Fig. 1: Do you like to experiment? Try
shifting a neck’s resonance by adding
temporary mass with a clamp.In their quest for tone, most bass players
experiment by swapping pickups, strings,
and tone controls, but rarely do they look
for a new neck ... and at least those owning
bolt-on instruments could.
Necks have two basic functions: They serve
as a platform for our playing and they handle
the tension of the strings. While the first consideration
leads to last month’s thoughts about
shape and radius [“What the Heck is with that Neck?,” November 2012], the second
consideration is about stiffness and adjustability—
factors that affect both playability and,
as we’ll see in a moment, tone.
Adjustability. Whether made of wood,
composites, or aluminum, almost all necks
have a truss rod. The truss rod’s only function
is to adjust the neck to a slightly forward
curvature against the tension of the
strings. There are also two-way truss rods
that act in both directions, but they’re usually
not needed on basses. That’s because a
4-string has a string tension of about 800N
(Newtons)—equivalent to the weight of an
80-kilo adult constantly pulling at the neck
over its entire (and hopefully long) life.
The truss rod provides a way to adjust
a fretboard to its ideal state, which is to be
slightly concave, thanks to a subtle forward
bow. The main reason behind this is that a
string’s amplitude decreases as we shorten
its vibrating length by moving further up
the fretboard. And if we aim for the lowest
action, the neck curvature has to follow this
non-linear change of amplitude. So a totally
straight neck would either mean fret buzz
in the lower register or an unnecessarily
high action in the upper register.
It’s easy to see the curvature by pressing
a string down against the first and last fret.
A distance of 0.5 mm between string and
fret in the middle of the fretboard is a good
number. Turning the truss rod nut clockwise
shortens the truss and causes the neck
to bend backward and vice versa. These
adjustments work the same way, whether
the truss nut is at the body or headstock,
although I have seen some differences in
accuracy between these systems.
Most truss rods don’t run through the
complete length of the neck. For example,
rods having the adjustment nut at the
headstock often end around the 17th or 19th
fret. That’s not a big deal because in that
region the neck is already thicker and stiffer,
and thus better able to withstand the pull of
the strings before the neck finally ends in the
solid neck pocket. Having the adjustment nut
on the body side sometimes means not being
able to adjust the softest part of the neck,
which is the area at the lowest frets. Those
who need to loosen the strings and detach the
neck simply to adjust it might want to put a
neck with a headstock-accessed truss rod on a
checklist for their next purchase.
Tonal influences. Judging by the number
of comments on forum threads, of all
the non-electric components, body wood
has the most impact on bass tone. It seems
players typically replace a neck if they don’t
like its feel or adjustability, but rarely make
the move for tonal reasons.
Which begs the question: How much
influence can a neck have on tone and
where in the sonic spectrum does it have the
greatest effect?
Being a bass player, your main focus
should be on the fundamentals and lower
mids. Whatever happens in the upper mids
or treble range is an extra. Without getting
too technical, we can explore the production
of fundamentals and lower mids using
a very simple model.
For starters, we’re talking about electric
solidbody basses, so the model is a vibrating
string with its two bearings at the bridge and
nut. Ideally, these are infinitely stiff, so any
vibrational energy ends up in our pickups.
Every less-than-ideal resonating part will
suck up certain frequencies, thus modifying
the resulting spectrum. Hopefully this creates
a better tone with more character and doesn’t
rob our bass of its fundamentals.
So we don’t have to deal too much with
mass, elasticity modules, or the quality of
our bearings in this discussion, let’s make it
easy and ask only one question: Which of
the two—body or neck—is more critical to
our fundamental tones?
Without getting bogged down in
technical terms, we can expect the lowest
resonances from the element that’s the
weakest or softest. Assuming neck and body
are made of the same material, their cross
sections give us a good impression of their
stiffness. The ratio of these is about 10
times more for the body, so we can expect
the lowest resonances within the neck. The
length and lever arm of the neck compound
this and make the resonances even lower.
The most annoying resonance on basses
is a dead spot—one that meets the fundamental
frequency, causing it to be rapidly
absorbed. When this happens, the frequency
shakes the neck and not the magnetic
field of our pickups.
This simple model tells us that one way
of getting rid of a dead spot is to get a
stiffer neck, which is why stiffer composite
necks are less known for dead spots. Adding
mass to the neck—especially to the headstock—
is another way to lift resonances.
If you’re dealing with a dead spot, try this
DIY experiment: Temporarily affix a clamp
to the headstock to change the neck’s mass
(Fig. 1). Changing to heavier or lighter
tuners can also shift a neck’s resonance.
Because they react to their multiple higher
modes, these resonances will also work their
way through the entire midrange.
I admit that the body-to-neck connection,
the headstock, and all the involved
masses factor into the tonal equation. But
our simple model is good enough to show
the crucial importance of the neck, and it
also suggests that the body’s sonic role is
way overrated.
Heiko Hoepfinger is a German
physicist and long-time bassist, classical
guitarist, and motorcycle enthusiast. His
work on fuel cells for the European orbital
glider Hermes got him deeply into modern
materials and physical acoustics, and
led him to form BassLab (basslab.de)—a
manufacturer of monocoque guitars and basses. You can
reach him at
chefchen@basslab.de..