Thermophone or the future of ultrasound transducers: Modelling of thermoacoustics generation in porous materials

Electroacoustic transducers along with piezoelectric devices are the most widely used methods for acoustic sound generation in gas and liquids. A mechanical movement of a membrane induces fluid vibration thus creating an acoustic wave. The thermoacoustic process on the other hand uses fast paces temperature variations in a sample to excite the fluid (generally air). The rapidly changing temperature generate a compression expansion of the air and thus creates an acoustical wave. Such materials are called thermophones. They were discovered in the same time period as traditional electroacoustic transducers but their limited efficiency coupled with the technological limits of fabrication prevented scientific craze at the time. In 1999 a new thermophone was presented with a significant improvement compared to the samples used a century prior. This article coupled with the newly found ease of access to complex fabrication process of nanomaterials rekindle the interest in thermoacoustic for audio purposes. In this work a thorough literature review is presented and a novel multilayer model for thermoacoustic sound generation is derived. This model was solved for plane wave, cylindrical wave and spherical wave generation. Another model based on a two temperatures hypothesis for plane wave generation is also solved to represent more accurately the generation of thick porous thermophones. An extensive analysis of those models allowed for a detailed understanding of the thermoacoustic sound generation: its strengths, weaknesses and differences with traditional speakers. Lastly, experimental investigations of porous carbon foams in partnership with CINTRA Singapore are presented. Validation of the models and insights about the handling of such flexible and lightweighted but fragile samples are presented as well at their potential applications for scientific or commercial purposes as broad band sensors.