Stroked Aluminum Rod
To illustrate longitudinal standing waves in an aluminum rod.
Aluminum rod with rosin.
Practice! This one is a bit weird at first, but is a very dramatic demonstration if done adequately.
Apply powdered violin rosin to your fingers or wear a rosined glove and stroke the aluminum rod firmly while holding it at a nodal point. Holding it in the center produces the fundamental, holding at 1/4 of the way from one end produces the second harmonic, holding at 1/6 of the way from one end produces the third harmonic, etc. The rod is about 6 ft long, and the speed of sound in aluminum is about 16,700 ft/sec, so the frequency of the fundamental is about 1400 Hz. The sound is very loud and lasts a long time; the Q for this system is around 100,000!◙
Lecture Demonstration, R. E. Berg, Instructions Sheet for Stroked Aluminum Rod. ◙Lecture Demonstration Calculations Sheet, "Q of Mechanical Oscillators." ◙Nobuo Naba, Observations of Longitudinal Vibration of Metal Rods, AJP 40, 1339-1340 (1972). ◙E. R. Pinkston and L. A. Crum, Lecture Demonstrations In Acoustics, JASA 55(1), 2-6, (1974).