By David Gage
Generally, an upright's string height goes up in the summer and down in the winter. In fact, over the course of a year, the string height on an upright bass changes probably more than on any other instrument. This is due mainly to its large, shell-like body, long and flexible neck, and its composition of several different woods. Wood is dimensionally unstable because it is
hygroscopic-it absorbs and releases water as the atmospheric humidity changes. All of the various woods of the string bass are changing at different times and amounts. String-bass top plates are usually made of softer, less dense wood (spruce or pine) than the back plates; as the humidity changes, the dense, hard woods (maple and ebony) tend to twist, bend, and warp more than the soft woods. As a result, in the winter-when air tends to be drier-string-bass backs typically bend in toward the body chamber's interior, pulling back the neck and moving the fingerboard closer to the strings, thereby decreasing the string heights. The top plate also bends inward, toward the body chamber's interior, moving the bridge down so the strings get closer to the fingerboard. In the summer the top and back plates bend away from each other, raising the string heights.
In addition to seasonal changes, string heights can sometimes change rapidly due to indoor humidity swings. In climates where the ground actually freezes in the winter, the indoor air is further dried when heated to room temperature. In New York, for example, the indoor relative humidity in the winter can change from 95 percent to five percent very quickly. Such humidity changes not only can cause body cracking in an upright bass, they also often greatly change the height of the strings from the fingerboard.
In order to maintain string heights, it is very useful and effective to have bridge adjusters installed in the bridge's legs.
How Bridge Adjusters Work
The most common type of bridge adjuster is a simple wheel with a pin passing through the center, perpendicular to the wheel. Usually one side of the wheel pin is threaded while the other side is not. When drilling the hole in the bridge leg, we start at the bottom of the foot and drill the length of the leg, "dead-ending" this hole before it exits at the bridge leg's top curve. We install these adjusters so that the threaded part is facing down and tapped directly into the leg's lower (or foot) section.
The upper (non-threaded) part is simply inserted into a hole in the bridge's top section; this part of the pin spins freely as the lower threaded section moves up and down within the lower leg. The hole for this upper pin should be slightly wider than the pin, as allowing some play in the adjuster fit enables the bridge foot to "seat" squarely on the bass top. It is a misconception that this non-threaded pin should be firmly held in its hole; it's more important that both bridge feet are sitting squarely on the top, on the same plane as the instrument top plate. Therefore it is important that the bridge top sits flat and evenly on top of the wheel when the feet are square to the bass top. This fit is what allows for a solid sound transfer to the instrument top. If the adjuster is threaded into the foot, it's possible (and easy) to remove wood from the upper leg section later, should it become necessary to reduce string height. In the case where the pin's threaded side is threaded into the upper bridge section, the hole going into the upper section stops before it exits, making it impossible to thread it the entire length. If the non-threaded pin goes into the upper section, the non-threaded pin can be cut down to only a few millimeters and still hold, making the "dead-ended" hole issue inconsequential.
I believe that adjusters installed with the thread going up, into the upper leg section, bind and wear more quickly. Some adjusters are designed with threaded inserts imbedded in the bridge leg to keep the thread from eroding from friction during adjustment. These inserts add unnecessary weight to the bridge, which means they can mute the instrument's sound-and they aren't necessary, as proper threading tapped directly into the bridge wood will last for years.
Bridge adjusters can safely raise the bridge about three-quarters of the threaded pin's length.
Higher than that, you run the risk of instability and sound loss. If you need to go higher, a new bridge isn't necessarily needed. Maple shims installed in the bridge leg (on the opposite side of the wheel from the threaded pin) can completely and permanently fix the problem without any negative effect to the sound.
Adjuster Materials
There are those who believe that bridge adjusters alter an instrument's "natural" tone and sound production. In Europe, bridge adjusters are just starting to be used more widely; to change string height, Europeans still commonly use two bridges of different heights. But in our shop we have installed hundreds of sets of adjusters in bass bridges, and many times we have played the bass before and after installation and could not discern a difference. If anything, the bass may sound louder with adjusters. All of our clients from the New York Philharmonic and the New York Opera Company use bridge adjusters. If adjusters detract from the sound, it's negligible-although I do think that the adjuster's weight, fit, and construction materials are important factors.
There has been a constant and growing debate as to the effect that adjuster materials have on the sound production. This debate can be very subjective; we all tend to hear what we see, and vice-versa. For example, if I feel that wood adjusters are more "organic" and in greater harmony with the wood in the bass, perhaps that's what I'll hear when I play the bass. If I feel that aluminum adjusters are going to sound "metallic," then maybe that's what I'll hear when I see them on a bass I'm playing.
In the next two columns I'll cover a complete, objective discussion of adjusters. This will include a detailed description, installation instructions, structural analysis, and finally an objective test of all of the commonly used adjusters. My colleague Sam Finlay and I have devised a sound experiment using Digidesign Pro Tools to record (and professional bassists to analyze) the differences in sound. Through a hands-on trial, we will also try to determine any differences in the feel of a bass with different adjusters.