Optimal tuning of classical guitar
part 2: balancing the bend
by Dr Tom Chalko
The increase in sound projection of modern classical
guitars has been achieved by thinning guitar tops and their bracing,
resulting in more fexible (and also more fragile) top membranes.
Fig 1. below illustrates the key problem. Forces of tensioned
strings and force that holds the bridge attached to the guitar
soundboard top do not act in the same plane. These forces produce
the bending moment F*h that bends the
Since timber and other composite materials used for guitar tops
are not perfect springs, bending deflections caused by this bending
moment increase in time and become permanent.
Symptoms of this bending include
- guitar becomes out of tune in high positions, because bridge
tilts, which reduces string lengths
- Sound response of guitar becomes compromised, because soundboard
that is subject to bending load no longer acts as a "optimal
- guitar and its tuning become increasingly sensitive to temperature/humidity
changes, because timber under bending load is more sensitive
to these changes than timber under pure tension or compression.
My solution involves installing 2 carbon-fibre
tubes 4mm in diameter through guitar soundhole as shown in Fig
2. Total weight of these tubes is about 4 grams.
The vertical tube is installed in a 4mm hole through the bridge,
missing bracing beams and string-tying holes.
The horizontal tube, parallel to strigs, is installed under compression
to produce force B on the tip of the vertical tube approximately
equal to B=F*h/H (see Fig 2 for notation). Silicone tubing "tube
feet" increase friction that holds loaded tubes in place.
After I installed carbon tubes in my Juan Hernandez 2012 Luthier
model guitar I held the guitar without strings for about 4 weeks
to let the timber "unbend" itself. After I installed
strings - the guitar top remains unbent.
Prior to "bend balancing", the Hernandez guitar was
powerful, but required a soundpost to control "wolf notes"
on G and B strings. After "balancing the bend" the soundpost
has become redundant. The acoustic response of the instrument
has become extraordinary. I could not stop playing it for several
days, ~10 hours each day. "Bright" sound typical to
"lattice-braced" guitars has become very rich and controllable,
like in best guitars. The longer I played the guitar - the richer
sound it produced. It seemed as if the timber was "learning"
to vibrate after it was relieved from bending load. Photo 1 shows
Then I implemented the "bend balancing"
system to my second guitar Yulong Guo Chamber Concert 2015 double
top (timber and nomex laminate top). This guitar has almost no
bracing (see Photo 2) and after 12 months of playing - the increased
top deformation has caused the bridge to "raise" and
alter the guitar "action".
Effect of "bend balancing" on my double-top guitar has
also been impressive. This is now my preferred guitar for recording,
because it has ecceptionally powerful and yet uniform response
in all registers.
On the basis of my testing, the "bend balancing" method
described in this article has potential to become a new method
of restoring sound to older guitars and preventing deterioration
of new and old guitars.
It is quite likely that the "bend balancing" technique
can open new frontiers in guitar design and construction with
luthiers pushing for even thiner tops and adventurous bracings
and using pre-load in carbon tubes to "optimize" performance
of their guitars.
Carbon-fibre tubes are removable.
You can also choose to balance "part" of the bending
moment (50% or 80% for example).
Instead of patenting I decided to share my method freely, but
I would appreciate acknowledgment of its implementation. Please
contact me for more details.