3D printed sound absorbers using functionally-graded sonic crystals

In this work, a thin functionally-graded sound absorber that achieves an absorption coefficient near unity is demonstrated. The sound absorber consists of a multilayer arrangement of an interwoven sonic crystal lattice with varying filling fractions, backed by a thin elastic coating that acts as a flexural acoustic element. The overall thickness of the sound absorber is about one tenth of the wavelength in air, and it was 3D printed with a thermoplastic polyurethane. Samples were fabricated and acoustically tested in an air-filled acoustic impedance tube, from which absorption and effective acoustic properties were obtained. A theoretical formulation for the effective acoustic properties of the sonic crystal lattice was used to guide the design process, and excellent agreement was found between measured and theoretically predicted results. A range of sonic crystal filling fractions and thicknesses were tested to verify the fabrication process and robustness of the theoretical formulation, and both were fo...