Prototype Gore Guitar Freq Request

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RonRapp
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Prototype Gore Guitar Freq Request

Post by RonRapp » Thu Mar 15, 2018 10:09 am

Hello Trevor!
I just completed the "box" based upon the falcate design in your book. I am using an Englemann Spruce top with Sapele back and sides. Attached is the frequency spectrum. The main top T(1,1)2 appears to be quite a bit lower than expected in the design book... Can you take a look and tell me your thoughts and how I best proceed, please? Be frank, you will not hurt my feelings...much...8-D Also, I used the spreadsheet to determine the initial plate thicknesses...Thanks in advance, Ron Rapp in Roxborough, Colorado USA.
Unfinished Box Freq Spectrum 1.pdf
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Trevor Gore
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Re: Prototype Gore Guitar Freq Request

Post by Trevor Gore » Sat Mar 17, 2018 1:07 pm

According to your chart, the T(1,1)2 is at ~175Hz, which I would consider too low at this stage of the game for a "normal" guitar.

First thing is to check your measurement set-up for VA and associated hardware so that you are confident you are measuring correctly.

If you are, you are likely to end up finished with a T(1,1)2 of ~ 160Hz, which is definitely on the low side, but maybe still a workable guitar, depending on your sonic tastes. The question to seek answers to is why is it that low? To help you answer that, I will need to know a lot more about how you put the guitar together; materials and dimensions etc..

RonRapp
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Re: Prototype Gore Guitar Freq Request

Post by RonRapp » Sun Mar 18, 2018 1:39 am

Trevor,
Thank you for the response!
The only variance to the book design are the materials in the falcate braces:
-The primary falcate braces are Englemann Spruce laminates using epoxy...maximum height of 9mm.
-The secondary falcate braces are California Redwood laminates using epoxy...maximum height of 9mm. I was having difficulty acquiring quartersawn Alaskan Yellow Cedar. Also, I couldn't get the Englemann I have to bend that sharply.
-The lateral and lower tertiary braces are solid Englemann Spruce, using Titebond.

With regard to correct measurements, I have a question...I am balancing the box on my desk in front of the microphone, and I am tapping around the bridge area (with the soundhole open and the bridge pin holes taped over). Does there appear to be an issue with my technique possibly?

I have set up VA9.0 per your screenshots...

As you may remember, I DO like more bass in my guitars, but I was hoping to try a guitar that you find pleasing...

Thank you for your time in pondering and responding. Maybe I will be able to meet up with you this summer after one of your classes in Denver...

Thanks,
Ron Rapp

RonRapp
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Re: Prototype Gore Guitar Freq Request

Post by RonRapp » Sun Mar 18, 2018 9:12 am

Trevor,
Also, regarding the initial measurements of the Englemann top...
F long (Hz) = 73
F cross (Hz) = 140
F diag (Hz) = 39
Mass, gms = 204.9
Length, mm = 568.325
Width, mm = 227
Thickness, mm = 4.6
Density, kg/m^3 =345.27
Elong, Gpa = 8.54
Ecross, Gpa = 0.8
G, Gpa = 0.5
Elong/Ecross = 10.7
Plate Thickness, mm = 2.96

Thanks, Ron Rapp

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Trevor Gore
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Re: Prototype Gore Guitar Freq Request

Post by Trevor Gore » Sun Mar 18, 2018 11:01 am

RonRapp wrote:
Sun Mar 18, 2018 1:39 am
I am balancing the box on my desk in front of the microphone, and I am tapping around the bridge area (with the soundhole open and the bridge pin holes taped over). Does there appear to be an issue with my technique possibly?
Possibly.

Try holding the guitar as you would when playing it (or as close as you can without a neck) and repeat your tap tests. Try to get just one tap per buffer. Post the results and I'll have another look.

RonRapp
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Re: Prototype Gore Guitar Freq Request

Post by RonRapp » Mon Mar 19, 2018 10:50 am

Hello Trevor,
I re-tapped the box with it in my lap, and attached is the resulting chart:
T(1,1)1 = 97
T(1,1)2 = 172
T(1,1)3 = 227

Thanks for looking into this again.
Ron Rapp
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Unfinished Box Freq Spectrum 3.pdf
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Trevor Gore
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Re: Prototype Gore Guitar Freq Request

Post by Trevor Gore » Mon Mar 19, 2018 4:17 pm

OK, thanks Ron.

The plot still looks a little raggedy, but much the same frequencies as last time. Why the T(1,1)2 is that low is hard to say. The plate properties and thickness prediction are exactly what I get in my spreadsheets. The braces are larger section than I use, but that should result in higher rather than lower frequencies.

Is the rest of the build according to the plans? Did you end up on the target plate thickness? Is the CF as specified, bridge plate as specified etc. etc.? Do you have any photos of the braced top you can post?

Otherwise, it's down to Section 4.11.

RonRapp
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Re: Prototype Gore Guitar Freq Request

Post by RonRapp » Tue Mar 20, 2018 4:00 am

Thanks Trevor,
Looks like it is back to ol' Section 4.11...

As you can see from the picture, my epoxy/carbon fiber skills need work, but I think my wood fitting skills are reasonable... :)

If you see any "Oh, THAT's the reason for the low frequency", please let me know...

I know this subject is exceedingly hard to teach to others, and I appreciate what you have done, and what you are doing!

Ron Rapp
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20180314_100919[1].jpg

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Trevor Gore
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Re: Prototype Gore Guitar Freq Request

Post by Trevor Gore » Tue Mar 20, 2018 8:47 am

Nothing jumps out from the picture. All I can think of that's left is the quality of the epoxy mix (were the proportions correct, was it mixed thoroughly, was it warm enough to cure properly, etc.?) And finally, how is the fit between the underside of the primaries and the bridge plate? Are they glued down properly?

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kiwigeo
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Re: Prototype Gore Guitar Freq Request

Post by kiwigeo » Tue Mar 20, 2018 6:49 pm

The Secondary braces are Californian Redwood. How does stiffness compare with the spruce you used for the primary braces?
Martin

RonRapp
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Re: Prototype Gore Guitar Freq Request

Post by RonRapp » Tue Mar 20, 2018 11:28 pm

Good question Martin, but I do not have a definitive answer for you...
The book had a passing mention of redwood bendability, but I did not see a stiffness comparison...
This MAY be an issue with the low frequency...

Thank You!
Ron Rapp

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Trevor Gore
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Re: Prototype Gore Guitar Freq Request

Post by Trevor Gore » Wed Mar 21, 2018 9:17 am

I've used King Billy Pine with an Elong of ~5GPa for all braces, using the dimensions given on the plans, and got strung guitars with a T(1,1)2 of 170Hz. Boxed not bound they come out ~190Hz. So whilst the redwood secondaries may contribute to the low T(1,1)2 you measured, I doubt that is the whole story.

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kiwigeo
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Re: Prototype Gore Guitar Freq Request

Post by kiwigeo » Wed Mar 21, 2018 3:33 pm

Trevor Gore wrote:
Wed Mar 21, 2018 9:17 am
I've used King Billy Pine with an Elong of ~5GPa for all braces, using the dimensions given on the plans, and got strung guitars with a T(1,1)2 of 170Hz. Boxed not bound they come out ~190Hz. So whilst the redwood secondaries may contribute to the low T(1,1)2 you measured, I doubt that is the whole story.
I mentioned this because I used KPB bracing on my first build but didn't check the Youngs Modulus of Elasticity for the stuff....it was very low.
Martin

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