...This adjustment information had its' beginnings with members of the Violin Society of America (the VSA), which I learned at their Portland, Oregon national convention (November, 2004 A.D.). The members who taught this technique felt it had great merit, I attended the seminars, took notes, spoke to the instructors, went home and tried the adjustment on a insturment. It seemed to help improve the tone, so I tried it on another; it seemed to be an improvement also. I have now adopted it as a basic foundational adjustment when building or adjusting an instrument.
... If the fingerboard tap tone (tap downward on the top of the scroll) is lower than the fundamental tap tone of the back plate (the goal frequency that we will base our adjustments on) tap tone, then the adjustment upward is easliy accomplished by evenly sanding the end of the fingerboard nearest the bridge which raises the scroll tap tone very rapidly with just a slight shortening of this end. Do this carefully and check your progress often by tapping the top of the scroll and the center of the back plate as the change is rapid and you do not want to over shoot the mark!
...However, if the fingerboard tone is too high (compared with a tap on the back underside of the scroll) then the adjustment is more complicated. Several methods are available to you:
...Perhaps the most straight forward method is to scrape the top surface of the fingerboard evenly until the desired tap tone (matching the center tap of the back plate) is achieved.
...Reidar Skjelkvåle, of Norway suggests this can also be accomplished by removing a small amount of wood from the underside of the fingerboard just behind the meeting with the neck wood:
"To reduce the fingerboard tone..., I do it simply by removing small
amounts of wood from underneath the fingerboard right behind the end of the
neck. If you remove wood here (3-4 cm closest to the neck), you reduce the
spring/stiffness and the frequency goes down. "(11/2008 posting)
...You can also add weight (mass) to the head (scroll) area or you can remove more material from under the fingerboard (over the top of the violin). This latter method is what the VSA exponents suggest and requires the removal of the fingerboard and a substitiue neck to be attached. It is complicated and a lot of work.
...You might want to consider adding weight to the head (peg box/ scroll area) instead as an alternative unless the fingerboard is just plain too thick and needs attention anyway. If the pegs are now boxwood, substituting ebony pegs may be enough to bring balance in the tap tones. I have had success in boring a hole into the head from inside the far end of the peg box (above the 'A' string) and filling it with an epoxy resin mixture into which I then drop line lead (bird) shot. {This is the tiny 'marbles' of lead that sportsmen load shot gun shells with and is available through a sporting good shop that handles ammunition.}
...A similar method might be to screw a small piece of lead sheet to the same head of the peg box with a small pan head wood screw. This method has the advantage of being easily adjustable and reverseable. If you raise the head frequency too high (above the end of the fingerboard tap) then simply unscrew your lead sheet weight and trim a bit off; or scrape some off with a small curved gouge.
...If how ever, the end of the fingerboard tap (which is also the top of the scroll tap tone) is lower than the back tap tone (which should be equal to the underside of the scroll tap tone), then this can be raised by evenly sanding the big end of the fingerboard (which is toward the bridge) with a sanding board until the tap tone is raised to match the back tap tone. At that point, all is in balance and the tone will be improved.
These are the places you tap with your special tapping tool (or your knuckel).
...I believe that what is being balanced acoustically is a 'spring pendulum':
Think of the violin when it is strung up...
...The string tension (of nearly 80 pounds), pulls the neck so that the head (scroll is moved upward and toward the bridge with the neck structure acting like a spring which conteracts against the spring of strings. My guess is that the scroll and fingerboard act like a pendulum and if the frequency at both ends is the same, then when the neck (spring) is osilated by the bowing action on the strings (during the act of playing), the action is smoother and the tone is not harmed as it might be by an unbalance here.