Mass Loading Sides

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Dominic
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Mass Loading Sides

Post by Dominic » Tue Sep 06, 2011 2:58 pm

Hi Trevor, you mentioned in the aging thread that mass loading the sides was a great way to tune your top frequency. I have reading about this in your book. I plotted the data from Fig 2.3-15 'Effect of adding mass to a medium sized steel string guitar sides' into excel. I then ran a little regression and get an elasticity (slope) of -0.0137. This can be interpreted as one gram of mass will result in a drop in T(1,1)2 of 0.0137Hz. And with this I can calculate the mass needed by entering the starting value and the desired end value for T(1,1)2. That's all just a bit of fun. I am enjoying following all the math and playing with the equations in excel.

However, what I was wondering was how consistent you have found that elasticity on different styles of guitar. Of course it would be easy enough to check myself by testing different guitars and their response to mass. But I wonder what your experience has been.

Also, it is a very linear relationship at the frequencies shown but does that linear relationship hold further up and down the scale. I would assume so but thought it wise to check.
Thanks
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Trevor Gore
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Re: Mass Loading Sides

Post by Trevor Gore » Tue Sep 06, 2011 8:47 pm

Dominic,

The mass loading of sides is amazingly robust and predictable. In Section 2.4.1.1 (para 3) I mention that "...this figure holds true for a number of different body sizes and shapes for both steel string and classical guitars..." Basically, Fig 2.3-15 has held true (in terms of the frequency shift per added mass) for every guitar we have tried it on (lots!), which, frankly, is quite surprising. I would always expect that there would be some effect, but that the gradient holds true on some very different guitars over a range of frequencies is what I find surprising. But that's how it is. Rejoice in this simplicity - it isn't always like that!

One critical thing in execution, however, is that the masses are intimately attached to the sides. i.e. when you bolt the side masses in, seriously tighten the bolts. If you have issues in execution, the masses not moving with the sides is the most likely cause. Also be aware that the bolts can "ease" over time as the wood creeps a little, but is easily fixed by re-tightening them.

Cheers, Trevor.

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Re: Mass Loading Sides

Post by ChuckM » Wed Sep 07, 2011 6:41 am

If I may ask the obvious followup question...

Can we then raise the Top resonance, even if just a little bit by removing mass from the sides ? When using a heavy wood like rosewood or ebony this may be a viable option if the desired change is small and the amount removed doesn't weaken the structure too much. It would be nice to have a few more options to raise the soundboard coupled resonance as almost every other technique lowers it.

FWIW, I did some sampling while working over the weekend and found that simply adding binding and a small amount of purfling raised a top resonance by 15Hz on a Macassar Ebony/Spruce classical. The bridge was glued on for both tests.
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Trevor Gore
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Re: Mass Loading Sides

Post by Trevor Gore » Wed Sep 07, 2011 9:39 am

Chuck M asked:

"Can we then raise the Top resonance, even if just a little bit by removing mass from the sides ?"

Yes, if you take the added mass out of the sides you get back to where you started. The process is entirely reversible. However, unless you started with really thick sides, you are unlikely to be able to remove enough mass of wood from the sides themselves to make much difference. I you look at the slope of Fig 2.3-15 you can work out how much mass has to come out to make much of a frequency change and it equates to a lot of wood. Much easier to build just a little stiff and come down in frequency. Going up is hard.

A number of people have reported major changes in resonance from boxed, to boxed with the binding and purfling rebate cut, to fully bound. I don't see much change at all. I think (don't know) that the difference is due to whether your rebate revealed the kerfs in the linings or not. I usually use thick laminated linings so I don't get the ring of perforations around the top edge when I cut the rebate. The air leakage you get seems to make quite a difference, notwithstanding the change in the edge stiffness.

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Re: Mass Loading Sides

Post by ChuckM » Wed Sep 07, 2011 12:34 pm

Yes, going up is hard, but when you're just shy of the target on the downward side, it's difficult to justify dropping 10 or 20 Hz (in the case of skipping over a doubled air resonance) to get down to the next target frequency.

The guitar I mentioned raising 15Hz (uncoupled) had laminated solid linings on the top. So far I haven't come up with a good reason why it would do that. The coupled top frequency only rose a couple or 3 Hz. It is currently at 94/199 with targets of 95/202. The 94 to 95 part is easy. I'd lowered it from 97 with some soundhole adjustment and I can easily move it back up a Hz, but the top... So close ...
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Trevor Gore
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Re: Mass Loading Sides

Post by Trevor Gore » Wed Sep 07, 2011 2:50 pm

The frequency you're trying to miss is G at 196.0Hz. 30 cents off is likely enough if that's where it stays. One way up is to lower the mass in the bridge/saddle. Another way is to raise the air (and/or back) and trust to the repulsion effect. Another option is to go 100Hz and 190 Hz which works well for me.

I can't add any more to the binding effect issue. It just doesn't happen with my builds. I only look at the fully coupled frequencies however.

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woodrat
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Re: Mass Loading Sides

Post by woodrat » Sat Sep 17, 2011 9:47 pm

I love the concept of being able to trim the frequency of the top by adding mass. I have incorporated the mass attachment points into my latest two builds. I am loving the books...I am all over the place in my reading though as there are so many goodies in there. I look forward with interest to seeing the results of this concept in action. :D

Cheers

John

Ps...My guitars are not always held together with tape! :wink:
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Dominic
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Re: Mass Loading Sides

Post by Dominic » Sun Sep 18, 2011 11:28 am

Good work John, I hope you can post some frequency plots with before and after comparisons if you actually end up adding mass.
Cheers
Dom
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