Cricket Inspired micro Speakers

Abstract

MEMS technology has ushered in a new era of miniaturized sensors and actuators. Many smart devices and systems are being developed using these sensors. Home automation is now a widespread reality owing to the development of affordable miniature devices. Wearables like smart watches and point-of-care medical devices have brought positive changes in the healthcare industry. Also, at global scale, these sensors and actuators find their place in tracking weather changes and remote sensing applications. Many of these micro and nano systems communicate with humans using electroacoustic devices. They can take in voice input, process it and give out voice instructions/suggestions using a system made of microphones and audio speakers. However, when we compare the sizes of all the different sensors and actuators with the size of an audio speaker, we see that audio speakers have not really achieved miniaturization. For example, in a standard smartphone a mini audio speaker is still 8 times larger in volume when compared with a MEMS microphone. An audio speaker is still struggling to get into micron size range. This further limits the extent to which a smart device can reduce in size. The size reduction of the audio speaker, if possible, will lead to an overall size reduction of smart devices. We inspect the intricacies involved in miniaturization of an audio speaker and explore a possible solution by combining silicon MEMS technologies with nature inspired design for the same. In this work, we present two unconventional approaches to build electrostatically actuated thin audio speakers. First, we present a bio-mimetic micro-speaker inspired by the sound production mechanism of field crickets. This design uses peripheral actuation unlike the usual full area actuation in the conventional electrostatic speaker designs or unlike the electrodynamic speaker designs where the diaphragm is directly actuated by magnet-coil partially covering the central area of the vibrating diaphragm. Also, as in the cricket’s sound production mechanism, we design to take advantage of the resonance. Our speaker essentially uses a silicon diaphragm created by etching out patterned cavities in the handle layer of an SOI wafer and controlled lateral etch of the buried oxide to create closely spaced top and bottom annular electrodes for peripheral actuation. These electrodes are used to drive the diaphragm with audio signal close to its resonance. The open cavity provides an incredible advantage in terms of increasing the pull-in voltage enormously. While we demonstrate the working of these micro-speakers with several audio signals, the development must continue with an array of such speakers for attaining a flat response over audible frequency range in order to make them commercially viable. The second novel design to build wafer thin loudspeakers is based on an accidental discovery we made during testing of the cricket inspired speakers. We demonstrate how two simple pieces of silicon stacked loosely together and actuated with appropriate electrical signal produce sound. The theoretical explanation is given behind the new design idea, whose foundation is electrostatic actuation. Also, a few initial results for the thin speakers developed with this design are presented