Re: [Phys-l] Bad physics? coupled oscillations

[curtis osterhoudt <flutzpah@yahoo.com>]

Yeah. The longitudinal vibration of the rod accounts for the vast majority of the sound produced. 


It's a very easy demonstration to do with _solid_ rods, too, and there are various other methods of showing that the organ-pipe resonance is tremendously-weakly coupled to the longitudinal vibration of the rod anyway (hint: symmetry arguments can play a large role here).


One can also drive (via other methods) the organ-pipe modes of the air column (not sure about the 9th or 18th one!), and show that they don't couple well or at all to the rod's motion. 


This is a bad explanation by physicsclassroom.com


 




> ________________________________
> From: Bernard Cleyet <bernardcleyet@redshift.com>
> To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
> Sent: Tuesday, September 6, 2011 12:10 PM
> Subject: [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.
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