How do you deal with vanish and pore filler later on wood for frequencies measurement ?

[rdpdo]

Hello,

I searched the forum for an answer to my question but couldn't find it. This question has certainly been asked many times so I apologize for asking it again!

I'm reading Trevor Gore's first book and I'm at the part that explains how to obtain wood characteristics like Young's modulus by making frequency measurements.

I was just wondering how you take into account that the wood is going to be varnished at the end? I suppose that the varnish will modify the mechanical characteristics of the wood, right?

It's the same for pore filler no ?

Do you use some uncertainty in your measurements to take this phenomenon into account? If yes, what percentage?

Do you use a technique that will allow you, once the guitar is finalized (varnished), to modify the frequencies? (I don't really know how, for example by having the possibility of removing mass in certain places once the guitar is finished???)

Thanks you very much !

[kiwigeo]

If you consider the thickness of a finish to the thickness of the surface its being applied to....I don't think it's going to contribute much to the total mass of a top. Stiffness......most decent guitars have a very thin finish which is flexible enough so as not to impede free vibration of the top.

Pore filling......most of the more commonly used top woods are not open pored and requiring filling. I make my instruments with active backs but I've noticed no difference in tap results pre and post pore filling and finishing (French Polish).

My overall take on the subject.....unless you're using an open grained wood for your top or you're using a really thick rigid finish on your tops compensating top thickness for effects of finish is not really a fruitful exercise.

If you read the books you will learn that the Gore method includes a means of trimming top (and back) frequencies...ie by adding mass to the sides and for the back by removing material from the centre of the lower cross brace.

Trevor will no doubt weigh in with some knowledgeable comments

[rdpdo]

Thanks you very mych, I take these very interesting remarks into consideration... This helps me a lot because this subject bothered me a little!

I haven't yet read the book dedicated to construction (volume 2) but I saw in the first volume that Trevor actually talks about added masses that he can "adjust" later... This technique does not seem to be explained in detail in the first volume but perhaps this will be the case in the second volume That said, I haven't read everything yet!

Thanks again !

[GregHolmberg]

Here's how I understand his system.

You start with a certain target thickness for the panel, which you calculate from the seven measurements you take of the rectangular, raw panel (length, width, initial thickness, mass, and three frequencies) using the formula in Equ. 4.5-7. Once thicknessed to that number, these half panels will in theory have the frequency you chose for parameter f, as in Table 4.5-2. Lets say, for example, you chose 75 Hz for the top.

From there, the top goes through many changes: glue halves together, cut to shape, glue in braces (taking a spherical shape), apply CF, glue to sides, scaping the outer 50 mm, finishing, gluing the bridge, stringing up, etc., which cause the frequency to change. He describes some of these changes in frequency in the Build book. In the end, you should end up at the target frequency, with the box closed and the top modes coupled to the air and back modes. Say, for example 170 Hz. You might have to make some adjustments on the finished guitar (shave braces, add side weights, etc) to hit it exactly.

Now, how did he choose 75 Hz? Trial and error, after having built many guitars, using his bracing system. Can you calculate that f=75 Hz will put you at 170 Hz in the end? No. You have to build your guitars using your techniques, and see where it comes out, and then adjust the parameter f to get closer to the target on the next guitar. Repeat until you can hit the 170 Hz consistently. Now you have a value for f that you can use for all your guitars going forward.

So, what thicknessing each piece of wood to f=some-value will do for you, is to produce a consistent guitar, even as the individual pieces change density and stiffness, or even species. This was the point of his "shed" guitar--crappy wood still produced the target frequencies in the finished guitar.

This is what I got from the book, anyway. Maybe others understood it differently...

Greg

[rdpdo]

Hi,
Thanks you for your advice.

You might have to make some adjustments on the finished guitar (shave braces, add side weights, etc) to hit it exactly.

Is it explained on the build book how you add weights or shave braces on the finished guitar, when box is closed and varnish is done ? Is is not difficult to access these area on a finished guitar?

[kiwigeo]

Is it explained on the build book how you add weights or shave braces on the finished guitar, when box is closed and varnish is done ? Is is not difficult to access these area on a finished guitar?

Yes all explained. Trevor is so thorough he even makes a plan before going to bed each night

Weights are added via T nuts set on inside of sides and accessible via sound hole. Central section of the lower transverse back brace is shaved by reaching in through soundhole.

[rdpdo]

Great thanks ! I look forward to read this