Honeycomb-shaped meta-structure for minimizing noise radiation and resistance to cooling fluid flow of home appliances

Meta-structures composed of honeycomb-shaped unit cells were built to have high sound-proof and heat-exhaust performance. Each unit cell operates as a Helmholtz resonator (HR) and a lumped inertance muffler (LIM), resulting in sound energy dissipation at specific tonal and broadband frequencies and minimal noise transmission. This cover structure allowed heat-exhaust through the open cross-sectional area. The acoustic characteristics of the meta-structure fabricated using ABS were measured by the four microphone method. The bandgap at specific frequencies and noise reduction at high frequencies were investigated. The significant change of negative effective mass density and the purely complex value of the wave speed were measured at the resonant frequency band of HR. The theoretical model was suggested to analyze the noise reduction mechanism. The designed meta-structure was applied as the covers of the refrigerator machine room and the top-loading washing machine for demonstrating a practical noise control applications. The suggested structures were designed to meet same geometric constraints as the original cover both in dimensions and open area for heat exhaust. To evaluate the noise reduction performances, the sound radiations from the operating refrigerator and washing machine were measured. The noise transmitted through the panel was significantly reduced by using the proposed cover in the specific frequency bands. For the washing machine, the efficiency depended on the type of the operating process. Noise reduction was large during the fill and wash process, whereas, minimal during drain and spin process. Because the resonance and cut-off frequencies of HR and LIM were designed as 570 and 300 Hz, respectively, the effectiveness was minimal for low frequency sounds. The temperature of the operating compressor of refrigerator and the spinning motor of washing machine showed minimal increase suggesting identical heat exhaust performances. The suggested meta-structure induced large noise reduction without influencing heat exhaust by similar open area for fluid flows as the original cover.