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.