Official Luthiers Forum!

Hello all, Im making some radius dishes, but I realized a mistake that I have made. I intended to make a dish with a 25' radius but accidentally made the dish 25' DIAMETER (12.5' radius). I am planning on doing a Live Back as shown in the Gore/Gilet book, which calls for a 10' radius. Would I be safe if I just used this 12.5' dish instead of 10' for my Live Back? I am worried about low structural stability and poor frequency response. -Alex

I have used both 15' and 10' radii dished for Gore braced live backs.
You will be just fine with 12.5'

What Jeff said.

Leave the main back brace a little higher in the middle initially. The sizing of that brace gives plenty of scope for adjustment.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I don't have Trevor's book ( obviously, I should) but what is a 'live back'?

Sent from my GT-P3113 using Tapatalk 4 Beta

_________________
Under Compensated Nut!

This is the simplified story:

If you look at the Chladni patterns on a guitar top (boxed up guitar), the monopole mode usually appears either two or three times, with matching peaks in the frequency response curve. The first appearance of the monopole mode (lowest frequency peak on the frequency response curve) is typically about 100Hz and is due to the fully coupled main air resonance. The second peak is typically around 190Hz and is due to the fully coupled main top resonance. A third peak, typically a few semitones higher in pitch, may or may not exist and if it does exist it is usually due to the back contributing significantly as part of a coupled oscillator with the top and air. If this vibration mode exists to any significant degree, you have a guitar with a live back. If it does not exist, it is usually because the back is too stiff/heavy to move very much and the guitar has a non-live back. Some people call this a reflective back, but for a number of reasons that is not a very suitable term.

Again very simplistically, a live back guitar gives "tone" whereas a non-live back guitar favours "volume". The reason for this is that a live back takes energy to move it, so less energy is available to produce sound, but the extra peak(s) in the frequency response curve due to a live back make(s) for a more interesting sound. A live back can also help you out of wolf note problems on high mobility, responsive guitars. There is no right or wrong way of "doing" a back, but each way has its own merits. A live back on a classical guitar needs to be carefully designed and implemented, because usually you're looking for as much volume as you can get, so there is a fine balance between that and the tonal benefits. The range of tonal acceptability is wider for SS guitars, so there isn't the criticality.

The details, of course, are where you'd expect.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks Trevor. I'm a little clearer on this now.

Sent from my GT-P3113 using Tapatalk 4 Beta

_________________
Under Compensated Nut!

So what you're saying is live backs can have less volume but cooler tone?

_________________
Old growth, shmold growth!

Yes, in a nutshell.

There's also other things that happen, too. For example, because the air, top and back all couple together, a live back guitar will sound "bigger" than a non-live back guitar, because it lowers the main air resonance to where it would be on a larger guitar with a non-live back. Also, because of the coupling effects, you can tune a top by working only on the back of the guitar. Backs (and sides, for that matter) have much more effect on tone than they're usually given credit for.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au