Method for retrieving effective properties of locally resonant acoustic metamaterials

Acoustic metamaterials can be described by effective material properties such as mass density and modulus. We have developed a method to extract these effective properties from reflection and transmission coefficients, which can be measured experimentally. The dependency of effective properties on the positions of the boundaries of the acoustic metamaterial is discussed, and a proper procedure to determine the boundaries is presented. This retrieval method is used to analyze various acoustic metamaterials, and metamaterials with negative effective properties are reported.