Mechanisms and applications of the transduction of heat into sound by arrays of thin metallic films

Abstract

The work presented in this thesis concerns the generation of sound via heat; namely, the thermoacoustic effect. This heat is typically provided by electric heating in thin film devices, resulting in non-resonant, broadband sound generation. The aim of this work is to develop a deeper understanding of the mechanisms behind this process, as well as identify potential applications. This is achieved via analytical and numerical simulations, and experimental measurements.

A novel analytical model is derived for describing the sound generated by thermoacoustic sources. This is validated by simulation, as well as electrical, thermal and acoustic measurements of a variety of sources.

Multiple sources are also considered, in the context of acoustic phased arrays. It is found that thermoacoustic sources can successfully reproduce typical phased array behaviour, such as beam forming and steering. It is shown that the unique characteristics of this transduction method make thermoacoustic sources highly favourable for this application. In particular, the sources are non-reciprocal, eliminating issues with element cross-talk. The thermal profile is spatially dependent on the local current flow, which offers the ability to define thermal sources by confining the current to specific regions in the film, such that a full phased array can be produced using a single film. Electrical coupling between elements, due to the nonlinearity of the source, results in an additional sound source that is highly sensitive to the configuration of the array. It is shown how this coupling can be used to infer properties of the array elements themselves by measuring the generated sound.

A new method of thermoacoustic generation via thermoelectric effects is also explored. Measurements using thermocouple devices show that this results in linear sound generation, at the source fundamental frequency, due to the reversible nature of the effect. This reversibility also presents opportunities to further optimise sound output via dynamic control of the device temperature.