Red Cedar top for a Parlour guitar - thicknessing!

[Munchkin]

Hi Everyone

Just getting round to making a parlour guitar (finally) using Trevor's equations.

I'm a little nervous about the dimension I got from the equations - 2.45mm seems awfully thin to me. I've attached a jpg of the input data to the spreadsheet with the output. The calculator has been fully tested and I successfully used it to make a Dreadnought last year.

I struggled to get a sensible cross vibration reading. Most of my taps generated about 45Hz with an occasional 127Hz observation, which I used in the calculation.

My question is two-fold. Is this a problem anybody else has had with cedar in getting the cross vibration reading? And is 2.45mm just too thin for a steel string guitar?

This is nice AAA set I've had for over 10 years.

Looking forward to your thoughts before I get my plane out!

Cheers

Martin

[Trevor Gore]

First, 2.45mm should be fine for a small WRC guitar provided you have it matched with appropriate bracing. I have a similarly sized shape which I use for classicals and get panel thickness of ~1.9mm (spruce) and they work fine. Any worry is really about the stress on the bridge glueline, if you have the right bracing, and the stress on a classical bridge glueline is typically higher than on a SS pinned bridge glueline.

However, it's worth thinking about what these equations do. For the same "f" number they're designed to give the same modal frequencies, irrespective of material properties and box size, presuming you're using an appropriate bracing scheme. Now, the thing about smaller guitars is that many people use such a box size to produce the characteristic "middly" sound you get from small guitars, which, to a degree, depends on them having higher modal resonant frequencies. So if you want to target higher modal frequencies, you need to lift the "f" number, which will give you a bigger number for the top thickness. How much you lift it is up to you and your taste in guitars. If you've built such a model before and have a preferred thickness range, you should be able to "reverse engineer" the equations to give you an idea of the "f" number you need. That's how these equations were "calibrated" in the first place.

As a starting point, I'd be staying close to the 2.45mm, because the reduced box size will give a higher main air T(1,1)1 resonance which will "repel" the main top T(1,1)2 resonance higher anyway. Try to stay close to the 2 degree bridge rotation criteria, but don't exceed it, and you should end up with a great sounding guitar.

Keep us posted as to how it goes!

[Munchkin]

Thanks as always Trevor for the thorough reply!

After posting I actually revisited tap testing my top and am still getting an ambiguous reading for the cross grain vibration. I've attached a spectrum which shows the challenge - 3 peaks!

Punching the 3 values into the calculator gives me 2.72 (46Hz), 2.66 (70.3Hz) and 2.5 (115.4Hz). The latter is more in the range expected for the test....

Do you sometimes get such ambiguity in your tops?

Martin

[Trevor Gore]

For the cross grain I'd be expecting ~100Hz. It might be worth breaking out the value for Ecross, so that you can see if you're in the expected range.

The way to suppress the unwanted peaks is to hold in antinodal positions for those modes. If, for example, you hold on the cross-grain centreline (half way down the long edge) when trying to excite for Ecross, you are on a node for the diagonal mode as well as a node for the cross marimba bar mode and a node for the long grain first partial (Table 4.3-1). So stay on the node line for the cross marimba bar mode and move off the centreline. Tap on the antinode of the mode you want. Small adjustments can give quite a bit more clarity. Check out Fig 4.5-3, but also beware Equ. 4.3-8.

[Munchkin]

I was holding the plate at about 35% and tapping in the centre widthways.... ECross= 0.73GPa at 115.4Hz (.54GPa at 100Hz)

I'm actually after a fairly conventional tonality so I'll leave f at 75.

The back and sides are to be cocobolo, which is amazingly heavy! And unfortunately the set I have cannot be made into a rectangle so I can't use your formulae for establishing the thickness. I was going to aim for 2.5mm with a live back.

Any thoughts or comments would be appreciated.

Cheers, Martin

[Trevor Gore]

Ecross ~0.75GPa sounds about right for WRC.

For the cocobolo, you could use a three point bend test to get Elong and use average figures for Ecross and G. The error will be relatively small. (See Section 4.5.4). However, the larger problem is the high density which means a back that is likely to have too low a monopole mobility to be live, just because it will be so heavy. Equ. 4.5-7 will get the frequency in the ball park for you, but not the monopole mobility if you use high density woods. So to get round that, you will have to use a thinner back to get the mass out and more bracing to get the stiffness up. The back mobility needs to be better than 7.0 s/kg x 10^-3 to be live. I'd be guessing at 2.0mm to 2.2mm for back thickness with no scalloping of the lower bout back brace. Then figure out what "f" needs to be for wood of that density range and you'll know for next time.

[Munchkin]

Hi Trevor & All

Time passes..... and the parlour guitar is finally there! I dimensioned the cocobolo back to 2mm as you suggested and did not scallop the brace. The body size is 485 X 355mm. The spectrogram is quite nice with T(1,1)1 at 112Hz and T(1,1)2 at 195Hz (I plan to work it down to 190Hz). However T(1,1)3 is non existent and I'm wondering whether it's worth my time trying to liven the back up by scalloping the brace with potentially destructive effects on the other resonances? I'm really quite pleased with the tone and the volume which is louder than I expected. The body resonances between 350 and 650Hz give the voice quite a bit of 'character'.

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Live Back.jpg (50.28 KiB) Viewed 12292 times

For your interest (and the gallery!) here's a couple of photos. I ended up doing a scoop cutaway as I couldn't bring myself to cut into the back! Not for the fainthearted I should say.... Customer is delighted, especially as the green celluloid lines look so great (his idea).

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Cutaway.jpg (127.66 KiB) Viewed 12292 times

Port.jpg (88.06 KiB) Viewed 12292 times

Looking forward to hearing your thoughts

Martin

[kiwigeo]

My thoughts....nice!! The cutaway and headstock are particularly appealing to the eye.

[Trevor Gore]

Nice work, Martin. Perhaps I could persuade you to post a couple of pics in the Book Builds Gallery, here?

It looks like you may have a little bit of back response happening at about 230Hz. It sounds like you've really pleased your customer, which is what matters at the end of the day!