  An acoustic negative bulk modulus metamaterial based on the concept of
expansion chambers is proposed. It is shown that addition of a neck region to
an ordinary expansion chamber improves its transmission loss characteristics at
low frequencies and the resulting structure displays a negative bulk modulus
behavior. Additionally, membrane based metamaterials are analyzed. Using FEM,
the negative density behavior of a membrane carrying a center mass and of a
tensioned membrane array is analyzed and the inherent similarity of the two
designs is discussed. Further, the modified expansion chamber is combined with
an array of stretched membranes and the resulting structure is analyzed for
double negative behavior.
