simple and definitive......but just for example you have a guitar which you perceive to be

a too bright and you would like to mellow the tops a bit
b not bright enough and treble needs a boost (classical problem)
c too Bassy/boomy
d needs some bottom end boost


Im also curious about how a soundport /hole size would affect the situations above, and the height of the strings above the top plate. 1/2 seems to be the target from what I've read...but what would be the difference if if two identical instruments had string heights of say 3/8 and 5/8?

I know thats lot of questions !
Thanks
Steve

"a too bright and you would like to mellow the tops a bit"

Try thinning the top behind the bridge toward the tail block.

"b not bright enough and treble needs a boost (classical problem)"

Thin the top outside of the bridge wings toward the edges.

"c too Bassy/boomy"

Hard to do at this point, unless you want to add some bracing. I'd think a PMTE would help, but I've never used one as an add-on.

"d needs some bottom end boost "

Assuming your 'main back' tap tone is more than a semitone higher than the 'main top' tap tone, try thinning the back bracing a bit.

"Im also curious about how a soundport /hole size would affect the situations above..."

A larger hole, or the addition of a port, will normally raise the 'main air' resonant pitch, and make the resonance itself stronger and more 'focussed' in frequency. Usually this makes the sound 'brighter', more 'forward' or more 'open', depending on how you use the terms.

Needless to say this is not an exhaustive catalog of ways to fine tune a completed guitar. Don't blame me if you try these and they don't work: guitars are all different, and respond differently to changes.

"...and the height of the strings above the top plate. 1/2 seems to be the target from what I've read...but what would be the difference if if two identical instruments had string heights of say 3/8 and 5/8?"

I'm in the middle of an experiment on that now (and, yes, I've been stuck in the middle of this one for a while: these things can take time!). My statistics guru has decided I have enough data to say something useful about the height of the strings off the top so...

Basically, raising the strings off the top increases the static torque on the plate, and also the leverage of the tension change signal an octave above the fundamental. The extra static force distorts the plate, and may make it harder to drive, particularly in it's fundamental, or 'main top' mode configuration; the one that looks like a loudspeaker. Thus a high saddle can cut down the power of the fundamentals of the strings. OTOH, the extra leverage increases the power of the second partial, and possibly some of the higher partials as well.

That's what I'm seeing in measurements of the spectrum of the sound output of the experimental guitar, which is a classical. I need to do some listening experiments with the recorded plucks to see if anybody can actually hear the differences I'm measuring. A steel string might be effected more than this classical, or less: I can't say on the basis of the data I have now. I rather suspect that the effects will vary in strength depending on the guitar. I'll note that in this case I raised the strings from 11mm off the top (just under 1/2") to 18mm (3/4"). Don't try this at home! A smaller change would be likely to have less of an effect. Don't forget that raising the strings off the top tends to peel the bridge up quicker, too, and puts more force on the front of the saddle slot!

Thanks again alan! can someone explain what a PMTE is ? thanks Jody

It's a little brace tucked in behind the bridge plate. It stiffens the bridge plate area and adds mass.

http://www.proulxguitars.com/buildup/build6.htm

Google made me look smart.

Something that can also work for the too bassy problem... at least on a Steel string...

It may not be that the guitar is too bassy, just that it doesn't have enough treble and midrange..... like say the guitar is still fairly stiff, but has a massively heavy bridge and bridge plate.... which can give it a really strong "Whomp"... (Like using brass bridge pins for example..)

1st... Change the strings... It could be that the 80/20's or PB's are dead... and this sort of thing happens -- strings go dead and the guitar sounds thumpy and wooshy... Put on new strings and it sounds good again.. or if you are really feeling wild.. Change to Cleartones and it goes "Ka-Zing!"

If the strings are fairly new....

2nd... Think about what sort of bridge pins you are using... Are they really heavy ones? If so, replace them with lighter pins...

Then.. If this doesn't help....

Thin the thick "Hump" on top of the bridge behind the saddle slot a little bit-- this seems to bring out the trebles some... Then thin the wings of the bridge a little to balance out the midrange.

In this case, you aren't actually decreasing bass as much as increasing treble and midrange...

Of course, you gotta be careful fiddling around with the bridge... as if you thin too much... you can leave funny crease marks on the top.... make it sound really weird and hinky and then you have to go pull the bridge and start over again! Not too much fun.

Thanks

John

Something else that I found contributed to "Too Bassy" is large gaps between the bridge plate and X-brace legs. Shimming these gaps made the guitar sound "Right" again

John

Fillipo,

While I am not Al... I did send some emails back and forth to William Cumpiano regarding building philosophy.

He tends to run in the "Don't make adjustments afterwards" camp.... I do understand his rationale, and I certainly believe this is a worthy goal...

For example... If your OOO guitar strung with Light strings needs an "Optimum" weight bridge + bridge plate combo with your particular stiffness top... Wouldn't it be best to target that "Optimum" weight and footprint rather than to make it either larger or smaller and then have to figure out how to "Voice" it to correct this problem? If you are shaving the bridge afterwards (Like I end up having to...) You never figure out what the "Right" bridge/plate footprint and weight combination really is... It is pretty hard to weigh all those shavings and sanding dust to figure out what you took off...

Same with top thickness.... If you voice and voice and voice the top... How do you know how thin it really ended up being in the different places for the Initial stiffness, thickness, deflection, X-angle, etc....

In that sense, I philosophically agree.... that we should strive to perfect the "Design." On the other hand... I spent a year as a High school teacher. I often wished that students would go back and correct errors and try to understand what went wrong before moving on to the next thing... Only to foul up the new material in the same way because they didn't get that 1st part right....

Personally... I am not good enough to get a guitar to come out right on the 1st try... and since I can't live with the way it sounds.... I end up fooling with it afterwords to try to get the sound balance better than it initially was.

But.. that is just me.

Thanks

John

A second thanks to Alan
That was an interesting read as usual!
John Im in complete agreement that I would like to be able to know in advance how the guitar will sound
But that takes alot of guitars...until then i might have to experiment. Yes strings pins etc are first line of enquiry
I dont agree that voicing after the box is built invalidates data...yes you probably wont weigh sanding dust but if you know dimensions at glue up you still have a clue what might be an improvement.....or not.
Im pretty sure that classical guitar makers and violin makers have been doing this forever
with all the best plate voicing in the world, they must change at glue up and it is still the glued up box,neck strings and all that we play and listen to.. Im not sure that I believe that a guitar "opening up" will fix a fundamental problem in its sound palette....
I wanted some more Bass on a small guitar I just built. The top is plenty thick so I planed a couple of swipes off the tone bars and I think it worked....or did it ?

I've been thinning the edges of the lower bout of my guitars after the box is glued up but before the binding goes on for a couple of years. I tap the top as I'm doing it and listen for changes. I think it's been a big help in getting the tone I'm looking for. You can see how thin you are getting by looking at the edge of the top. Got the idea from Dana B's voicing article.
If anyone wonders what role the edge of the top plays in tone just hold the closed box up and tap on the top, then rout the binding channels and do it again. It'll sound like a cigar box. After the binding is on it sounds like a guitar again.
TJK

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I try to get the voicing 'right' before I put the box together, if I can. For one thing, it's a whole lot easier to thin the top or shave braces when you've got the thing on the bench that when you have to reach inside. I only tend to do post hoc voicing if there's a real problem, and the idea is to have fewer and fewer of those as time goes by (right?).

Terrence:
If you find yourself thinning the edges all the time, why not do it before you put the top on the box? You've got more control, it's easier to do, and you can record your tap tones or whatever to make it easier next time. I used to thin out behind the bridge a lot before binding, and then I just started doing it on the bare top.

Since Terence got the idea from Dana and I'm doing the same thing (actually I learned it from John Slobod when we were both working at Pantheon), I'll take the liberty to respond.
It's impossible to do it before hand. I know John (and by extension Dana since John learned it from Dana) was looking for a certain "springiness", especially at the bridge location on the top. Tapping there one should get a very specific "feedback" from the top, like it's bouncing back to the finger. Hard to explain but doing it will clear things out.
I'll talk for me and will say that it's different for every guitar. Each has a "sweet" spot, thus knowing how much to thin the edges on the lower bout before putting the guitar together would be a very wild guess. Recording the taptone would be useless, as the pitch here is irrelevant (as it is throughout the building process IMHO).

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Laurent
Do I take it then you do this before glueing the bridge on?
I found that quite amusing and would love to hear more but that probably another thread!

Before routing the binding/purfling channels.


It's one school of thought http://www.youtube.com/watch?v=sUN2x5iK8LE&feature=related

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

The before and after are not mutually exclusive of course. I started thinning the edges and extreme lower bout after the guitar was together. Lately as I have been using the Chladni testing I have been more able to get it tuned so that when I join the top and back to the rims it is more or less right where I want it(bouncy-ness wide, at the bridge). Not all the time of course and when it isn't I go ahead and sand away. I always sand the the top first and get it where I want it and then work on the back to match it to the top. On the back also, I leave the upper bout alone. I want that heavier and stiffer, like the upper bout of the top. There is alot of string pull acting there as well. Sanding the lower bout and edges, as with the top, will effectively drop the back pitch enough in my experience.

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Burton
http://www.legeytinstruments.com
Brookline, MA.