Demonstration of a highly directional acoustic source

Abstract

Highly directional light sources such as flashlights, spot lights, and lasers are common objects well known to most people. In contrast, a highly directional acoustic source, or, in other words, a source of sound that is audible in only a very narrow region of space, is far less common. Most people have never experienced such a source, and the phenomenon is not found in nature. A highly directional source of sound known as a parametric array is used underwater for sonar applications, but the frequency (pitch) of the sound is often above the hearing range for human listeners. Similarly, highly directional, focused sound sources are regularly used in medical applications, but, again, the frequency is too high to be heard. The narrowness of the acoustic beam cannot be experienced by human listeners. Recently, parametric array technology has been commercialized for use in air at frequencies in the human auditory range. These devices produce very narrow beams of audible sound. When pointed at one person, who can be listening contentedly, the sound is inaudible to another person only a few feet away. Such a device will be demonstrated and the basic physics behind its operation will be explained.