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

In the framework of the research project Comfortable Cabin forLow-emission Aircraft (COCLEA), a low-frequency noise protector concepthas been developed consisting of a double wall with integrated so-calledmembrane-type acoustic metamaterials (MAMs).MAMs are composed of small unit cells with a thin membrane layer andsmall rigid masses attached to it. These structures exhibit tunablenarrow frequency bands with transmission loss values considerably largerthan the corresponding mass law. This can possibly be exploited toimprove the otherwise poor low-frequency performance of a standarddouble wall.Analytical and numerical studies of the noise protector conceptindicated substantial improvements in the low-frequency regime.Subsequent laboratory measurements, however, showed that some importantpractical aspects need to be considered in a technical realization ofthe concept.The present contribution summarizes the most important findings of thisresearch activity. Based upon the analytical, numerical, andexperimental results, the relevant design parameters of the concept areidentified.On this basis, an advanced design of the noise protector concept hasbeen realized and validated in laboratory measurements.