Stiffness measurement of the top

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Dave M
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Stiffness measurement of the top

Post by Dave M » Thu Jan 08, 2015 8:47 am

This may not be quite the right place for this but I couldn't see a more appropriate one.

Having finally finished my Fleta type classical based on the book design I am trying to explore the issues with it.
Today I have been measuring the stiffness of the top in order to get at the monopole mobility. The figure I get for mobility is improbably high, particularly given the low volume of this instrument so I am trying to check the whole process. It may be my measurements or my arithmetic...

My question is: what sort of deflection do people get for a given loading? I have been seeing deflections of about 0.02 to 0.16 mm for loadings of around 100 g to 500 g. Is this the sort of figure most makers are seeing? Is it in the right ball park or am I doing something really silly?

Thanks in advance Dave
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Re: Stiffness measurement of the top

Post by Trevor Gore » Thu Jan 08, 2015 8:30 pm

Craig posted a few numbers in this thread, which might help, but read the whole thread.

I get deflections of ~0.2mm for loads of ~1.2Kg force (~120 N). Make sure you have the loads in proper units of force, e.g. Newtons and the right zero count for the deflection in metres.

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Re: Stiffness measurement of the top

Post by Dave M » Fri Jan 09, 2015 6:37 am

Trevor thanks that was just what I needed.

Dave
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Re: Stiffness measurement of the top

Post by Woodsy23 » Mon Jan 12, 2015 12:26 pm

Trevor,
I am also ineterested in the top stiffness you are getting with your guitars as I am attempting to build your falcate SS.

Are the deflection and load you quote above only typical for your Fleta type classical guitars? If so, what are the typical deflection and load for your falcate SS design?
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Re: Stiffness measurement of the top

Post by Trevor Gore » Mon Jan 12, 2015 6:41 pm

Conventional classicals (including Fleta's) run out at ~ 20x10^-3 s/kg, (Fig 1.7-9) which is what the numbers given above refer to. My falcate SSs run out at ~18x10^-3 s/kg (Fig 1.7-8), but the uncoupled top resonance is ~20Hz lower.

You should be able to figure it from there! :D

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Re: Stiffness measurement of the top

Post by Woodsy23 » Wed Jan 14, 2015 11:29 pm

Trevor,

I think I can work out the answer using equation 1.7-1 but I'm not sure what the typical uncoupled top frequency would be. From the table of test data in the link to Craig's post that you gave above, I see that the uncoupled top frequency for the Fletta classical guitars varies from 203 Hz to 230 Hz, say an average of about 215 Hz. In Section 1.7.3 you mention that several of the guitars in Figure 1.7-9 have a T(1,1)2 close to 190 Hz so the uncoupled top frequency would be less than that. So, is the uncoupled top frequency of your steel string is about 195 Hz (20 Hz lower than 215)? From Section 2.3.8 and Fig 1.7-6 of the design book, I would have thought that it would be under 170 Hz. If the guitar discussed in 2.3.9 is your design, it has an uncoupled top frequency of 157 Hz.

I'm a little confused. Hope you can help.
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Re: Stiffness measurement of the top

Post by Trevor Gore » Thu Jan 15, 2015 3:09 pm

Woodsy23 wrote: From the table of test data in the link to Craig's post that you gave above, I see that the uncoupled top frequency for the Fletta classical guitars....
Craig's data was for Selmer/Maccaferri (gypsy jazz) guitars. I referred to that post because Craig shows quite a bit of his workings, which I thought might help Dave out. (Plus there were a couple of other bits of discussion which were interesting)
Trevor Gore wrote:Conventional classicals (including Fleta's) run out at ~ 20x10^-3 s/kg, (Fig 1.7-9) which is what the numbers given above refer to.
The numbers I was referring to were these:
Trevor Gore wrote:I get deflections of ~0.2mm for loads of ~1.2Kg force (~120 N).
..which together with this:
Trevor Gore wrote:Conventional classicals (including Fleta's) run out at ~ 20x10^-3 s/kg, (Fig 1.7-9) ...
...means you should have enough info to figure things out.

For a Fleta, you have target monopole mobility and you have k (force/deflection). Using Eqn 1.4-29 you can get m. Using Eqn. 1.7-1 you can get f (all for a Fleta).

...then...
Trevor Gore wrote: My falcate SSs run out at ~18x10^-3 s/kg (Fig 1.7-8), but the uncoupled top resonance is ~20Hz lower.
... lets you convert the number to a Falcate SS target, which should be in the ball park.

If this doesn't make any sense when you try the numbers, get back and I'll be a little less cryptic!

(It's never a bad thing to run the number backwards and forwards a bit so you get a feel for how thing behave and their sensitivities)

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Re: Stiffness measurement of the top

Post by Woodsy23 » Fri Jan 16, 2015 6:46 am

Trevor,

Ok. I understand now.

So if the conventional classicals have a typical monopole mobility of ~ 20x10^-3 s/kg and a deflection of 0.2mm for a load of 1.2 kg = 12 N (I think you meant) or 11.7 N more accurately, the typical uncoupled top frequency would be 187 Hz. For your steel string, the typical uncoupled top frequency is 20 Hz lower at 167 Hz and the typical monopole mobility is ~ 18x10^-3 s/kg so the deflection for a load of 11.7 N would be ............................................................... 0.2mm - the same as the typical classical. Now I have the answer to my original post - I think.

I enjoyed the challenge you set. Maybe you could set a quiz question each week on this forum, based on your design book, and award prizes for the best answers. LOL.
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Re: Stiffness measurement of the top

Post by Dave M » Fri Jan 16, 2015 8:14 am

Woodsy

Oh no no no! This is hard enough already. I have enough questions buzzing around without someone actually thinking up new ones!

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Re: Stiffness measurement of the top

Post by Trevor Gore » Fri Jan 16, 2015 9:16 am

Well done Woodsy! I think you've got it. Of course, it would help if I put the decimal points in the right place. Sorry about that.

Things do vary a bit, in real life. If you can get to around 15 ms/kg whilst keeping the structure under control and the resonances in the right places you'll be doing fine. This takes some high quality work, particularly in making sure all your glue joints are really sound (if you'll pardon the pun).

Good luck with it all and when you've finished, post some pics in the book builds gallery.

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Re: Stiffness measurement of the top

Post by Deems Davis » Fri Feb 19, 2016 5:04 am

I just finished building the rig to measure monopole mobility nearly identical to Trevors latest version. After fiddling with the spreadsheet and pouring thru the threads on the topic. I'm getting what I feel are accurate numbers.

I'd like to verify some things regarding the units that are input into the formulas.

deflection = mm .266 in my case
Load = newtons 12.106 using 1235 gms
Freq = hz 151.4 (plugged Top)

The implication that I get from fig 1.7-8 in the book and discussions on the threads, is a higher mobility (s/Kg x 10^-3) implies a 'better' guitar. "better" in this sense meaning more responsive.

just doing some sensitivity checking of the formulas, indicates that the greater the deflection --- the greater the mobility. In my case this came out to 20.9 s/Kg x 10^-3. So I could look at this and compare to fig 1.7-8 and figure I'm the world best guitar builder (SS Falcate). And could conclude that I should just build tops that are thin and deflect well.
However... my bridge rotation is only 1.5-1.6 degrees (the bridge is too wide in order to properly position the saddle due to measurement error) And the positioning of the main freqs is not optimal due to other mistakes (getting the soundboard too thin is one of them).

So is it better to say or think in terms " A high mobility is a great indicator of the responsiveness of the guitar IFF AND ONLY IFF the other criteria for modal tuning are satisfied"? And taken by itself could be misleading?

I'm a new builder and as I try to get my head wrapped around these concepts I sometimes need to echo back / reflect my understanding of them in order to solidify things in my head. I appreciate the time and feedback that folks in this forum provide.


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Re: Stiffness measurement of the top

Post by Trevor Gore » Fri Feb 19, 2016 9:08 am

See Design Section 1.7.7. paragraph 4.

In Chapter 2 (somewhere) I talk about responsiveness, even timbre, no dead notes (wolf notes) plays in tune to the even tempered scale etc. etc,

There's no one single "measurement" that indicates a good guitar for a particular purpose; your "perfect guitar". Design Chapter 2 goes heavily into placing of modal resonances and monopole mobility, whilst Chapter 3 talks in depth about the things required to articulate what is meant by a "perfect" guitar; playability, musicality, structural integrity. You need all of those things. Getting the modal resonances right and reasonable monopoly mobility is a good start, but that's when you start to run into the tuning problems and wolf notes...

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Re: Stiffness measurement of the top

Post by kiwigeo » Fri Feb 19, 2016 9:29 am

Dave M wrote:Woodsy

Oh no no no! This is hard enough already. I have enough questions buzzing around without someone actually thinking up new ones!

Dave
The propeller on top of your hat isn't spinning fast enough!!
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Re: Stiffness measurement of the top

Post by Woodsy23 » Sat Feb 20, 2016 10:16 am

Deems Davis wrote:I'd like to verify some things regarding the units that are input into the formulas.

deflection = mm .266 in my case
Load = newtons 12.106 using 1235 gms
Freq = hz 151.4 (plugged Top)
The monopole mobility you calculated from these numbers is correct, by my spreadsheet.
Deems Davis wrote:In my case this came out to 20.9 s/Kg x 10^-3. So I could look at this and compare to fig 1.7-8 and figure I'm the world best guitar builder (SS Falcate). And could conclude that I should just build tops that are thin and deflect well.
However... my bridge rotation is only 1.5-1.6 degrees
In my falcate ss build, I got similar mobility numbers to you. I did three tests and obtained 19.0, 21.4 and 22.5. I took deflection readings at several loads and graphed load versus deflection in Excel and got Excel to determine the gradient of the line of best fit. I recall that my rotation at the bridge was also in the order of 1.4 to 1.6 degrees (I can't find my records). When I got those numbers, I also thought for a second that I was the greatest guitar builder in the world (Can I be joint best guitar builder in the world with you? LOL). But of course we both know that can't be the case.

In sections 4.9.2 and 4.11 of the white book, Trevor talks about the importance of good gluing practice and workmanship and how his guitars end up better than his students. I would like to speculate that high quality workmanship, where all the joints are well prepared, fitted and well glued etc, actually results in more rigid construction and that means reduced deflection of the top for a given load. The mobility is reduced correspondingly. However, the higher quality workmanship is also likely to reduce vibrational damping and that will enhance responsiveness. It doesn't make sense that you can achieve a more responsive guitar by poorer workmanship that results in a less rigid top with greater deflection.

So, the challenge is to achieve the monopole mobility number in Figure 1.7-8 for the Falcate SS but in a guitar with a high quality of craftsmanship (which works against increased top deflection).

This is all speculation and hypothesis on my part, of course, and I've lived long enough to know that just because something sounds logical and reasonable doesn't mean it is true.

Trevor's answer to my question in this post is also worth reading.
viewtopic.php?f=33&t=6886
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Re: Stiffness measurement of the top

Post by Deems Davis » Sat Feb 20, 2016 11:38 am

Thanks Richard,

It's nice to get feedback as it helps solidify what I think I've learned. Being a new builder my skills are definitely "immature" I'm mid way through another SS Falcate should have it boxed in a couple of weeks and am anxious to see if this one come closer to the target.

Deems

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