Robert E. Apfel
Dept. of Mech. Eng., Yale Univ., New Haven, CT 06520-8286
The efficiency of a vibrating object in converting fluid motion into sound and of an object in reflecting sound depends on whether the fluid in contact with the object has time to get around it during one half an acoustic period. If it does not have adequate time, the fluid is compressed and sound is produced (or reflected). To simulate this invisible process for a classroom of students, a viscous liquid with a large number of small plastic balls is placed in a transparent glass baking dish sitting on a transparency projector. Small, medium, and long paddles sloshing back and forth in the liquid show different behaviors. If the paddle is large and oscillating at a modest or high rate, the particles are seen to be trapped in front of paddle and appear to be compressed. For the small paddle and sufficiently low vibration rates, the particles have enough time to slosh around the paddle. With a minimal amount of analysis, students will quickly learn to appreciate how the efficiency of vibrating surfaces or reflecting objects depends on the relative magnitudes of the object size and the acoustic wavelength.