Test Equipment Setup
Our test procedure was conceptually very simple. On a very
massive piece of wood we mounted a guitar string with a nail on one end and a
guitar tuning peg on the other end. One third of the total string length from
the tuning peg end we mounted an electric guitar pickup with two one-inch
screws. On either end of the mount, on the same face as the guitar pickup,
were two small pieces of Dynamat noise-absorbing material to isolate the test
pieces of wood from the heavy base piece. The various pieces of test wood were
centered on top of these Dynamat pads; the string was then tightened with the
tuning peg such that the string firmly rested on the pieces of test woods,
suspended about a quarter-inch above the guitar pickup.
The output of the guitar pickup consisted of a single output wire with a ground
shield surrounding it, much like a piece of coaxial cable. We grounded the
ground shield at the power supply and ran the signal line to the input of an
741 op-amp buffer. Electrical guitar pickups have very large source impedences,
meaning that the instant one tries to draw a current out of the signal, the
voltage becomes practically unreadable. The op-amp buffer circuit therefore
protected the output voltage of the pickup so that we could both read it with
the data acquisition software on the lab computers and amplify it so we could
hear the output signal with a lab loudspeaker.
Pieces of Test Wood
We used four pairs of pieces of test wood. The first pair, the control, was a set of
medium-density fiberboard (MDF) blocks with additional Dynamat on top of the
pieces such that the string rested against the Dynamat instead of the MDF.
Dynamat has extremely good vibration damping characteristics, more so than any
other material that we could have used as a control subject. In addition to
the control test, we collected data on the three test woods: maple, ash and
Before each test, we detensioned the string, substituted the appropriate two
identical test wood blocks into position on the Dynamat, and aligned the string over
the same pole of the electrical guitar pickup. We then retensioned the
string and continuously plucked it while monitoring the readout on a guitar tuner that
"listened" to the amplified loudspeaker output. We adjusted the tension until the tone of the
output sound was 440 Hz (the musical note D). As soon as possible after the
tuning, we captured a single, standard pluck (an impulse) with the LabVIEW program "Recorder"
using a sampling rate of 10 kHz. We chose 10 kHz because it allowed four seconds of record
time (LabVIEW limits data to a maxiumum of 40000 samples
long) while still allowing considerable frequency headroom for all the harmonics
of the pluck. We plucked the string such that the amplitude died out just as
we ran out of room in the data array. These files were saved as .dat files and
uploaded to Owlnet for analysis with Matlab.