An acoustic guitar was excited by a 4 millisecond long
"tap" from a mechanical shaker that struck the D-string to simulate a
single string being plucked. The velocity of the string was measured using a Polytec PSV-400 scanning laser Doppler vibrometer. For this measurement, there was a capo on the 6th fret, just out of the field of view of the photograph
and vibrometer measurement. The video shows the first few reflections as the wave bounces back and forth along the string.
Technical Details
Photograph showing Washburn guitar with a mechanical shaker that was used to tap the string
The guitar used was a Washburn D-27S steel-string acoustic guitar. It was hanging from a guitar stand from rubber bands attached to the tuners.
Fiberglass insulation was placed under the guitar to damp mechanical vibrations.
An Agilent function generator produced a pulse which was amplified and sent to a Bruel & Kjaer 4810 mini shaker.
A Bruel & Kjaer 8203 force transducer at the end of the
stinger allowed measurement of the time of impact of the stinger with the string.
This was about 4 ms in duration and occurs between about 10.4 ms and 14.4 ms in this video.
Since there was no damping of the string, to allow the note to decay, there was a 8 second delay after each tap.
The vibrometer measurement laser was pointed at 128 scan points along the string, and the time domain samples were averaged for 10 taps at each scan point. The velocity at each scan point was sampled 256 thousand times per
second. To measure the entire data set took several hours!
The graph at the bottom of the video is the measured time-domain velocity for a point just above the
impact point.
