There is a lot going on inside the box that guitarists hear, but will never see without a mirror. Out of sight, out of mind. But not for the luthier. What you see below is the most fussed over part of the instrument, for this is what gives the guitar its voice. The principle is simple: reduce the mass of the vibrating part of the soundboard to its absolute minimum, yet retain enough strength to resist the 180 lb. pull of the strings indefinitely without distorting. It’s a very sophisticated engineering and acoustical challenge. A background in physics is helpful. There are many variables to consider: the stiffness of the spruce and its musical quality, as demonstrated by its tap tone; the thickness of the soundboard to the thousandth of an inch, and its taper; the stiffness of the bracing material; the size and placement of the braces, and how they are carved; the angle of the x-brace; the arch of the braces; the placement of the bridge; the size, thickness and grain orientation of the bridge patch; the scale length of the strings; the choice of back and side material; the size and depth of the box; the neck material, to name a few. Changing any one of these variables will change the sound of the instrument. A lot of thought and planning goes into every detail of my instruments.
The soundboard is voiced, or acoustically analyzed and altered, three different times during the building process: during the carving of the braces, as shown above; just before the back is attached and the guitar is like a drum, as shown below; and then again when the box is complete, just before final sanding. It is this process of voicing that separates fine hand made guitars from expensive factory made instruments. I probably spend more time working on the voicing of my guitars than factories spend making an entire instrument.
I also take pride in how clean my instruments are on the inside. There are no errant drops or blobs of glue. All interior edges are rounded and fine sanded to be acoustically efficient. It’s the details that make the difference.