Experimental Confirmation of Non-Positive Refraction in the Passing Band of a Phononic Crystal

Abstract

A sonic crystal composed of open ended copper pipes mounted in plastic in open air is found to non-positively refract sound of frequency approximately 10.05 kHz incident at approximately 23.3°. This is confirmation of the predicted refractive properties of such crystals upon incident sound with frequencies within their passing bands at certain angles of incidence. The measurements were obtained by examining the spectra of sound obtained from a microphone whose position was varied relative to a speaker apparatus projecting a pure sine wave created by a function generator, with and without the crystal in the way. The horizontal shift in the profile of sound due to the crystal was then examined and it was concluded that the crystal did not positively refract it, considering it to be acting as a flat lens. Furthermore, it is confirmed that such a crystal causes incident sound to undergo minimal distortion to its profile by observing that a collimated beam incident upon the crystal results in approximately the same profile as output.