Acoustic camouflage for military vehicle based on an adaptive Helmholtz-Resonator System

In military vehicles, the acoustic emission of internal combustion engines has a significant impact on the acoustic camouflage. For example, engine fans or exhaust streams exhibit low-frequency components which can be detected at a great distance. These correspond to the amount on pistons in the engine either at driving speed or in idling mode. In the latter case, a stationary acoustical signal is analyzed for peaks in the frequency spectrum. For the semi-active reduction of these narrowband frequencies, an acoustic metamaterial based on Helmholtz-resonance is developed. A Helmholtz-resonance-based acoustic metamaterial has been manufactured with real-size dimensions reaching for the frequency regime below 200 Hz by four neck-cavity pairs in total. Furthermore, one of the four pairs is extended with an adaptive control of its adjustable cavity volume. Thus, a narrow band within the low-frequency regime can be actively detected and mitigated. The adaptive controller is based on a feedback algorithm and the acoustic wave separation method. Consequently, the system is adaptable to an emitted acoustical signal.