Porous Materials and Kelvin Cell Foams in Hearing Aids

Hearing aids often face challenges in mitigating noise and unwanted vibration, which can significantly impact their performance and user comfort. Porous materials, known for their excellent sound absorption and vibration damping properties, are highly regarded in the fields of acoustics and mechanics and are widely used to control noise and vibration. These materials are characterized by their interconnected pore structures, which enable energy dissipation through viscous and thermal interactions. This thesis explores the use of porous materials, particularly Kelvin cell foam structures, to address the challenges of noise and vibration control in hearing aids. The research investigates the acoustic and mechanical properties of porous materials using a combination of experimental analysis and numerical simulations. A detailed review of porous material models is conducted, including equivalent fluid models and Biot’s theory of porous media. Measurement techniques, such as impedance tube and transmission tube methods, are employed to evaluate sound absorption, transmission loss. Mechanical properties of porous materials also introduced. A key focus is placed on the Kelvin cell foam model, which is analyzed for its acoustic and mechanical performance. Parameters such as porosity, flow resistivity, high-frequency limit of the dynamic tortuosity, and elastic modulus are calculated to establish relationships between the geometric structure of the foam consisting of identical unit cells or a variety of unit cells and its functional properties. The classic model is compared with simulation results, highlighting its limitation on this application. The characterization of acoustic-mechanical parameters of foam structures composed with multiple layers of different species of Kelvin cells is also discussed. The final phase of the research applies the findings to hearing aid design. Multilayer Kelvin cell foam structures are proposed as an innovative solution to isolate the receiver in hearing aids, effectively reducing noise and vibration transmission. The proposed design demonstrates improved performance through optimized acoustic and mechanical properties. This work provides a foundation for integrating advanced porous materials into hearing aid technology, offering a pathway to enhance device functionality and user satisfaction. Future research directions include further refining the model, increasing the design freedom of the model by adding randomness, and exploring practical applications in other acoustic and vibration sensitive devices.