A. Quartersawn Wood: The Soundboard should be Quartersawn first of all. This will be a good indication that the top will possess some degree of stiffness. Try to select a top that limits the vertical grain to a maximum of 20 degrees off vertical.
B. Number of Growth Rings: There should be a high number of annual growth rings per inch, consistently along the guitar top. Personally I like to see greater than 20 per inch, But more than 12 will give you a nice stiff top.
C. The Soundboard Should be Stiff: You should be searching for a very stiff top here and if it meets the criteria of both A and B above it is very likely to be stiff. Pick up the top plate and try to flex it parallel to the grain and across the grain. If it feels like a piece of cardboard or spongy, it will have a muddy, mushy sound. It will not have clear highs and more likely overpowering lows. If that is the sound you are after, then you found your Soundboard. If you are looking for a well-balanced tone with clear highs and balanced lows, go with a stiff top.
D. Tap Tone? What is that? As you make more instruments and test more Soundboards, you will grow accustomed to picking out a great sounding Soundboard by tap tone testing. Here is how you do it. Pick up one half of the Soundboard set. Hold it between the thumb and index finger of one hand and let it hang freely. Try to hold it about 1/4 to 1/3 of the way down one edge of the Soundboard, about 1″ in from the edge. With the index finger knuckle of the other hand, give the top several sharp raps, dead center on the wood plate. You should hear a clear, bell-like response that has some sustain (lasts for a bit). If it sounds more like a “dull thud”, the top will not be very responsive to high notes and could be very disappointing. Another thing that is experienced during this tapping is you can feel the vibration or the resonating quality of the wood. After all, getting a top to vibrate freely or resonate well is what we are striving for.
Another great quality about a stiff Soundboard with a great tap tone is you can mill the top thinner than that of a softer top. Less wood equals more resonance and greater vibrations and you will likely wind up with a superior quality Soundboard.
E. Wood Species: The species can have a great deal of influence on a tap tone too. If you hold up a AAA grade Western Red Cedar Top and compare it to a AAA grade Sitka Spruce top, the Spruce top will win the stiffness and high-pitched tap tone contest most every time. This is because the inherent qualities of Ceder are that it is a softer wood and the softness provides some insulation quality to mute the sound. You should analyze comparable woods like cedar against cedar or spruce against spruce, to select the right top for your guitar.