http://video.mit.edu/watch/tuning-forks-resonance-a-beat-frequency-11447/
http://www.math.harvard.edu/archive/21b_fall_03/tacoma/index.html
I found some wrong methods http://www.strand7.com/html/naturalfrequency.htm http://peer.berkeley.edu/education/files/worksheets_friendship/Natural%20Frequency%20Worksheet.pdf
but none I think are right. Do you know why wind induced oscillations are resonant? Imagine I have a string. It is resonant 100% of the time. Now when the wind causes the string to oscillate it causes the tension on the string to fluctuate within some level of tolerance say +-1 Newton. By changing the tension on the string I have also changed the fundamental frequency of that same string and it resonates between that range of values.
There are things I don't think are right though assume I have 52Fe and 52Cu http://upload.wikimedia.org/math/7/3/c/73cb6f0121da6b745df4141181907357.png
says that they should have an identical resonant frequency since they have the same mass however we know they are not the same.