Ray Model of Sound Focusing with a Balloon Lens

Document Type

Article

Publication Date

3-2012

Publication Title

Journal of the Acoustical Society of America

DOI

10.1121/1.3681124

ISSN

0001-4966

Abstract

A weather balloon filled with carbon dioxide gas is used as a positive spherical acoustic lens. High frequency but audible sound from a circular loudspeaker ensonifies the balloon and produces increased sound pressure levels in a region along the principal axis according to a ray acoustics model. This enhancement was measured experimentally and was found to agree with theory. The possibility that interference from reflected sound off walls or the floor could mask or mimic the expected focusing was countered by calculating and measuring within a “shadow zone” in which only direct rays or rays refracted by the balloon exist by the method of Fresnel volumes. The experiment described in this paper would be a suitable learning experience for junior high and high school students showing how rays and Snell’s law apply to sound as well as light and giving them a measurable predicted focal region for enhanced sound pressure levels.

Comments

Copyright 2012 Acoustical Society of America. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the Acoustical Society of America.

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