Effect of Membrane Flexibility on Acoustic Properties in Periodic Structures

Kala, Shivaprasad and B, Venkatesham (2019) Effect of Membrane Flexibility on Acoustic Properties in Periodic Structures. Masters thesis, Indian institute of technology Hyderabad.

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Abstract

Honeycomb structure is commonly used to achieve high stiffness to weight ratio for structural design. Design of panel with low mass and high stiffness and good acoustic performance at low and mid frequencies is one of the requirement. The honeybee hive has good sound absorption with low weight and high stiffness. Typical hive structure can be approximated as two periodic layers with central membrane. A good amount of literature work was available in understanding periodic layers but minimal on membrane in periodic structures. This motivated to develop membrane, which is sandwiched between two periodic layers. Different membrane configurations with various periodic core layers are studied. Periodic core layers prepared using Additive Manufacturing (AM) technology, which include 3D printed ABS plastic using Fused Filament Fabrication (FFF) technique, 3D printed Accura plastic using stereo lithography technique. Other type of periodic core layers prepared using polypropylene tubes (straws) joined through hot wire cutting, paper honeycomb made by hand using Honeycomb Before Expansion (HOBE) method and aluminum honeycomb layers. The membranes are prepared using 80 GSM A4 paper, aluminum foil and natural wax used in bee foundation sheet. The periodic core layers with different cell size, wall thickness are prepared for parametric study. In addition, the membrane is prepared with different materials, thickness, and perforations in the membrane. Normal incidence absorption coefficient of the samples is measured using impedance tube. Different parameters effecting the sound absorption in the periodic structures with membrane are identified. These parameters are classified into geometry, manufacturing methods, materials aspects. Vibration analysis of the membrane is carried to find the dynamic properties of the membrane. Based on the small sample experimental results, a large sample with dimensions of 0.3m x 0.3m is fabricated. Also developed small sized reverberation chamber to test the random incidence absorption coefficient of large sized samples.

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