Transmission loss of membrane-type acoustic metamaterials with coaxial ring masses

Transmission loss (TL) of membrane-type locally resonant acoustic metamaterials (LRAMs) with added ring masses was analyzed using both finite element analysis and experimental techniques. LRAM structures traditionally display high TL (over 40 db) over a narrow frequency band. TL peak frequency in membrane-type LRAMs is tuned by variations in mass and membrane properties, with the objective of increasing the breadth of the TL peak. The addition of a ring mass to the structure either increased the bandwidth of the TL peak or introduced multiple peaks, depending on the number of rings, the distribution of mass between the center and ring masses, and radii of the rings. The addition of ring mass increased the TL frequency bandwidth by up to 360% (at a sound level 30 dB) compared to a structure of equivalent overall weight with only a central mass. Finite element analysis was used to predict TL behavior of several ring configurations, and TL for these configurations was measured to validate the model predictions. Finally, finite element analysis was used to predict the mode shapes of the structure under single-frequency excitation to understand the mechanisms responsible for the TL peaks. (C) 2011 American Institute of Physics. [doi:10.1063/1.3665213]