back braces

When Martin started building heavier braced guitars in the 1940s, they stopped the scalloped and parabolic braces, replacing them with braces that look like most braces seen on backs today--even height along their length, except for the curved down ends where they meet the sides. 
These heavier braces dampen much of the back, preventing it from reinforcing the mid-range and bass, except for frequencies that happen to be the same as the tap tones over the braces.  This is partly why some strings (the A on most Martins) are louder than others.  When a back’s braces are reshaped parabolically, they can allow the entire back to work with the top, and bring out a greater range of sound.