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Grain Direction in Braces.

jdowning - 3-29-2012 at 10:19 AM

On modern acoustic guitars the grain direction - as viewed in cross section of the braces - is vertical.
On old lutes the grain direction is horizontal i.e. running parallel to the sound board surface. In my opinion the only benefit of this arrangement might be that it provides a stronger glue joint between brace and sound board.

The pros and cons of this topic have been discussed in the past on this forum.

I came across this article yesterday that I thought might be of interest to the luthiers on this forum.

http://finelystrung.com/2011/01/14/stiffness-of-soundboard-braces-h...

farukturunz - 4-3-2012 at 08:17 AM

Quote: Originally posted by jdowning

On modern acoustic guitars the grain direction - as viewed in cross section of the braces - is vertical.
On old lutes the grain direction is horizontal i.e. running parallel to the sound board surface. In my opinion the only benefit of this arrangement might be that it provides a stronger glue joint between brace and sound board.

The pros and cons of this topic have been discussed in the past on this forum.

As everybody knows there are several bracing patterns and several methods applied properly to the soundboards and their results are quite typical and generally matchless. These authentic and original solutions have been put forward by some creative luthiers.

If a maker is following these authentically designed patterns then he/she must copy the authentic applications in brace beams like grain direction.

I’d made an experiment to find out the rigidity difference according to the grain direction long before I established my authentic bracing method which was named as Brace Tuning Method.

I prepared a Spruce block with the dimensions 70x20x400mm. Then I cut it into two parts to obtain two similar blocks whose grain directions were vertical to each other when they lay on their wider sides. The dimensions of each were 40x20x400mm.

Tapping each one in the direction vertical to the wider surface I measured the specific frequencies of them using a piezo mic.

The result was as follows:

A- Cross section vertical- specific frequency: S.fr.vert: 587Hz + 7cents
B- Cross section horizontal- S.fr.hori:584Hz+20cents

So what?

I am sure there are too many luthiers who will annotate the outcome of my frequency difference experiment.

My dear friend djowning, it seems to me that we need to be acquainted with some general concepts of "luthiery" which is the application of some other disciplines like physics, specifically acoustics, static and endurance, etc. Otherwise we can be neither con nor pro. in this very specific case. Of course this is only my personal idea and I am neither con nor pro... I apply both ways: vertical grain and horizontal as well.

Moreover, I am using composite braces sometimes...Especially for bracing some of the Double Soundboards. A very thin carbonfiber sheet is interposed between two balsa welts and it has no grain direction appearantly. I just need to know how rigid it is. To understand the rigidity I am using my "frequency method"

By the way, my inner voice suggeting me reveal the main idea of my method. It has been misinterpreted due to many rasons... I am not "tuning" the soundboards!!! yet, dealing with a "tuning method" and naming my method as "Brace tuning"

Main aim of my method stands on the idea of organizing the differently rigid areas throughout the whole soundbord.

With respect.

jdowning - 4-3-2012 at 12:23 PM

Thank you farukturunz.

Your conclusion - based upon your hands on trials - confirms that grain direction in brace cross section is not significant - at least as far as structural stiffness is concerned. This agrees with the experimental findings of luthier Christopher Martin in his article previously posted.

The question here, of course, does not address the infinite possibilities of 'luthier' ideas of bracing patterns or sound board 'tuning' or alternative non-traditional bracing /sound board materials - based upon scientifically valid data or otherwise.

farukturunz - 4-3-2012 at 11:27 PM

Thank you jdowning for bringing forward such an interesting question which is highly mind reviving.

Regards

jdowning - 5-11-2012 at 12:22 PM

Having been experimenting for some weeks now with a simple method (resonant frequency) to determine speed of sound in Sitka spruce sound board/brace materials (and hence stiffness, acoustic 'goodness' etc) the method was tested this afternoon to verify if the grain direction in a brace cross section (vertical or horizontal) made any significant difference to the longitudinal stiffness of a brace.

As we have seen so far luthier Christopher Martyn from mechanical bending tests has concluded that the cross section grain direction makes no difference to brace stiffness longitudinally. Faruk Türünz has also arrived at the same conclusion by accurately measuring 'tap tone' frequencies of sample braces with both vertical and horizontal cross section grain.

For this acoustic resonance trial, 8 test pieces - four with vertical grain and four with horizontal grain - were prepared from a piece of fine straight grained spruce (construction grade) measuring 46 mm x 39 mm x 719 mm. There was some slight grain deviation from perfectly straight longitudinally but no significant grain 'run out' due to spiral growth of the tree (determined by splitting a test sample).
The results of the acoustic tests - calculating speed of sound velocity of the samples measured longitudinally - based upon the first two harmonic frequencies - gave an average SOS in the horizontal grain samples of 5442 metres/second and 5404 m/s for the vertical grain samples - all measured frequencies in each group of samples agreeing to well within 2% (the majority within 1%) of each other.

So with the results of the SOS vertical and horizontal cross section grain being within 1% of each other (average 5423 m/s) it might be concluded that cross section grain direction in a brace makes no significant difference to its longitudinal stiffness (stiffness being proportional to the speed of sound squared or SOS x SOS). This results, therefore, supports the conclusions reported by Christopher Martyn and Faruk Türünz.

The material selected for this test was found in my wood scrap pile - a piece of a construction grade '2 x 4'. The wood felt 'heavy' (relatively high resin content?) and the density based upon dimensional measurement and weight gave a density of 514 kg/m³ - quite high for spruce. From this the acoustic goodness or radiation factor R = SOS/density computed to 10.6 - a bit too low for 'tonewood' quality material (which should range from 12 to about 17).
The higher density increases the value of SOS and hence stiffness which is fine for a timber used in general building construction but less desireable for more highly stressed structures - including musical instruments.

jdowning - 5-14-2012 at 11:59 AM

The acoustic resonance tests were carried out with samples of typical brace cross section - i.e. narrow in width and deep. See attached image of vertical and horizontal grain samples.

Further support for the conclusion that grain direction in the cross section of a brace does not affect the longitudinal stiffness properties of the wood comes from some earlier acoustic tests on square section, long thin rods to determine the effect of the length of a sample in measuring speed of sound longitudinally.
Clearly the cross section grain direction here makes no difference to the SOS measurement - it is the same in a square section rod regardless of whether or not the grain runs vertically - or horizontally if the sample is laid on its side. See the attached image - the crack seen in this sample is the result of testing for any longitudinal grain 'run out' (due to spiral tree growth) by splitting the sample perpendicular to the growth rings. In this sample there was zero 'run out'.

So there you have it - direction of grain in the cross section of a brace be it vertical, horizontal or any other angle makes no difference to the longitudinal stiffness properties of the brace wood. What does make a significant difference to brace longitudinal stiffness properties is the presence of grain 'run out' and (less so) grain deviation from perfectly straight - both resulting in reduced stiffness properties.
Heavier more dense wood results in increased stiffness but this is undesirable from an acoustic 'goodness' perspective as higher density results in a relatively lower R (acoustic radiation factor) value.

Brace Cross Grain (600 x 282).jpg - 38kB