Silicon-based MEMS microspeaker with large stroke electromagnetic actuation

This paper reports on the design, manufacturing process and characterization of a new silicon-based microspeaker. The objectives of this work were to get better sound quality, higher acoustic power and higher efficiency than that of conventional microspeakers. An optimized microstructure for the silicon mobile part enabled the sound radiator to be extremely light and rigid. The suspension was designed to enable large out-of-plane displacements of the radiator. The radiator was actuated by an electromagnetic motor composed of a fixed permanent magnet and a planar coil located on top of the silicon membrane. The electromagnetic structure was designed and optimized regarding efficiency and linearity. The obtained upward and downward motion, close to a perfect piston movement, was very beneficial for the sound quality. Electro-mechano-acoustic characterization showed that the suspension can stand out-of-plane displacement higher than 1200 μm. For an injected power of 0.5 W, the microspeaker was capable to generate a sound pressure level of 80 dB at 10 cm from 330 Hz up to 20 kHz frequency. The efficiency reached 3×10^-5, that is to say three times higher than typical efficiency of conventional microspeakers. Moreover, very low intermodulation effect was detected. This contributed to the remarkable sound quality of this silicon-based microspeaker.