Bass bar adjustment (shaping, adjusting for acoustics)...
...An important part of any violin family stringed instrument is the bass bar.
...It is so named because it is attached just under the 'bass' (lowest toned string) side of the bridge, that is, the left side foot.
...It gives added support to the top plate (belly) on that side and helps to distribute the sound vibrations down the plate in both directions.
...One of the important items in making an instrument that will have an excellent tone, response, and power is this bass bar.
...It is made of spruce wood; and is quarter sawn when looked at from the end; that is the annual growth rings appear to go up and down, toward and away from the plate wood.
...Typically this bar is carved from a blank which is cut to the desired width before-hand. Usually this width is uniform through out its length; 5mm (0.197") wide. The bottom side (toward the top plate) is carefully shaped to exactly follow the plate's inside contour (which is finally shaped before the bass bar is fitted and glued on. When perfectly fitted, the bar is glued into place with violin making glue, usually perpendicular to the predominant contact surface (as the curvature changes throughout the length of the bass bar).
... I prefer to pre-heat the bass bar and top plate to about 100 degrees F. before gluing; this ensures the glue will stay liquid and give the closest bond possible. This gluing operation should take no longer than 30 seconds if all is pre-fit and in readiness.
...Once the glue dries (overnight) then the top surface can be carved to its final and permant shape.
... The remainder of this section is about doing this final shaping or making adjusting changes to affect certain tone realationships to the top plate:
...Once the bass bar is glued into place, we can begin to shape the bar. It will be thickest in the center and taper toward each end.
...The completed bass bar will look something like this...
...Our goal is to make the top surface (tapping down) tap-tone of the bass bar to be the same from one end to the other. We want this uniform tap tone to match the top plate frequency when it has matured to its final stable frequency. Since the plate frequency will increase at first with exposure to U.V. light (sun light) and the effects of the varnish ground coat and final varnish coats, we have to plan what this final top-plate frequency will be and design our bass bar with that frequency; even though it will be higher than the plate at this stage of development. The bass bar will not be increasing in frequency over time... this is it!
...The ideal frequency is 322 HZ.
(Other frequencies that resonate with carbon are 300, 285, and 262 HZ) This is the frequency we eventually want the top plate to be at when mature.
...If the bass bar is too low at one point, the frequency there can be raised by slightly tapering the sides of the bass bar (leave the bottom surface, which is glued to the top plate, its full width). This will increase the downward tap tone. If the tap tone is too high in a certain place, then scrape or sand a little material off the top surface at that point to lower the tone there.
...Doing the final bass bar shaping with this method will result in a bass bar of the classical shape and you will have built your violin with a bass bar that matches the instrument acoustically, for better quality sound, adjusting for the sound characteristics of the particular pieces of wood that you are working with.