http://www.phys.unsw.edu.au/~jw/guitarintro.html
http://entertainment.howstuffworks.com/guitar4.htm
http://www.phys.unsw.edu.au/jw/Helmholtz.html
http://en.wikipedia.org/wiki/Helmholtz_resonance
Lapp, David R.
The Physics
of Music and Musical Instruments. Medford, Massachusetts