Live back too low when uncoupled

[Thibaud]

Hi,

I'm currently building my first acoustic and I went for an indian rosewood live back.

Today I have completed step 12.4 "gluing the back panel". At the end of the paragraphe it says to adjust the uncoupled back frequency.
It turns out that mu back is at 169hz. Looks like i'm way too low?

Any idea if that is normal or if there is a solution to rise the frequency up?

Thanks for your help

Thibaud

[kiwigeo]

I'd do a full tap test on the back and top once the bridge is on before deciding on any actions to adjust T(1,1)3 frequency.

[Thibaud]

Hi kiwigeo,

Thanks for your reply.

I’ll follow your advice and continue my build. If worst come to worst I’ll pop the back out later.

Thibaud

[kiwigeo]

If the back has the radial bracing plus main brace over lower bout then you have the option of reducing stiffness of the back by taking material out of the centre of the back brace. To raise stiffness you can try sticking weights to the back. You can try raising the stiffness now if want by blue tacking coins to the inside of the back and seeing how it changes the tap test results.

Hi kiwigeo,

Thanks for your reply.

I’ll follow your advice and continue my build. If worst come to worst I’ll pop the back out later.

Thibaud

[Trevor Gore]

If the back has the radial bracing plus main brace over lower bout then you have the option of reducing stiffness of the back by taking material out of the centre of the back brace.

That's correct, but reducing the stiffness will reduce the T(1,1)3 frequency. The OP wants to raise it.

To raise stiffness you can try sticking weights to the back. You can try raising the stiffness now if want by blue tacking coins to the inside of the back and seeing how it changes the tap test results.

Sticking weights to the back will not alter the stiffness, but it will increase the mass, which will also lower the T(1,1)3 frequency.

If the back frequency is too low, the way to increase it is to raise the stiffness of the back. This can be done by capping the back's lower bout brace, or replacing it with a taller (or un-scalloped) one. For a 000 sized guitar with the lower bout back brace unscalloped and built to the specs in the book, the B(1,1) frequency should be typically somewhere around 240Hz.

[kiwigeo]

Thanks Trevor...my post typed after a few too many IPA's.....

[Thibaud]

Hi,

Thanks for your answers I have closed the box and ended up at:

the main air 88hz
Main top 170hz
Main back 209hz.

It looks like I’m too low overall but I will follow Trevor’s advices in the book. Next Time I will follow the first advice and don’t scallop too much…

I have made the mistake to scallop the lower boot brace before closing the box so I have glued a piece of spruce back on. I will see what happen tomorrow.

Gluing fiber carbon over a X-Brace to increase stiffness would be an option? I don’t know if anyone tried before.

What is the consequences of having the main frequency too low?

Thank you very much for your help
Thibaud

[Trevor Gore]

Gluing fiber carbon over a X-Brace to increase stiffness would be an option? I don’t know if anyone tried before.

See page 11-12 in the Build book.

What is the consequences of having the main frequency too low?

"Low" is a matter of taste. The lower you take the main resonances, the more bass you get. See Design Section 3.2.2 et seq.

[Thibaud]

Thank you Trevor.

Your book is true gold. I’m not sure one life is enough to understand it all! Having you helping us is a massive help! Thanks for that

By the way, here is the guitar currently being built.

I’ll keep this feed updated when the guitar is complete.