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[Phys-l] Bad physics? coupled oscillations



"Another common physics demonstration that serves as an excellent model of resonance is the famous "singing rod" demonstration. A long hollow aluminum rod is held at its center. Being a trained musician, teacher reaches in a rosin bag to prepare for the event. Then with great enthusiasm, he/she slowly slides her hand across the length of the aluminum rod, causing it to sound out with a loud sound. This is an example of resonance. As the hand slides across the surface of the aluminum rod, slip-stick frictionbetween the hand and the rod produces vibrations of the aluminum. The vibrations of the aluminum force the air column inside of the rod to vibrate at its natural frequency. The match between the vibrations of the air column and one of the natural frequencies of the singing rod causes resonance. The result of resonance is always a big vibration - that is, a loud sound."

Resonance

http://www.physicsclassroom.com/Class/sound/U11L5a.cfm


I'm a bit puzzled. The ratio of sound speeds (both longitudinal) is ~ 17 Therefore the fundamental (1/2 wavelength both ends free) of the tube must excite the ~ 17 harmonic of the both ends open "organ pipe" resonance -- doesn't seem very likely.

However there is a report on the mode switching caused by the thin tube (organ pipe) resonances. The pipe tho is transversely vibrating, IIRC.


ScienceDirect - Journal of Sound and Vibration : Pitch and level changes in organ pipes due to wall resonances


http://www.sciencedirect.com/science/article/pii/S0022460X03006436


bc thought he knew much about driven oscillators.