Acoustically shaping the Neck, for neck to fingerboard matching ...
...What I am refering to here is the acoustical matching of the tap tone of the neck wood with that of the fingerboard at their junction (glue line).
...Remember that first the fingerboard is shaped in such a was as to produce a constant tone all over the top surface of the fingerboard of 188 hz. This means that the side tone will also be consistant all along its edges, at 188 HZ.
... By tapping the wood of the neck just below the glue line of the fingerboard and comparing this tap tone with the side of the fingerboard we can determine if they are in harmony, and if the neck shape needs to be adjusted to make this acoustical match.
...Tap with the edge of the wood (unsharpened) end of our low-toned tapping pencil just above the glue line on the side of the fingerboard and then immediately just the other side of the glue junction on the neck wood. The fingerboard tone is fixed (once it is adjusted to be even all over) and is not to be changed anymore, so all the changes will be make to the neck shape to make the neck wood side tap tone at the glue joint match the fingerboard side tap tone.
... Go ahead and ensure that the neck thickness is correct and then we will begin to fine tune the neck shape.
... If the neck side tap tone is too high, then material is scraped or sanded off the immediate side shape near the glue line to the fingerboard, to lower the tone.
...If however the tap tone from the side is too low, then this tone is raised by rounding off the side curve (or shoulder) area.
...Remove only small amounts of wood and continually check your work progress by tap tone testing as the frequencies change rapidly.
...When the tap tones all along this fingerboard to neck joint are in harmony, on both the fingerboard and neck, your job here is done!
Using a sanding board to keep straight lines...
Using a scraper to refine neck shape
Match these two areas acoustically : the side tap of the neck wood at fingerboard joint will be at 198HZ; while the side tap all along the length of the fingerboard will be an even 181HZ .When alternatively tapped, they will sound in harmony.
Tap these two places and compare...
Material is removed here to raise the neck side tap tone at the glue joint.
Material is removed here to lower neck side tap tone...
Set the neck section thickness first, before fine tuning...
..Maple of the neck.
..Ebony wood fingerboard.
...This is the sound specturm from a tap on one plate of a recent violin, and will serve as an example of what a tap tone is composed of and what we are to listen for:
...On the left is sound level in Db; on the bottom you see the frequencies generated by a tap in the center of the top plate of my Opus #22 violin. The first prominent sound peak is the 'fundamental tap tone' that we are to listening for. However, our wonderfully designed ears (brain, nerves, etc.) hear all these generated sound frequencies at once. The highest pitches (in this case 4,000 hertz, vibrations per second) is the easiest for your ears to hear, as our hearing is more sensitive to high frequency sound. The 'fundamental' tap tone is the tap tone frequency that we want to hear and compare.
...Given the DB scale for loudness, an increase of 10 DB is twice the sound loudness (unless I misunderstand).
...Cross Section of the neck with the fingerboard down:
..A smooth transition of tap tone...
from 176 HZ to 198 HZ.