The Energy Flow for a Spherical Acoustic Lens: Experimental Results Avoiding Interference Effects

A simple classroom demonstration consists of a weather balloon filled with carbon dioxide, a sound source, and a microphone. Since the speed of sound is slower in carbon dioxide than in air at room temperature and pressure, the balloon acts as a positive spherical acoustic lens. Preliminary experimental results have been presented previously [C. E. Dean and J. P. Braselton, “The energy flow for a spherical acoustic lens: ray and wave methods vs. experiment.,” J. Acoust. Soc. Am. 125, 2627 (2009)]. The possibility of interference effects from the reflection of sound off surfaces was brought up in the ensuing discussion. The current results have been measured in a way that minimizes the effect of interference due to reflections off walls, floor, or other surfaces. © 2010 Acoustical Society of Americ