David's Corner

On Setting up the Bass, Part II: Bridge, Bassbar and Soundpost

David Gage

I began our discussion of setup talking about strings. The string vibration is 'interpreted' by the bridge of the string bass while it is relaying the information to the top plate of the instrument. This top plate is the surface piece on which the bridge sits and is most commonly made of spruce or pine. Air dried spruce is a wonderfully resonant wood. The top is the single most important component to an instrument's quality of tone.

Set up is work we do to get the most out of that pre-existing body or box of the string bass. We've already discussed strings, let's go on to the bridge and it's supporting cast, the bassbar and soundpost.

Bridges are generally made of soft maple (traditionally Yugoslavian) which is considerably harder than spruce. Any wood softer than maple, made into a bridge, would warp or break under the pressure from the strings. Even with maple the installer must be careful not to make the bridge too thin or leaning forward or back due to potential warping problems. The bridge from front to back is shaped like a tall pyramid. It is my experience that the thickness at the bottom should be no less than 22 millimeters and at the top no less than 5 millimeters. It would be prudent to start with 24-25 m.m. at the bottom and 6-7 m.m. at the top, perhaps removing the last millimeter or so after the instrument has been checked through playing. The ratio of bridge top height to leg height should be about 1 to 2. Bridge adjusters or risers are necessary to regulate string height due to playing demands and to counter string change due to humidity fluctuations. We recommend aluminum rather than wood adjusters. The hole drilled for aluminum adjusters is smaller and the bridge is therefore less vulnerable to cracking. Aluminum is a much better sound conductor than dense hardwood strong enough to withstand the string pressure. The density of the wood is an important consideration. The denser, harder the wood, the more bright or treble the sound of the bass. The softer the wood, the darker the sound.

The location of the bridge on the top is important. The instruments of the violin family are theoretically symmetrical in design. If you could fold the top down the center the f-holes would line up with each other. Since we feel that the f-holes create the dynamic center of the instruments, the outside edges of the feet of the bridge should be equidistant from the inside edge of the f-holes. The bridge size should be wide or narrow enough to allow the bass or E foot stem to be directly over the bassbar. The E or bass foot should be over the thickest part of the bassbar. This allows the bridge to most efficiently drive the top plate. Generally the thickest part of the bassbar is at the intersection of an imaginary line between the inner nicks of the f-holes and the bass bar.

The bassbar runs along the length of the top and has the same function and design found in all the violin family. The function is to support the top from collapsing under the string pressure and it 'tunes' the top to reproduce the most desirable resonance. It helps dictate where the top vibrates The design is a long tapered beam generally made of spruce, shaped and glued to the inside of the top. The thickest part is at the center (at the nicks) tapering to either end. In my opinion many makers and repairmen 'spring' or push up the top too much when fitting a bassbar. This can quicken the response of sound but cause structural damage by pulling down the center extremities of the upper and lower bouts. This can be evidenced by an impression or dip in the top at either end of the bassbar. When buying a string bass it is always good to check the integrity of the top to bassbar union.

The soundpost is the cylindrically shaped post, also generally made of spruce, which supports the treble side of the top. This post is fitted between the top and back plates of the instrument. In a sense, the sound post is the antithesis of the bassbar in that it dictates where the top doesn't vibrate. At the point where the soundpost touches the top is a nodal point or a spot of no vibration. An apt image of the top vibration around the soundpost is that of the ringed ripples that move away from a stone thrown into a calm lake.

Not only the location but the tightness of the post is important. A too tight or too loose post can not only affect the sound but it can also compromise the structure. A post that is too tight can actually crack the top creating a serious and expensive sound post crack while a post that is too short can cause the treble bridge foot area to cave in.. The thickness of the top at the soundpost area must be thick enough so that that area will not collapse around it.

At the start, the soundpost should be fitted at approximately a soundpost width behind or below the treble foot and centered to the stem of the foot. The grain of the soundpost should be perpendicular to the grain of the top. We move the soundpost according to how the instrument sounds and feels. Generally, if the post is moved toward the bridge it gets more focused, brighter with less volume. Away from the bridge it gets darker with more volume and more 'spread' to the sound. Generally, if moved from the starting spot toward the center of the instrument the upper strings get louder, toward the f-hole or outside the lower strings get louder. This having been said, each instrument reacts differently to post moves. We start with the original setting then adjust and listen.

The soundpost's fit is crucial. It should be flush to the inside of the top and the back of instrument. This improves the linkage between the two plates and helps generate the maximum resonance from the whole box.