Design of flat broadband sound insulation metamaterials by combining Helmholtz resonator and fractal structure.

A new sound insulation metamaterial (SIM) is designed by combining a Helmholtz resonator and a fractal structure for sound insulation in the frequency range of 600 to 1700 Hz. Using a fractal-based internal structure, the designed hybrid SIM shows a relatively constant value of sound transmission loss (STL) over the target frequency range. The basic concept of this work is to develop two functional parts working in each effective frequency range. One is a Helmholtz resonance structure that operates with a high value of STL from 600 to 1000 Hz, and the other is an internal fractal structure that shows a high STL at higher frequencies in the 1000 to 1700 Hz range. In this work, the proposed metamaterial has a mean of 16.34 dB with a standard deviation of 1.73 dB within the wide target range of 600 to 1700 Hz. Additionally, the proposed SIM is approximately 40 % smaller in size than the previous resonator and has a higher mean STL with a small ΔSTL of 1.7 dB between highest and lowest values. [ABSTRACT FROM AUTHOR]