No abstract

The thermoacoustic sound generation offers a promising wideband alternative to mechanically driven loudspeakers. Over the past decade, the development of nanomaterials with new physico-chemical properties promoted a wide interest in the thermophones technology. Indeed, several thermophone structures based on suspended nanowires, graphene sheets, highly porous foams or sponges have been investigated. At the same time, theoretical models have been developed to predict the frequency and power spectra of these devices. However, most of models have taken into consideration a solid homogeneous material for representing the thermophone generating layer, and its microstructure was therefore neglected. If this assumption holds for thin dense materials, it is not acceptable for thick and porous thermophone devices. Hence, a model able to describe the behavior of highly porous foam-or sponge-like generating layers is proposed. It is based on a two temperature scheme since the thermal equilibrium is not typically attained between the foam material and the embedded air. To do this, the fluid equations for the air are coupled with the heat equation for the solid foam through boundary conditions mimicking the energy exchange at the contact surface between them. The behavior of the main physical variables within the porous generating layer is explained and comparisons with recent experimental results are thoroughly discussed.

In the past decade, a lot of research has been conducted on the potential of carbon nanostructured materials to emit sound via thermoacoustics through both simulations and experiments. However, experimental validation of simulations for three-dimensional graphene (3D-C), which has a complicated 3D structure, has yet to be achieved. In this paper, 3D-C is synthesized via thermal chemical vapor deposition and its microstructure and quality tested using Scanning Electron Microscopy and Raman spectroscopy respectively. Then, a two temperature model is used to predict the effects of numerous parameters: frequency, input power, sample size, connection area, connection path, pores per inch, thickness, compression as well as the addition of a backing on the acoustic performance and temperature of the 2 sample. The experimental results presented in this paper validate the predictions of the adopted two temperature model. The efficiency of 3D-C is then compared with results presented in other studies to understand how the presented 3D-C fared against ones from the literature as well as other carbon nanostructured materials.

A thermoacoustic sound generation model, based on the classical balance equations of the continuum mechanics, is here developed for the cylindrical and the spherical thermoacoustic wave generation. In both geometries, the model considers an arbitrary multilayered structure, where each layer can be fluid or solid and it is characterized by the fully coupled thermo-visco-acoustic response. It means that the viscous behavior and the thermal conduction are considered in each layer. The model is based on a unified representation of cylindrical or spherical thermoacoustic waves, which is valid for both fluid and solid phases. Thanks to the continuity of temperature, particle velocity, normal stress and heat flux between adjacent layers, the model can be implemented by means of a versatile matrix approach, allowing flexible analysis and design of cylindrical or spherical thermophones. Any thermoacoustic variable can be determined at any position, any frequency and for any input power. The results are compared with the models already existing in the literature and the underlying physics is thoroughly discussed. The analysis is focused on the better understanding of the thermoacoustic generation with application to the state of the art of the thermophone technology.