Issue 9

FLAT BACK vs. ROUND BACK
The Technical Stuff

Part II

In 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.

Round-backed 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.

Typically, 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. That’s 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, severe changes.

In 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.

Flat-backed 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.

When 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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