Enhancing the low-frequency noise reduction of a double wall with membrane-type acoustic metamaterials

摘要

In the framework of the research project Comfortable Cabin for Low-emission Aircraft (COCLEA), a low-frequency noise shield concept has been developed consisting of a double wall with integrated so-called membrane-type acoustic metamaterials (MAMs). MAMs are composed of small unit cells with a thin membrane layer and small rigid masses attached to it. These structures exhibit tunable narrow frequency bands with transmission loss values considerably larger than the corresponding mass law. This can possibly be exploited to improve the otherwise poor low-frequency performance of a standard double wall. Analytical and numerical studies of the noise shield concept indicated substantial improvements in the low-frequency regime. Subsequent laboratory measurements, however, showed that some important practical aspects need to be considered in a technical realization of the concept. The present contribution summarizes the most important findings of this research activity. Based upon the analytical, numerical, and experimental results, the relevant design parameters of the concept are identified. On this basis, an advanced design of the noise shield concept has been realized and validated in laboratory measurements.