University of New South Wales (Australia)
Univ of New South Wales: Musical Acoustics: Strings, Standing Waves, Harmonics
The production of a standing wave in a string is presented and discussed in this site. Exceptional diagrams are accompanied by thorough explanation and mathematical analysis to explain the various modes of vibrations.
Michigan Reach Out
Sounds Like Science: Guitars
At this site explore the relationship between audibility and frequency of sound, and the difference between frequency and volume.