Hearing The Shape Of A Sound: The Physics Of Colliding Bars

Francesca M. F. Mascarenhas

The phenomenon of colliding bars or rods has a long and fascinating history. In contrast with similar experiments involving colliding balls, the kinetic energy of the system after the collision is not conserved -- sound waves are produced due to compression and tension in the bars when they collide. By using two sets of bars of different cross-section, this experiment sought to quantify the frequencies produced during a collision. The goal was to investigate whether by just listening to the bars ring we could tell the difference betweeen the sound produced by a bar of circular cross-section compared with the sound produced by a bar of square cross-section. Indeed we were able to audible differentiate between the sounds produced by the two bars. Our FFT analysis of the sound waves generated showed that the ringing in the bar was dominated by the longitudinal and first three flex modes. Applications of theory to experimental data provided good agreement to within 10% of all the modes that we identified as responsible for the ringing.