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Hello,

I'm working through the Gore book and I'm at the point where I want to figure out the target plate thickness for a given top. I have a set of sitka that is:

Plate Length (Ll) 593
Plate Width (Lc) 223
Plate Thickness (h) 3.42
Plate Mass (m) 202

I set up VA to use a sampling rate of 11.025kHz and sample size of 16384 per Appendix I in the design book. Below are the results for fl, fc and flc.







The numbers I'm getting don't seem to line up with the examples in the book and what I've seen others post here. Am I not set up correctly in VA, not understanding the results, or...?

Admittedly I've not read the whole design book. I have read the whole build book. Mostly the design book makes me feel like an idiot.

Thoughts?

Brad

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Can you turn down the gain of you microphone. It looks like you are clipping the input. Then you should look for a peaks in the range that makes sense.

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http://www.Harvestmoonguitars.com

Normally, I use G-tune when tapping plates, but VA can be used as well. The frequencies you are looking for are given in Table 6-1 in the Build book. Follow the tapping instructions given in that section (Section 6.2). Looking at your plots, you seem to have mostly the right stuff there. If you switch off the Phase plot (View/Phase), you will get a larger Spectrum Plot to look at.

You seem to have some mains hum going on at 60Hz on all three plots. On the fL curve the first "real" peak is at ~ 70 Hz, which is where it would be expected. For Fx, you still have a lot of the 60Hz mains hum and the 70Hz fL going on (which means you are not holding the sample in the best place) and seem to have suppressed the cross grain vibration mode. Play around with different holding and tapping positions as per Section 6.2, tapping in line with the holding position ( i.e. if you're holding at 60% from one end, tap at 60% from that end). Listen for a clear tone. When you have it, record it. It should be in the range given in Table 6-1. I'll also say at this point that if you're getting clean tones easily, you can use the Frequency Meter function in VA to give a direct frequency readout from a single tap.

Looking at your flc plot, you have the 60Hz hum but the signal you're looking for is lower frequency than that. You may not have it because your mic hasn't got a frequency response that goes low enough. However, if you hold a corner of the panel close to the mic and tap the other side of that corner, you should get a reading. This one is usually easy to see (as is the first one), Generally, it's getting a good read of fc that can be problematical.

Regarding tapping the closed box when you get to that stage, too much (most likely) or too little signal are the most usual problems once you've got as far as you have. The level setting is very much computer dependent because most computer makers seem to have more than the basic Windows drivers on their systems. For the HP laptop I'm using right now (with Win 7x64), the level setting is done in an app called DPS Audio Control Panel. Level setting can also be done via the speaker icon in the tool tray (bottom right of screen). "Wrong" click on it, select "Recording Devices", select "Recording" tab, click on the mic you're using (which should be shown in the list), click on "Properties", click on "Level" and set your gains there. Since Windows XP, I don't think the mic level sliders in VA have worked (probably due to the other apps that different makers have loaded over the top of the Windows stuff).

There's some more detailed instructions about setting up VA on my website here.

I tend to use VA version 9.0.1 (an early version) for some reasons I talk about here.

Good luck and let us know how you go.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Appreciate your help John and Trevor.

I *think* I have the data collection issue with the mic sorted out. Loose XLR cable. Here is my setup:


Here is the new data:

Long Grain


Cross grain


Twisting


Here is my data in the cheat sheet spreadsheet (sorry, Trevor, I'm trying to digest the material and still keep building going ):


Does this look better / correct? It aligns better with the examples in table 6-1 of Build and table 4.5-3 of Design.

Thanks for the help!
Brad

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Yep. That looks a lot more like right. Well done!

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au