Metamaterials-Inspired Aerospace Structures (MIAS)

The vibroacoustic characteristics of structures are vital in determining the operational envelope and mission feasibilities. The sources of vibroacoustic excitation are mainly due to noise generated by the launcher during ignition, lift-off, and atmospheric flight. Typically, foam or fiberglass claddings and cores or acoustic liners which incorporate resonating chambers are used to prevent the transmission of sound through such structural locations. However, this approach is found to be ineffective for vibroacoustic sources with dominant frequency content below 400 Hz. It is proposed to develop a metamaterial-inspired composite structure incorporating low-frequency vibro-impact resonating elements coupled with conventional high-frequency acoustic absorbers. The idea is to employ structurally-integral tuned resonators to pick up energy from incident low-frequency sound waves and utilizing the mechanism of frequency up-conversion via impacts, transfer the energy to higher modes in the sandwich primary structure for subsequent dissipation with conventional acoustic absorbers. The advantage of the proposed structure would be in reducing the transmitted pressure of low frequency waves, for which conventional methods are ineffective. Our initial bound for the attachment mass is within 5 to 10% of the baseline structure to show significant peak pressure reduction for LF waves. The state-of-the-art conventional absorbers provide about 10-20% sound transmission loss (STL) in the 100-150 Hz range. Our performance objective is to achieve STL of about 50-60% in frequency range below 400 Hz with 5-10% mass increase without deteriorating stiffness response of the structure. Successful completion of Phase I work will result in a "proof-of-concept" MIAS unit cell. In Phase II, detailed design and fabrication of the MIAS prototype panel will be completed.