ted haskell wrote:
" To expand on what Tom wrote, my experience has been that higher
strings tend to darken (mellow) the sound, and shorten the sustain."
Right. There's a physical reason for this, that takes a little
explaining: the relative 'impedance' of the string and top.
Impedance refers to how difficult it is to get something moving at a
particular frequency. With conductive strings, either solid metal or
the lower, wound strings on a classical, it's possible to rig up an
electromagnetic driver. You put an alternating current through the
string, which passes between the poles of a magnet, and it vibrates at
whatever the frequency of the current is. This is a fun experiment, but
I'll warn you that you can cook a D string pretty quickly from the
resistance in the windings, and it never will sound the same afterward!
If you're driving at some frequency other than the pitch the string is
tuned to, or a harmonic of that, you don't get much motion for a given
current, but at resonance, the amplitude is greater. Mechanical
impedance is defined as the ratio of (velocity/force) at a given
frequency: velocity is related to the amplitude, and force is
proportional to current in this setup. Thus, off resonance, the
impedance is high because it takes a lot of force to achieve a given
You can do something similar for the top of the guitar at the bridge,
of course. As with the string the impedance is lowest at the
frequencies of any resonances that involve motion at the bridge
location, and higher at other frequencies. Unlike the string, though,
the complex structure of the top causes the resonances to be at all
sorts of unrelated pitches. The 'dips' in the impedance curve can be
measured, but they're hard to predict on a guitar top, unlike those of
The things that determine the impedance of an object are its mass,
stiffness or tension, and the amount of 'loss' in it. Higher tension
strings are heavier then low tension ones, and are also tighter, so
they have higher impedance. The top of the guitar, of course, weighs a
lot more than the strings do, and it's pretty stiff, too, so, in
general, the impedance is higher. However, it's not a simple matter of
addition. Remember that the impedance of something has to be specified
at a particular frequency. That's because the 'stiffness' and 'mass'
terms actually can cancel each other out at specific frequenceis,
leaving the total impedance equal to the 'loss' associated with
friction and things like sound radiation. This is why thicker strings
of a given material need to be at higher tension to have the same
pitch: you've got to get the larger mass term to cancel out with a
larger tension one.
One thing that knowing the impedance can tell you is how easy it is for
the energy of vibration to travel from one object to another. When the
impedances match, the energy transfer is complete, when they don't,
some energy is reflected at the boundary. This is why you have to match
the impedance of loudspeakers with that of the amplifier; the
efficiency suffers if you don't. But it's not just the efficiency: the
frequnecy response can change as well. That's because the impedance
varies with frequency. As a general thing, impedance is higher at high
frequencies, but that's a really general generality.
The upshot of all of this is that it's harder for a thin string to get
it's energy across the 'impedance barrier' of the bridge to drive the
top than it is for a thicker string. However, the thin string tends to
do relatively better at higher frequencies than at low ones. Thus,
higher tension strings tend to have a little more output at all
frequencies than lighter ones, but, in particular, they give more bass
response. They also tend to lose their energy faster to the top, so the
sustain is less.
It has been suggested that a top that is 'impedance matched' exactly to
the strings would give the most volume. This is probably true, as far
as it goes. However, it's the mismatch in impedance at the bridge that
'tells' the string how long it is, and thus, together with the tension,
what pitch to make. One outcome of too close an impedance match between
a string and a bridge is the 'wolf' note found on many 'cellos. The
same sort of thing can happen on guitars, although it's usually not as
pronounced or disasterous. The closest impedance match between strings
and the top is generally found on the banjo, and that's a big part of
the reason for that characteristic banjo tone.
Finally, none of this is to ignore the notion of 'choking' of the top
by heavier strings. That's a little harder to think about physically,
but is not an unlikely thing.
Alan Carruth / Luthier