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And I have a few questions and comments. First off, I really wish there was a discussion of units in the book when they talk about the procedure. It took me a while to figure it out till I saw the table on page 4-63. But when you start getting numbers like 12,654,334,231 you scratch your head thinking somethings got to be wrong!

But anyway...

The other thing was determining which peaks to use. I guess there was some background noise in the room I was recording in because my graphs are not the greatest. So I basically 'made sense' out of it in looking at the numbers that should be expected rather than what I think I saw. I used a Rhodes NT3 condenser mic into Audacity through a Delta 10/10 sound card. Charts are below:

Long Freq (Fl):


Cross Freq (Fc):


Long Cross Freq (Flc):


In particular it was the Fc value. As you can see there is a peak at 60 but the suggested numbers for that are much higher so I threw that one out and went with the next larger peak at 108. I guess maybe that 60hz peak was the Fl value sneaking in.

Ultimately the final target thickness based on the calcs is 2.52mm or about .99in. I had a feeling this would be a thin top too because while it is light in weight it felt very stiff in my hands. It will be interesting to compare these results with my standard deflection testing.

I welcome any comments from others who use this testing procedure.

Also... As a computer programmer by day I could write something to help figure this stuff out but I don't want to reinvent the wheel either. I'm sure someone has already written a program to help spit out the numbers. Does anyone have such a thing? I'd like to avoid this in the future should I adopt this method

There is a spreadsheet that's been floating around. I use it to plug the numbers and get the thickness. PM me your email and I'll send it to you.

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I'm curious as to how it relates to your deflection method as well. FWIW, 2.52 is very much in my wheelhouse for a fairly stiff/dense top...

Does audacity tell you the exact freq, or are you left to judge 'ish' along the graph at bottom?

What is this sorcery ?!


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You move the cursor right over the peak and it tells you the number. So I think it's pretty accurate. The photo's do not show that though.

The density of this top is 388.13 Kg/m^3

But yeah I felt like the results were about right given how stiff this top feels.

Get the Gore/Gilet books man, you're beyond basic carpentry excersises at this point:)

In a nutshell, the G/G method is to have target freqs for top and back. By measuring the freqs of the half panels in exact rectangular form, you can measure the existing freq, add the dimensions to the spreadsheet, and find how thick your top should be to land on your target freq. Math/physics, not sorcery...

One should always be skeptical of data at 60 Hz when working in the United States. Our electrical grid operates at 60 Hz, so that is often just noise in your measurement system!

It can be quite difficult to excite particular modes that you want to measure without exciting ones you want to exclude. Do the books give a recommendation for that? It is certainly possible to tap on the node line of one particular mode (requires an assumption as to its shape) so as to exclude it from excitation, but that may not be a great place to excite the mode you are looking for. Complicated business.

0.99" target thickness for a top? That doesn't sound right. If you took 60 Hz as one of your mode peaks that could be your problem. I've only done something similar to this once, but I don't think you will find anything as low as 60 Hz unless you are already way too thin. Try ignoring 60 Hz completely. Again, I'm no expert, but I would guess your Fl is 108 Hz and then peaks around 160 Hz and 200 Hz are Fc and Flc, not sure which is which.

EDIT: I think you did another unit conversion wrong. 2.5mm is around 0.1", not 0.99". You must have meant 0.099" so that thickness makes more sense now. Still, watch out for 60 Hz! It looks to be exactly 60, and it's in all of your measurements, I think that is likely electrical noise.

Jfmckenna,

That makes sense, I had thought rough guessing from the chart would be odd...

I'll try measuring it again with the lights off!

Let us know, but I wouldn't be scared of a .99 top...if the data is correct.

I just repeated the test with the lights and PC monitor off. I won't bother posting screenshots of the plots because they were EXACTLY the same. I guess there is of course still current running through the shop, at least to power the PC. but according to the book 60hz is not unheard of either for the Fl value. The next peak over is 110Hz which is outside of the range. Table 6-1 on page 6-8 has typical mode Freq Ranges for Englemann Spruce and the Fl values go from 50-75.

If 60 Hz is in the range suggested in the book, then it may be right. How thick is the plate you are measuring? The frequency will depend highly on the thickness, as well as the rectangular dimensions.

The "noise" I was suggesting isn't acoustic noise traveling through the air into your microphone, it is electrical noise in your measurement system. If nothing in your measurement chain is plugged into the wall (using a laptop?) then you would be fine. If you are taking the data into a desktop computer then you probably can't eliminate it. This is one instance where using your phone to record the data could be quite useful!

EDIT: I dug up my notes from when I did this in the past. I was testing a piece of Englemann at 8-15/16" x 23-7/16" x 0.16". I noted the first mode (2,0), which is probably equivalent to your Fl, at 70 Hz. So you are probably on the right track!

Yeah the mic is plugged into a PC which is plugged into the wall. So you are on to something, it's a bit suspicious. I don't know if my phone would be good enough to record but I could try it. The top is currently h=3.4mm, L=564mm, W=210mm mass at 156.3g. Density was calculated based on those numbers.

See my edit above. Yours is thinner than mine was, other dimensions are close, would expect a little lower than mine. So that 60 Hz is starting to seem real to me. I did my measurements with my phone, worked surprisingly well!

Be careful of room reflection also. The reflection can interact with the spectra changing your results. I not saying that happened, just that there's a potential for it.
I recently discovered this. You can see how it effected some of my spectrum at http://www.anzlf.com/viewtopic.php?f=1&t=8353.
Also you can check the 60 hz by taking a spectrum of your background noise.

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FWIW, I also had some hum issues because my mic was going XLR -> Mixer and then Mixer -> PC over USB. When I moved to a Blue snowball, the problem went away. I was able to clean some of it up with a better XLR cable, but it was / is best with the USB mic.

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Here is a link using the OLF Amazon link: https://www.amazon.com/Blue-Snowball-Mi ... e+snowball

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Hmm, ok. I didn't think about that. It is indeed an XLR connection but it is pretty decent recording we equipment too. I guess I should be able to analyze the background noise. I'll try that later today.

Brad--

If you don't mind me asking, what brand/model mixer were you using to send an AD converted signal to the computer via USB?

I don't think a decent interface would introduce hum as part of a digital audio signal to a computer. I mean, a Snowball has an OK, but relatively modest, AD converter built in. If it performs better than the mixer in this regard, that's probably saying something about the mixer. No offense intended.

I am very interested in this thread! I am tempted to follow the Gore/Gilet processes to determine optimum top thickness in the next few guitars I build, so any tips would be very welcome.

I use Visual Analyzer on PC but as for phone apps, I find Spectrum Analyzer for android to be very close and simple to use.
A very basic spectrum analyzer but for this purpose the peaks are the same as when using VA. You need need nothing only
phone or tablet.

Here is the spectrum over 4 seconds of background noise. There is a peak around 60, more like 58Hz. They all also have that bulbous initial wave isn't necessarily from hitting the record button because I snipped the very start out. So I'm a bit confused as to what that is.

It's this one: https://www.alesis.com/products/view/multimix-8-usb-fx

It's a cheap way to hook a turntable, my sons electronic drum kit, a mic, etc... to the PC. Your're right though, it could really even been the mic (a Blue bottle mic) or something totally unrelated. I just had best luck with the Blue snowball going into the PC with VA running on the PC. No worries, I'm hard to offend.

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60 Hz... maybe from mains line frequency??

"Here is the spectrum over 4 seconds of background noise. There is a peak around 60, more like 58Hz."

Since that's the background noise, I would suspect mains hum. Have you tried lifting the ground wire for that computer? Or, if you have a laptop handy, run the software on the laptop powered by its battery.