Active noise control using an Active Acoustic Grillage

An Active Acoustic Grillage (AAG) is an array of sound sources and microphones forming an acoustic barrier trapping sound but allowing air and light to pass through. AAG concept can be used for noise cancellation in ducts with relatively large cross-section. The original grillage designed by Hood Technology Corp uses distributed single-input-single-output (SISO) design to provide an inexpensive and expandable system. This design assumes "diagonal" dynamics and does not address the cross coupling between AAG cells. The objective of research in this report is to apply multi-input-multi-output (MIMO) design methods and increase the system robustness while retaining or improving the performance in comparison with SISO design. Work on the grillage is proceeding along two parallel lines: (1) development of a single cell experiment for evaluation of the "best" arrangements and types of speakers and microphones, and (2) development of a prototype grillage incorporating multiple cells for evaluation of geometry and control schemes. Serious controller design challenges are imposed by non-minimum phase behavior and lightly damped acoustic modes. Several linear quadratic and robust controllers have been developed that provide 10-15 dB attenuation in the frequency range of 100-300 Hz. Benefits of MIMO design are shown on a test grillage installed at the opening of a duct. The work demonstrates benefits of MIMO design in noise control systems with multiple sensors and actuators where cross coupling is significant. These results can be expanded to future applications like "acoustic curtains" or active baffles based on MEMS that are inherently multidimensional.