The Singing Aluminum Rod
Many retort stands consist of an aluminum rod screwed into a base.
Remove the rod from the base.
Balance the rod on one finger to find the linear centre of the rod and mark this with a small dot.
Rub crystals of Rosin up and down the surface of the rod (or use powdered rosin as used with a violin bow).
Grip and hold the rod at the linear centre with two fingers.
With two fingers of your other hand, stroke the rod from the centre to the end with gentle pressure. It will begin to “sing” with a high-pitched vibration.
Increase the speed and the pressure of the stroking action to increase the volume of the sound.
Damp the sound by toughing a finger to the end of the bar.
Holding the rod at the centre of mass will produce the fundamental tone.
Hold it at 1/4 of the length to get the second harmonic.
Hold it at 1/3 of the length to get the third harmonic.
Most vibrations that students experience are transverse … guitar strings etc.
This longitudinal vibration is unusual enough to be a discrepant event for most students.
Since the two ends of the rod are the sources of the sound, twirling it as if it were a baton will produce changes in the perceived frequency due to the Doppler Effect.
Students are impressed with the volume of the sound and are fascinated by the ability to damp it by touching the end of the rod with a finger.
Speed of Sound in Aluminum = 5082 m/s
If L = 1.5 m then wavelength = 2L = 3 m
Fundamental frequency = 1695 Hz
This link will access an impressive study of the phenomenon by Prof. Steven Errede, Dept. of Physics, University of Illinois at Urbana-Champaign IL, USA.
Please respect his copyright restrictions.