Just closed the box - strategy?

[gerryt]

Hi all

Still working away on my first "the Books" build. I'm at the stage that I've closed the box, trimmed it flush and captured a spectrum as in the image below.



The peaks as I see it (please correct me if I'm wrong) are

T(1,1)1 - 112Hz
T(1,1)2 - 199Hz
T(1,1)3 - 247Hz

The guitar is a traditional OM shape built with falcate top bracing 7-ish mm high at the peak of the primary and secondary falcates. The back has Book style hybrid ladder/radial style bracing and the lowest of the ladder braces is full height apart from a semi circular groove in the middle which I can use as a starting point if I have to lower that brace at all. Materials are Adirondack Spruce top and IRW back and sides. I have side weight mounts in there ready to go!

Spectrum was taken using a rubber on a stick hitting at the saddle location on the top. I was holding the guitar with one finger through the side sound port on the upper bout. The mic (a Shure SM58) was behind the back as I tapped on the front.

I haven't done anything to the box at all yet but I was wondering what if anything might be a good strategy at this point in proceedings. Are those frequencies an ok starting point? Will I be able to get down to target frequencies once the bridge and neck are in place?

Still pretty new to this science-y maths stuff so any help would be much appreciated.

Cheers, Gerry

[Trevor Gore]

Gerry,

Check out the last line of the last paragraph, p AI-3, (Design) for guitar holding instructions whilst tapping, then post another tap plot.

Thanks, T.

[gerryt]

Will do Trevor thanks. It's late here though so tomorrow it'll have to be.

All the best, Gerry

[gerryt]

Ok so I've found and read the instruction for tapping the box and the above is the result. I tried only to touch the sides and not dampen the top or back. The guitar body was on its side on my lap. The two big peaks are at 113Hz and 200Hz. Struggling to see the back in the spectrum. I wonder if the bump around 250Hz is it? I turned the guitar body round and tapped the back in the same way and got a big spike at 238Hz.

[Trevor Gore]

The plots don't look much like mine but I think we can work with them. It looks like the T(1,1)1 is close to 113 Hz and the T(1,1)2 is close to 200Hz. The B(1,1) is at 238Hz, which implies that the T(1,1)3 would be around around 240Hz or a couple of Hz higher, if it were there. I guess you're using a typical sound hole size (~100mm), but the T(1,1)1 is still fairly high, which implies a stiff box, which implies a stiff, non-live back currently.

Figure where you would like your final T(1,1)2 (probably 180Hz or 190Hz) and move on to next steps (edge thinning, binding etc.) with that target in mind, then work the T(1,1)3 (if you want a live back) to suit. Try to avoid ending up with the T(1,1)1 on A at 110Hz, which is a likley problem which you won't want. Probably the best way to avoid that is to leave yourself a largish sound hole overhang on the fret board, designed so that you can trim it away to tune the T(1,1)1.

[gerryt]

Thanks for that Trevor - much appreciated.

I had wanted to bind the fingerboard but I won't be able to if I leave a trimmable fretboard overhang. And I guess the thing with the overhang is to do it so it doesn't look like a mistake!

Is there a particular way I could deal with the back? Is it working the lowest ladder brace down? Or do I take a random orbital sander to it and thin it down?

[Trevor Gore]

You could "blind bind" the fretboard - bind in the same wood as the fretboard, no purfling lines. That covers the tangs. Leave the end unbound.

If you thicknessed the back according to the formula, working the back brace should get you there.

[gerryt]

So the latest spectrum is after perimeter thinning the top and scalloping the lower ladder back brace.

I'm getting T(1,1)1 as 109Hz now. Incidentally out of curiousity I taped a martin stylle ebony belly bridge I had lying around to the top and tapped again and got T(1,1)1 as 104Hz. The bridge weighed 33g. Am I likely to get similar or greater reduction with a lighter Books style laminated low density Rosewood/CF bridge? Is 100Hz+ a likely T(1,1)1 finished frequency for this guitar? Is there anything I can do to affect this? Unfortunately I think reducing the soundhole a little will be a fruitless exercise as I have a side sound port (probably my last build with this!).

T(1,1)2 is now reading 192Hz which I think is getting into the right ballpark at this stage?

And now I seem to be getting an identifiable spike for T(1,1)3 reading 228Hz. Or is that bump not really a spike worth noting?

Last of the many questions is what effect will mass loading the sides have on any of this?

[Trevor Gore]

Taping bridges on doesn't tell you a whole lot. You add the mass but not the stiffness. The effect on the T(1,1)2 of adding bridges is shown in Fig. 4.6-3 (Design). For the T(1,1)1, adding the bridge typically raises its frequency a small amount on my guitars. Adding the stiffness reduces the amount of air moved so raises the frequency more than the added mass that lowers the T(1,1)2 pushes it down.

Mass loading the sides will have the effect shown in Fig. 2.3-15.

[gerryt]

Ok so mass loading the sides has a marked effect on T(1,1)2 and none on T(1,1)1. And adding the bridge has about 0.4Hz of a drop to the T(1,1)2 per gram of bridge for a medium steel string guitar which mine is more or less. The stiffness of the bridge adding a little to the T(1,1)1.

What I don't understand is why my T(1,1)1 is so high - currently at 109Hz - when the T(1,1)2 and T(1,1)3 are ballpark ok. Is this simply a function of the properties of the Adirondack spruce top and the Sitka braces? Could I have exacerbated the stiffness of things with my less tidy than I'd have liked epoxy and carbon fibre technique?

Is there anything that can be done re the T(1,1)1? Is it something I need to be concerned about? If it ends up say at 108 or 109Hz what effect on the eventual sound can I expect or is that a "how long is a piece of string" question?

My thanks for your patience Trevor as you take this newbie through his first of hopefully many Books builds!

[Trevor Gore]

What I don't understand is why my T(1,1)1 is so high - currently at 109Hz

Well, don't forget the effect of the sound port. Depending on how big and whether you compensated by reducing the sound hole size, could be worth 5-10 Hz.

[gerryt]

Covering the sound port with my hand takes the T(1,1)1 down to 104 Hz so yep a 5Hz difference. I don't think I'll use a sound port again to be honest.

Is the 109Hz a thing to be worried about? Or will the guitar just be less than optimally responsive?

Thanks again.

[Trevor Gore]

The audience will likely miss out on some bass response, but the player might not because the port will act as a "monitor" and so the player may hear a stronger bass. Maybe.

[gerryt]

I'm guessing it's a suck it and see situation!

My next falcate guitars will be using perhaps more suitable woods - Englemann and Oregon Myrtle and Englemann and Apple wood. Might get a bit closer with these. The current build has been a massive learning experience for me - so many new things!

[Trevor Gore]

As long as you're having fun...

[gerryt]

Aye Trevor the sort of fun you have on a rollercoaster!