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