In my old parlor there is either three or four straight ladder braces in the bottom. Almost always three time the size needed and often cut too close to the side kerfing and poking out from the side ;-)
Anyway. I know about the five brace pattern used by Martin, brace 3 and 4 is wide, low and flat. I'm curious about why the wide braces are used. I would guess that the wide braces either give the bottom more flex and bass or the big glue joint is needed to keep the rosewood from cracking.
Thinking about testing one or two flat braces in a small parlor back, but I'm not sure if it will change the tone (to the better). Anyone care to elaborate on the subject?
There are two primary schools of thought on backs, reflective and reactive. Reflective are stiff, thick and..... well they reflect. Reactive have some flexibility and that's usually where you see wide, lower braces.
For Reactive think bellows and a pump. Reflective backs typically are thicker too and reactive are of course thinner.
Yes, I know about the two schools of thought :-) So the Martin guitars are made to have an active back and the flat braces are there primary to make the back more flexible.
The old parlor I'm working with has maple backs (often too thick) and they are made to be stiff. Or simply made overly strong to hold... I do like to have an active back. I will try a flat bottom brace for the longest brace the next time and see what happens.