Conventional acoustic absorbers like foams, fiberglass or liners are used commonly in structures for industrial, infrastructural, automotive and aerospace applications to mitigate noise. However, these have limited effectiveness for low-frequencies (LF, <~500 Hz) due to impractically large mass or volume requirements. LF content being less evanescent is a major contributor to environmental noise pollution and induces undesirable structural responses causing diminished efficiency, comfort, payload integrity and mission capabilities. There is, therefore a need to develop lightweight, compact, structurally-integrated solutions to mitigate LF noise in several applications. Inspired by metamaterials, tuned mass-loaded membranes as vibro-impact attachments on a baseline structure are considered to investigate their performance as an LF acoustic barrier. LF incident waves are up-converted via impact to higher modes in the baseline structure which may then be effectively mitigated using conventional means. Such Metamaterials-Inspired Vibro-Impact Structures (MIVIS) could be tuned to match the dominant frequency content of LF acoustic sources. Prototype MIVIS unit cells were designed and tested to study energy transfer mechanism via impact-induced frequency up-conversion and sound transmission loss. Structural acoustic simulations were done to predict responses using models based on normal incidence transmission loss tests. Simulations were validated using experiments and utilized to optimize the energy up-conversion mechanism using parametric studies. Up to 36 dB of sound transmission loss increase is observed at the anti-resonance frequency (326 Hz) within a tunable LF bandwidth of about 300 Hz for the MIVS under white noise excitation. Whereas, it is found that under monotonic excitations, the impact-induced up-conversion redistributes the incident LF monotone to the back plate’s first mode in the transmitted spectrum. This up-conversion could enable further broadband transmission loss via subsequent dissipation in conventional absorbers. Moreover, this approach while minimizing parasitic mass addition retains or could conceivably augment primary functionalities of the baseline structure. Successful transition to applications could enable new mission capabilities for aerospace and military vehicles and help create quieter built environments.
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