12 String and Large Backpacker Modes

The animations on the following pages are the results of some recent measurements I've made on two instruments. One is a traditional Dreadnought-shaped 12 String guitar made by Fender. It has a laminated spruce top and a laminated mahogany back. The second instrument is a large version of the "Backpacker" guitar, recently made famous by the Martin guitar company. This instrument was built by Bob McNally (The inventor of the Backpacker guitar) of McNally Instruments. It has a solid spruce top and a solid mahogany back. This instrument is also a 12 string.

The measurements of the top plate vibrations were made by exciting each instrument at the bridge next the high E string pair with a small shaker driven with white noise. The shaker location was chosen so as to approximate the location of the drive point on a guitar in normal use (i.e.the string location). The top plate velocity was then measured over a high resolution grid using a scanning laser doppler vibrometer. (The backs were measured similarly, however, since their is no bridge on the back plates the shaker was simply placed at a somewhat off-center location chosen to (hopefully) maximize response). The measurments were made with full string tension. The strings were damped by placing foam between them and the finger board.

The animations displayed are the vibration patterns at various peaks in the frequency response curves. Some are not actually modes at all, but rather travelling waves in the instrument. Some actually show the same mode responding at slightly different frequencies (i.e. the 303-355 Hz range of the 12 string top).

In the future, I will be measuring the remainder of my acoustic guitars in a similar manner. My goal is to try to correlate different modal characteristics of guitar plates to subjective "sound quality" judgements. Hopefully this can lead to different acoustic guitar designs. If you have any questions or input, please contact me at mdaley@sabine.acs.psu.edu.

12 String Top Modes

12 String Back Modes

Large Backpacker Top Modes

Large Backpacker Back Modes