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Noise attenuation at low frequency is one of the major challenges in industry. Many treatments ranging from micro perforated panel to heterogeneous blanket based acoustic meta-materials are available for high frequency noise absorption. The longer wavelengths relative to the material dimensions make it difficult to absorb the noise. For instance, to absorb a noise of frequency of 200 Hz a porous material of thickness 43 cm is required. Therefore, existing acoustic treatments are not effective at this range. As a new idea for an acoustic meta-material, it is proposed to embed an array of Helmholtz resonators in a poro-elastic medium. But these need to be small in size and yet work at low frequencies. The idea of miniature Helmholtz resonators was realized using disposable syringes of 2.5 ml to 10 ml range. These resonators were embedded in a 20 mm thick PU foam and its absorption coefficient was determined using an impedance tube. An array of such resonators with different combinations is shown to enhance low frequency absorption, from 250 Hz to 500 Hz. After this the meta-material is tested in a cylindrical cavity for its acoustic performance. The material developed is light in weight and holds promise for use in payload fairings, where there is a need for enhanced low frequency noise reduction with minimum increase in system weight.
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