Noise mitigation with active acoustic metamaterials

The potential of active metamaterials to address long-standing challenges such as noise mitigation has been recognized for several decades. However, the research of active structures capable to meet these challenges has been slow due to issues such as stability and limited bandwidth. This work will feature a class of active metamaterials composed of independent unit cells with sensors and actuators connected through electronics that impose desired transfer functions between these elements. We experimentally demonstrate two implementations of these active metamaterials for the application of noise mitigation. In the first case, a two-dimensional metamaterial is digitally programmed to behave with complex effective bulk modulus and mass density that have matched phases. With these properties, the metamaterial achieves very high sound absorption, but is well-matched to the background and thus scatters very little. Moreover, its acoustic behavior can be switched from opaque absorber to transparent medium on very small time scales. In the second case, broadband noise absorption in a duct is targeted through the design of a one-dimensional active metamaterial with analog circuitry. The transmission loss can be increased by adding cells to the metamaterial while maintaining stable performance.