A compact and low-cost MEMS loudspeaker for digital hearing aids

A microelectromechanical-systems (MEMS)-based electromagnetically actuated loudspeaker to reduce form factor, cost, and power consumption, and increase energy efficiency in hearing-aid applications is presented. The MEMS loudspeaker has multilayer copper coils, an NiFe soft magnet on a thin polyimide diaphragm, and an NdFeB permanent magnet on the perimeter. The coil impedance is measured at 1.5Ω , and the resonant frequency of the diaphragm is located far from the audio frequency range. The device is driven by a power-scalable, 0.25-μm complementary metal-oxide semiconductor class-D Σ △ amplifier stage. The class-D amplifier is formed by a differential H-bridge driven by a single bit, pulse-density-modulated Σ △ bitstream at a 1.2-MHz clock rate. The fabricated MEMS loudspeaker generates more than 0.8-μm displacement, equivalent to 106-dB sound pressure level (SPL), with 0.13-mW power consumption. Driven by the Σ △ class-D amplifier, the MEMS loudspeaker achieves measured 65-dB total harmonic distortion (THD) with a measurement uncertainty of less than 10%. Energy-efficient and cost-effective advanced hearing aids would benefit from further miniaturization via MEMS technology. The results from this study appear very promising for developing a compact, mass-producible, low-power loudspeaker with sufficient sound generation for hearing-aid applications. Copyright copy; 2009 IEEE.