About Neck and Fingerboard Alignment
The neck and fingerboard of the string bass are an integral part of the total sound and feel of the bass. It is important that their installation and condition are sound. The neck should be vertically centered down the top of the instrument so that the bridge can be concomitantly centered on the fingerboard while on the lateral center of the top. The overstand and angle or projection of the neck should allow for a bridge height of no more than 7" and no less than 6 ½". The fingerboard should be thick enough so that it keeps the neck straight and allows for a curve to be planed on the playing surface. The scroll, pegbox and tuners should be solid enough to hold the strings firmly.
Correctly setting a neck in the body is not easy. It's not uncommon to see a new, wonderfully made instrument with an 'out of line' neck. When buying a bass, old or new, check the neck alignment. Sight down the fingerboard toward the tailpiece where it projects over the top. It should be on a line down the center, equidistant from the f-holes. If not, the bridge will either have to be pushed over or the top will have to be cut or tilted over to compensate. There is a good chance that this will create an imbalance of sound and/or feel. The overstand is the distance from the top of the bass to the fingerboard at the neck butt where the fingerboard leaves the neck. In my opinion, this overstand should be 1" to 1 ¼". This allows for the correct (not too steep) angle of the strings over the bridge top and for clearance of the hand and arm when going into thumb position or vibrating the E and F on the G string.
The neck is usually made of maple or beech (sycamore) and is very flexible without the harder, stiffer fingerboard. The fingerboard not only provides a durable playing surface but also stiffens the neck and keeps it from warping. Unlike humans, a bass with a stiff neck is a good thing. In my opinion, a more flexible neck will actually absorb string and body vibration making the instrument quieter and slower. When assessing the condition of a fingerboard, we always check the fingerboard to neck union. The line between the fingerboard and the neck should be straight. An aluminum yardstick (provided it's straight) works well when checking this. If this line is not straight, most of the time it can be completely corrected with a new fingerboard or by refitting the existing board. The wood of the fingerboard is best if it's denser and stiffer than the neck wood. Ebony and rosewood are the traditional choices. Honduras mahogany and maple are used as less expensive substitutes but are not of adequate strength or density to do the best
The electric bass has truss rods to ensure the proper playing curve along the length of the string while providing stiffness to the neck to avoid unwanted warping. The string bass relies solely on the fingerboard for both of these demands. It needs to be much thicker than the relatively less structural electric bass fingerboard. It's also important that the end of the string bass fingerboard, which extends by itself over the body, does not flex when playing in thumb position. The playing surface needs to have a specific curve to maximize sound evenness and playing comfort. There needs to be enough wood to make the correct curve without too much thinning, sacrificing strength. If there's too much curve and it will be difficult to hold the string down and there will be less sustain than possible. Too shallow and the curve will not allow for the full excursion of the string, producing buzzing and string 'clacking' The correct curve will maximize 'growl' and sustain. The exact size of this curve depends upon the demands of the player and the response of the instrument. The larger the string excursion the larger the curve. For example, the E string needs a larger curve than the G string.
We first plane the desired curve into the fingerboard with a well sharpened block plane. First we plane a minimal curve then set the bass up and check it by playing with a bow and pizzicato. Gradually we work into the least curve that the bass will allow when playing strongly without general string noise. The next step we hand sand out any bumps or inconsistencies along that curve, using finer paper as the bumps get smaller. This will ensure an even sustain and sound throughout the length of the string. To locate the bumps we use a 5 ½" long aluminum straight edge which we rest on then slide along the fingerboard in the direction of the strings. By shining a 25-watt bulb between the straight edge and the fingerboard we can see the fingerboard surface bumps magnified as shadows in the thin band of light in the fine space between the straight edge and the fingerboard.
The string bass was designed to be bowed, thus the bridge top is arched. So that the inner strings aren't too far away from the fingerboard, the fingerboard is arched to come up to the inner (A and D) strings - in other words, bring the mountain to Mohammed! There are two common archings 1.) the continuously rounded and 2.) the ¾ round to a flat or bevel along the E string, which leaves a ridge the length of the fingerboard between the E and A strings. The bevel is probably a vestige of earlier 3 string bass days. The continuously round fingerboard feels more even during string crossings and are now most popular.