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Stiffness or low weight?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=24174		 Page 1 of 1
Author:  Alexandru Marian [ Wed Oct 14, 2009 11:00 am ]
Post subject:  Stiffness or low weight?
Having finished a lot of deflection testing on nearly 20 different tops or various densities, I have some ideas sworming within :)

Let's say you make a good or excellent guitar using a top of medium or high density.

Your next top is going to be light weight, and deflection testing shows a better stiffness to weight ratio than the piece before.

Great, but how are you going to take advantage of this?

1. Thin the top to same deflection measurement/stiffness as the one before. This is going to end up lighter than the one before.
2. make the top the same weight as before, this is going to be stiffer.

It seems either variant is an improvement. Which one would you choose? Or maybe a little of both? Which trend gives more power or volume, which gives more complexity and beauty of tone?
What if the "free stiffness" ruins the balance and you get a loud but too bright guitar?
Author:  David LaPlante [ Wed Oct 14, 2009 11:18 am ]
Post subject:  Re: Stiffness or low weight?
I think of the correct degree of stiffness as the constant, at least in terms of a goal considering that string pull is the same (with a little variation given the type of strings) from instrument to instrument.
If I can achieve this same correct degree of stiffness with a little less mass, all the better.
Approaches such as doming the top etc. are aimed at this and if you have a particularly light and stiff piece of Spruce, that is an advantage.
Author:  Brock Poling [ Wed Oct 14, 2009 11:29 am ]
Post subject:  Re: Stiffness or low weight?
Stiffness is my primary measure.
Author:  Markus Schmid [ Wed Oct 14, 2009 12:08 pm ]
Post subject:  Re: Stiffness or low weight?
Alexandru Marian wrote:
1. Thin the top to same deflection measurement/stiffness as the one before. This is going to end up lighter than the one before.

Yes, I would go for the same stiffness. Less weight will give you more volume (easier to excite top). More stiffness will change the quality of sound (not necessarily better or worse, but always different).
Author:  Alain Moisan [ Wed Oct 14, 2009 12:12 pm ]
Post subject:  Re: Stiffness or low weight?
There is one aspect that raises a question (at least for me!): will a thiner top of the same stiffness will hold against top deformation just as well as a thicker one?
Author:  Steve Saville [ Wed Oct 14, 2009 12:26 pm ]
Post subject:  Re: Stiffness or low weight?
When ever you put an "or" between two conditions, make sure that you should not be putting and "and" there instead.
In this case, why not choose low weight and stiff?

These "or" statements have become to me what I call a suckers choice. I work on not choosing one or the other and choosing both instead.
Author:  the Padma [ Wed Oct 14, 2009 12:30 pm ]
Post subject:  Re: Stiffness or low weight?
Alexandru Marian wrote:
Which one would you choose? Or maybe a little of both? Which trend gives more power or volume, which gives more complexity and beauty of tone?
What if the "free stiffness" ruins the balance and you get a loud but too bright guitar?



Ah Alexander... I am sure others will post to the technical BS (belief syatems) of these questions....

However this to me is the delema of building ... makeing "educated guesses" and then living with them. gaah

and so we learn.

blessings
the
Padma
Author:  Alan Carruth [ Wed Oct 14, 2009 12:37 pm ]
Post subject:  Re: Stiffness or low weight?
Alain Moisan asked:
"will a thiner top of the same stiffness will hold against top deformation just as well as a thicker one?"

It should.

In theory, the main thing that limits how thin you can make a guitar top is the bridge torque pulling it up. My understanding is that the long-term deformation of the top will be about 3x the initial level: if the top sinks .015" at a point 2" in front of the bridge when you string it up, that suggests that the long-term sinkage will be about .045". This _may_ vary somewhat depending on the wood, as some woods do seem to resist long term cold creep a bit better than others. I would not, for example, suggest using willow for a guitar top. I don't think there's a lot of difference in that respect between the softwoods we normally use for tops, though. If that's the case, then all you really need to worry about is that initial deflection, and it won't matter structurally how you get it.

The Young's modulus along the grain of all the softwoods I've tested tracks the density pretty nicely (with some, scatter, of course), in a linear manner. The stiffness of the top will depend on the product of the Young's modulus and the cube of the thickness. Thus, making a low-modulus, and low density top a little thicker to get the stiffness up usually results in less weight. Normally one would expect this to improve the power and high-end response of the instrument, all else equal.

The main benefit of high density seems to me to be a somewhat greater dynamic range. Heavier tops don't seem to 'top out' as much as light ones.

Which way you elect to go will depend on what you're after. On classical guitars I'm always looking for the lowest density soundboard wood I can find that has a reasonably high Young's modulus. If I'm making a steel string that's going to be driven hard in praise services or a Bluegrass band I'll want to find a denser piece of wood. It's not a 'one size fits all' sort of thing.
Author:  Alexandru Marian [ Wed Oct 14, 2009 1:01 pm ]
Post subject:  Re: Stiffness or low weight?
Thanks all for the replies.

It happens that the light spruce that raised my eyebrow is not only stiffer at the same weight, it is stiffer at a lower weight too, while having more thickness. Here are the current results, using a rectangular brace of the same width and length. Only thing varying is the height of the brace (hence the weight, ofc)

Spruce A:

-density: 420Kg/m3
-brace weight: 2.66 g
-brace height: 3.6mm
-compliance: 15.3mm

Spruce B:

-density: 360Kg/m3
-brace weight: 2.4 g
-height: 4.1mm
-compliance: 14.3 mm

I've repeated these tests using a low weight (bending it just a couple mm), also on the whole tops not just a tiny brace, and it's about the same.

The interesting part, however, is that while I can't claim to be able to discern the different compliance when using the same "muscle flex", I can say that, to the touch, without flexing too hard, the heavier spruce has a better feel. It feels "better", more "tensioned" while the soft spruce feels a little bit "rubbery". Maybe it is normal, just by looking at the very low weight it has.

I've made a good guitar with spruce A, also used it for braces, so I am quite curious about using this new batch. It will take more than 1 year till then but in the mean while I bought most of the higher grade tops remaining form that tree gaah
Author:  Kent Chasson [ Wed Oct 14, 2009 1:33 pm ]
Post subject:  Re: Stiffness or low weight?
Alan Carruth wrote:
My understanding is that the long-term deformation of the top will be about 3x the initial level: if the top sinks .015" at a point 2" in front of the bridge when you string it up, that suggests that the long-term sinkage will be about .045".


Thanks Alan, that's a very useful number I've never seen before.

Can you say any more about where that came from? Steel string and nylon? Average of a bunch of guitars? I'm assuming you have to know the original deflection and humidity and let the older guitar equilibrate to the humidity it was built in before you measure?
Author:  Alain Moisan [ Wed Oct 14, 2009 2:14 pm ]
Post subject:  Re: Stiffness or low weight?
Alan Carruth wrote:
Alain Moisan asked:
"will a thiner top of the same stiffness will hold against top deformation just as well as a thicker one?"

It should.

In theory, the main thing that limits how thin you can make a guitar top is the bridge torque pulling it up. My understanding is that the long-term deformation of the top will be about 3x the initial level: if the top sinks .015" at a point 2" in front of the bridge when you string it up, that suggests that the long-term sinkage will be about .045". This _may_ vary somewhat depending on the wood, as some woods do seem to resist long term cold creep a bit better than others. I would not, for example, suggest using willow for a guitar top. I don't think there's a lot of difference in that respect between the softwoods we normally use for tops, though. If that's the case, then all you really need to worry about is that initial deflection, and it won't matter structurally how you get it.

The Young's modulus along the grain of all the softwoods I've tested tracks the density pretty nicely (with some, scatter, of course), in a linear manner. The stiffness of the top will depend on the product of the Young's modulus and the cube of the thickness. Thus, making a low-modulus, and low density top a little thicker to get the stiffness up usually results in less weight. Normally one would expect this to improve the power and high-end response of the instrument, all else equal.

The main benefit of high density seems to me to be a somewhat greater dynamic range. Heavier tops don't seem to 'top out' as much as light ones.

Which way you elect to go will depend on what you're after. On classical guitars I'm always looking for the lowest density soundboard wood I can find that has a reasonably high Young's modulus. If I'm making a steel string that's going to be driven hard in praise services or a Bluegrass band I'll want to find a denser piece of wood. It's not a 'one size fits all' sort of thing.


Thank a lot Alan.

As Kent does, I'm thirsty for some more info around this.
Author:  Todd Rose [ Wed Oct 14, 2009 8:14 pm ]
Post subject:  Re: Stiffness or low weight?
Alan Carruth wrote:
The main benefit of high density seems to me to be a somewhat greater dynamic range. Heavier tops don't seem to 'top out' as much as light ones.


Would you also say that a stiffer top doesn't top out as easily a as less stiff one (bracing included, and let's assume the same total mass of the two tops)? If so, which is more determinant of "headroom" - the stiffness of the top or the mass? If you construct your top (through wood selection, thicknessing, bracing, nomex lamination, and/or whatever) to be light AND stiff, how will it compare, with regard to headroom, to A) another top that's the same stiffness but heavier, B) another top that's the same weight but not as stiff, and C) another top that's heavier and not as stiff?

I welcome anyone's experience and observations, as well as Al's.
Author:  Mike Collins [ Fri Oct 16, 2009 5:21 pm ]
Post subject:  Re: Stiffness or low weight?
I have always asked that question also !

I always went with stiffness -especially for steels.

With Nylons low weight was important !
I replaced a top on a 70's Classical from a very well known & respected maker.
I used a flexible top(like rubber) with low weight because the owner like the looks of it and he's bought 6 guitars from me .
Well -you can add stiffness-that's what cross grain braces do !

I used a 5 fan with 2 cutoffs style(Barbero) and the center of the top was .100-tapered to .080 at the edges.
The # 2 brace brace below the s.h. was angled to the treble side.

the fans crossed the top grain at a 15 degree angle.
The cutoffs at a 45 degree angle.
It sounded great !
Loud & clear with good balance!!
SO--I did a couple more tops like that on my own guitars and -they sounded great !!!!

We can always add stiffness !
my 2 cents
;)
Mike
Author:  Alan Carruth [ Fri Oct 16, 2009 9:30 pm ]
Post subject:  Re: Stiffness or low weight?
Kent Chasson asked:
"Can you say any more about where that [rule of thumb about creep in wood] came from? "

I actually got that from an Experimental Aircraft Association publication on wooden aircraft structures. Where they got it...

Todd Rose asked:
"If so, which is more determinant of "headroom" - the stiffness of the top or the mass? "

I wish I knew for sure. My first inclination is to chalk it up to 'impedance', which will be proportional to the square root of (tension x mass). The denser top will have higher impedance at the same stiffness. The rate of transmission of energy from the strings into the top is a function of the relative impedances of the two. In either case the impedance will be relatively high off resonance, and lower at resonant pitches. The problem is that, while the resonant pitches of the string are pretty easy to predict, the top is more complicated. Impedance can be measured; it's the ratio of force/velocity at a given frequency, and that's the usual way to find the impedance of the top. You need a certain level of impedance mismatch to avoid 'wolf' notes and get some sustain. The problem here is that, so far as I know, impedance is not amplitude dependant, or should not be in a linear system.

The problem is I don't know what's happening to the top when it's being 'overdriven'. I'm assuming it's getting into some sort of non-linear response region, but that's no more than a guess at this point. It will be 'fun' trying to figure out the experiment to look at that.
Author:  Todd Rose [ Tue Oct 20, 2009 7:58 am ]
Post subject:  Re: Stiffness or low weight?
Thanks for your reply, Al. Much appreciated.
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