BACK vs. ROUND BACK
part 1 I spoke about the sonic and playability differences
between flat and round-backed basses. Here I will concentrate
on the physical differences between them, and the effects
of these differences on the potential health and maintenance
of the bass.
basses are less susceptible to back damage than flat-backed
ones. There are a three reasons why flat backs are more fragile:
1) Flat backs require bracing, because the back wood is thin
and has no arch to speak of. This bracing tends to work loose
over time: 2) The thin back wood is more easily cracked or
punctured between the braces: 3) The traditional method of
gluing the braces at 90 degrees to the back wood tends to
cause deformation and cracking of the back wood. That is because
changes in relative humidity cause the braces and the back
wood to expand and contract in opposite directions from each
other. This is a result of wood expanding and contracting
mainly across its grain. When this opposite reaction occurs
in a severe way, and the bracing does not break loose, cracking
or shape distortion in the back wood takes place.
flat backs bulge inwardly in the winter, and become flat again
in the summer. In some instances, this bulging can cause tension
and even cracking in the ribs and top table. The more variable
a climate is, the more potential trouble one can expect from
a flat-backed bass. Thats not to say that owning a flat-backed
bass will be a nightmare of problems and expensive repairs.
But being forewarned, owners of flat-backed basses are wise
to be more careful about moving them about. They also need
to be more aware of fluctuations in climate, especially fast,
the early 1800s, Abraham Prescott (American string instrument
maker) experimented with using X bracing in flat-backed
basses. I suppose the theory was that the angled braces, which
crossed the grain of the back wood at about 30-40 degrees
(rather than 90 degrees) were less likely to break loose with
climate changes. These X-braced backs do indeed
seem to be more stable. I think the X brace works well for
sound production as well. The main brace in a flat-backed
bass is placed under the soundpost; it supports and distributes
the tension placed on the bass by its bridge and strings.
With an X or angled brace, the vibration spreads
in a different way than with a typical cross-brace. Several
modern makers are now using either the X brace,
a single angled brace, or a single angled brace plus another
angled stiffener or two. I personally would be very reluctant
to build a flat-backed bass nowadays with traditional bracing.
Climate variations are intensified these days by the amount
of air travel musicians experience; their bass can be in summer
conditions one day and winter conditions the next.
basses which are kept in a temperate climate with distinctly
different seasons and a considerable level of indoor heating
in the winter, will usually benefit form different winter
and summer soundposts. The inward bulging of the brace under
the soundpost (along with other shrinkage) in winter, can
cause changes in the distance the soundpost needs to bridge
of 3 mm (about 1/8) or more. When the soundpost is too
loose (generally in summer), tone and playability suffers,
and wolf tones can be exacerbated. This can also cause sinkage
of the treble-side f-hole. When the soundpost is too tight
(generally in winter), the tone and playability can also suffer,
and the bass top table can crack from the slightest
impact (or none at all). A tight post can also cause a nasty
bulge or dent in the top table. Round-backed basses also expand
and contract, but usually not as much. The rounded back is
more flexible and tends to adapt better to a single soundpost
used year-round. Ideally, when a bass is fitted for a single
year-round soundpost this fitting should take place in the
spring or fall.
weighing the purchase of a new bass, think carefully about
your ability to care for that instrument. If you live in an
extremely variable climate, or do lots of traveling with your
bass, you may want to limit your shopping to basses with rounded
backs. That said, if you fall in love with a flat-backed bass
(or already own one), use a bit more care as regards winter
dryness, severe humidity changes, and cartage. Happy thumping!
Kindly refer any questions you may have to ArnoldS@aesbass.com.
I look forward to answering them in future issues.
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